Pressure
involves the concept of force and surface area.
1. Pressure on an area, A is the normal force, F, which is being applied perpendicularly to the area.
2. Pressure on an area, A is expressed as the normal force, F per unit area, A.
3. P = (F/A)
4. This SI unit for pressure is the pascal, Pa, where 1 Pa = 1 N/m2 (metre square).
5. Pressure is increased:
1. Pressure on an area, A is the normal force, F, which is being applied perpendicularly to the area.
2. Pressure on an area, A is expressed as the normal force, F per unit area, A.
3. P = (F/A)
4. This SI unit for pressure is the pascal, Pa, where 1 Pa = 1 N/m2 (metre square).
5. Pressure is increased:
- if the force, F applied to a given area, A is increased
- if a given force, F is applied to a smaller area, A
6. If a balloon is pressed against by a finger, the balloon will only change its shape a bit.
If the balloon is pushed against by a needle with the same force, the balloon will burst.
This is because a finger has a larger surface area (A) than a needle. Hence, the needle exerts much pressure than the finger and perforates through the surface of the balloon, making a hole and freeing the air inside the balloon (or pops the balloon instead!)
Pressure in Liquids
1. For a liquid at rest, the pressure at a certain point in
the liquid is the same in all directions.
2. The pressure in a liquid is due to
a) Density of the liquid, p.
b) Depth of the liquid, h, below the surface liquid.
c) Acceleration of the gravity, g.
3. The pressure on a liquid is proportional to the density of the liquid, p and the depth, h, at which the liquid is measured.
4. The pressure in a liquid at rest (static liquid) is independent of the shape (area and slope) of the container.
5. The applications of pressure in liquids are:
i) Dams
ii) Domestic Water supplies
i) Dams
Dams are very much thicker at the bottom than at the top, since the pressure at the bottom is the greatest.
Large dams are built for the hydroelectric generation of electricity.
The high pressure on the deep-water side of the dam causes water to flow through these holes at great speed turning the turbines in the holes and generate the electricity.
ii) Domestic Water Supplies
The main water comes from a reservoir but in order to maintain a constant high pressure to the consumer, it is pumped to the top of a water tower located on high ground.
The main pressure is determined by the height, h.
a) Density of the liquid, p.
b) Depth of the liquid, h, below the surface liquid.
c) Acceleration of the gravity, g.
3. The pressure on a liquid is proportional to the density of the liquid, p and the depth, h, at which the liquid is measured.
4. The pressure in a liquid at rest (static liquid) is independent of the shape (area and slope) of the container.
5. The applications of pressure in liquids are:
i) Dams
ii) Domestic Water supplies
i) Dams
Dams are very much thicker at the bottom than at the top, since the pressure at the bottom is the greatest.
Large dams are built for the hydroelectric generation of electricity.
The high pressure on the deep-water side of the dam causes water to flow through these holes at great speed turning the turbines in the holes and generate the electricity.
ii) Domestic Water Supplies
The main water comes from a reservoir but in order to maintain a constant high pressure to the consumer, it is pumped to the top of a water tower located on high ground.
The main pressure is determined by the height, h.
Gas pressure and Atmospheric pressure
1. Existence of Gas pressure:
Imagine when someone throws a stone to you. It feels painful isn't it. Imagine all of your classmates throw stones at you, will you feel less or more painful? That's the same thing with gas. Gas molecules moving at a high speed in a large number will cause greater pressure on a wall or surface (but considerably less than that of the stones perhaps??)
2. Kinetic Theory of gases
Imagine when someone throws a stone to you. It feels painful isn't it. Imagine all of your classmates throw stones at you, will you feel less or more painful? That's the same thing with gas. Gas molecules moving at a high speed in a large number will cause greater pressure on a wall or surface (but considerably less than that of the stones perhaps??)
2. Kinetic Theory of gases
The kinetic theory of gases is based on the following assumptions:
The molecules in a gas move freely in random motions and possess kinetic energy.
The forces of attraction between the molecules are negligible
The collisions of the molecules with each other and with the walls of the container are of elastic collisions.
The molecules of a gas in a container move in all directions to fill the entire space of the container
until they collide with its walls.
The collisions of the gas molecules with the walls of the container are elastic collisions and the molecules rebound with the same speed which results in a change in momentum for each molecule.
The total change of momentum when the gas molecules collide with the walls of the container in one second produces a force which acts on the walls of the container.
By the definition of Pressure = Force / Area (P = F/A)
3. Factors Affecting Air or Gas Pressure
a. Pressure increases when the density of gas increases.b. Pressure increases when temperature increases due to kinetic energy of molecules increases.
Atmospheric Pressure
1. Existence of Atmospheric pressure
.According to the kinetic theory of gases, gases consist of molecules which are far apart and in random motion at high speeds.
The gas molecules have mass and experience the gravitational pull. The result is that gases
have weight.
The atmosphere is a thick layer of air that surrounds the Earth. You may probably have known this.
The atmosphere exerts a pressure called atmospheric pressure which is caused by the weight of the thick layer of air above the Earth's surface.
Atmospheric pressure acts on every object on the surface of the earth. No one is in exception.
Activity to show the existence of Atmospheric Pressure
Fill a glass with water to the brim. Cover it with a thick cardboard. Invert it downwards. The water does not fall down. Why? because atmospheric pressure supports the cardboard (and water) from falling. The resultant force on the cardboard is greater than the weight of water. Even in the existence of gravity!
Boil an empty tin half-filled with water. Cap the tin. Let it cool under running tap water. Wallaa....the tin will get crumpled as the water cools down. As the steam condenses, the pressure
inside the metal tin decreases, The external atmospheric pressure which is Higher, crushes the tin.
Mercury Barometer
1. A mercury barometer consists of a thick-walled glass tube, which is closed
at one end.2. The tube is completely filled with mercury and inverted several times to remove air bubbles.
The tube is then completely filled again with mercury.
3. After all air has been removed, the open end of the glass tube is inverted into a container of mercury.
4. The mercury column drops until it reaches a height of about 76cm above the lower surface. The space between the top of the mercury and the end of the tube should contain no air; it must be in a complete vacuum.
5. The column of mercury in the tube is supported by the atmospheric pressure and its height depends on the magnitude of the atmospheric pressure.
6. Since the atmospheric pressure at sea level can support a vertical column of mercury 76 cm or 760 mm high, we can, for convenience, express mm Hg as a unit of pressure. 1 Standard atmospheric pressure (1 P atm) = 76 cm Hg or 760 mm Hg (also known as one atmosphere).
P atm = 76 cm Hg = 10 000 Pa.
1 P atm = 76 cm Hg = 10 000 Pa = 1 bar.
7. In unit m water: P atm = 10 m water.
Application of Atmospheric pressure
Applications of atmospheric pressure:
Drinking Straw
1. When drinking with a straw, one has to suck the straw. This causes the pressure in hte straw to decrease.
2. The external atmospheric pressure, which is greater, will then act on the surface of the water in the glass, causing it to rise through the straw.
Rubber Sucker
1. When the rubber sucker is put onto a smooth surface, usually a glass or tiled surface, the air in the rubber sucker is forced out. This causes the space between the surface and the sucker to have low pressure.
2. The contact between the rubber sucker and the smooth surface is airtight.
3. The external atmospheric pressure, which is much higher, acts on the rubber sucker, pressing it securely against the wall.
Siphon
1. A rubber tube can be used to siphon liquid from a container at a higher level to another at a lower level. For example, we can remove petrol from the petrol tank of a vehicle or dirty water from aquarium.
2. The tube is first filled with the liquid and one end is placed in the liquid in the container A. The other end is placed at a level which must be lower than the surface of the liquid in container A.
3. The pressure in the rubber at the lower end is equal to atmospheric pressure plus the pressure due to h cm column of liquid. As the pressure at the lower end is greater than the atmospheric pressure, the liquid flows out.
Vacuum Cleaner
1. vacuum cleaner applies the principle of atmospheric pressure to remove dust particles. When it is switched on, the fan sucks out the air from space inside the vacuum (space A). Space A then becomes a partial vacuum.
2. The atmospheric pressure outside, which is greater, then forces air and dust particles into the filter bag. This traps the dust particles but allows the air to flow through an exit ath the back.
Lift Pump
1. A lift pump is used to pump water out of a well or to a higher level. The greatest height to which the water can be pumped is 10 m. This is equivalent to the atmospheric pressure.
2. When the plunger is lifted, the upper valve closes and the lower valve opens. The atmospheric pressure, acting on the surface of the water, causes water to flow past valve B into the cylinder.
3. When the plunger is pushed down, the lower valve closes and the upper valve opens. Water flows above the plunger.
4. When the plunger is next lifted, the upper valve closes again and the lower valve opens once more. the atmospheric pressure, acting on the surface of the water, forces water past the lower valve into the cylinder. Simultaneously, the water above the plunger is lifted and flows out through the spout.
5. This process is repeated until sufficient water is obtained.
Drinking Straw
1. When drinking with a straw, one has to suck the straw. This causes the pressure in hte straw to decrease.
2. The external atmospheric pressure, which is greater, will then act on the surface of the water in the glass, causing it to rise through the straw.
Rubber Sucker
1. When the rubber sucker is put onto a smooth surface, usually a glass or tiled surface, the air in the rubber sucker is forced out. This causes the space between the surface and the sucker to have low pressure.
2. The contact between the rubber sucker and the smooth surface is airtight.
3. The external atmospheric pressure, which is much higher, acts on the rubber sucker, pressing it securely against the wall.
Siphon
1. A rubber tube can be used to siphon liquid from a container at a higher level to another at a lower level. For example, we can remove petrol from the petrol tank of a vehicle or dirty water from aquarium.
2. The tube is first filled with the liquid and one end is placed in the liquid in the container A. The other end is placed at a level which must be lower than the surface of the liquid in container A.
3. The pressure in the rubber at the lower end is equal to atmospheric pressure plus the pressure due to h cm column of liquid. As the pressure at the lower end is greater than the atmospheric pressure, the liquid flows out.
Vacuum Cleaner
1. vacuum cleaner applies the principle of atmospheric pressure to remove dust particles. When it is switched on, the fan sucks out the air from space inside the vacuum (space A). Space A then becomes a partial vacuum.
2. The atmospheric pressure outside, which is greater, then forces air and dust particles into the filter bag. This traps the dust particles but allows the air to flow through an exit ath the back.
Lift Pump
1. A lift pump is used to pump water out of a well or to a higher level. The greatest height to which the water can be pumped is 10 m. This is equivalent to the atmospheric pressure.
2. When the plunger is lifted, the upper valve closes and the lower valve opens. The atmospheric pressure, acting on the surface of the water, causes water to flow past valve B into the cylinder.
3. When the plunger is pushed down, the lower valve closes and the upper valve opens. Water flows above the plunger.
4. When the plunger is next lifted, the upper valve closes again and the lower valve opens once more. the atmospheric pressure, acting on the surface of the water, forces water past the lower valve into the cylinder. Simultaneously, the water above the plunger is lifted and flows out through the spout.
5. This process is repeated until sufficient water is obtained.
Atmospheric pressure and altitude
Atmospheric pressure and
altitude
1. Atmospheric pressure decreases with altitude, or the height above of sea level. At higher altitudes, the density and temperature of the air are lower. As a result, the frequency of collisions of the molecules is decreased (lower). Hence, atmospheric pressure is lower.
Total pressure below the surface of a liquid.
1. The formula for liquid pressure, P = hpg, is used to determine the additional pressure due to the weightmof the liquid at any point below the liquid's surface.
2. As a result, the total pressure acting at a depth, h below the liquid's surface is the sum of the pressure due to the weight of the liquid (P) and the atmospheric pressure acting on the liquid's surface.
Total pressure acting on an object below a water with a depth of h = atmospheric pressure + hpg.
1. Atmospheric pressure decreases with altitude, or the height above of sea level. At higher altitudes, the density and temperature of the air are lower. As a result, the frequency of collisions of the molecules is decreased (lower). Hence, atmospheric pressure is lower.
Total pressure below the surface of a liquid.
1. The formula for liquid pressure, P = hpg, is used to determine the additional pressure due to the weightmof the liquid at any point below the liquid's surface.
2. As a result, the total pressure acting at a depth, h below the liquid's surface is the sum of the pressure due to the weight of the liquid (P) and the atmospheric pressure acting on the liquid's surface.
Total pressure acting on an object below a water with a depth of h = atmospheric pressure + hpg.
Instruments for measuring atmospheric pressure
Instruments for measuring atmospheric pressure:
1. Mercury Barometer
A mercury barometer consists of a thick-walled glass tube, which is closed at one end.
The tube is completely filled with mercury and inverted several times to remove air bubbles. The tube is then completely filled again with mercury.
After all air has been removed, the open end of the glass tube is inverted into a container of mercury.
The mercury column drops until it reaches a height about 76 cm above the lower surface. The space between the top of the mercury and the end of the tube should contain no air; it is a complete vacuum.
The column of mercury in the tube is supported by the atmospheric pressure and its height depends on the magnitude of the atmospheric pressure
2. Fortin Barometer
A fortin barometer is a type of mercury barometer which has a higher accuracy.
This barometer has a vernier scale which gives a more accurate reading of the atmospheric pressure. The mercury level in the container can be adjusted by a screw until the pointer touches the surface of the mercury. This eliminates the zero error.
The atmospheric pressure is measured in mm Hg.
3. Aneroid Barometer
An aneroid barometer does not use any liquid. It consists of a sealed metal chamber in the form of a flat cylinder with flexible walls. The chamber is partially evacuated and a spring helps prevent it from collapsing.
The chamber expands and contracts in response to changes in atmospheric pressure. The movement of the chamber walls is transmitted by a mechanical lever system which moves a pointer over a calibrated scale.
The Aneroid Barometer can be used a an altimeter (to determine altitude) by mountaineers or pilots to determine an airplane's altitude. The scale can be calibrated to give readings of altitude equivalent to a range of values of atmospheric pressure.
- an aneroid barometer is also used as a weather glass to forecast the weather.
Rain clouds form in large areas of lower pressure air, so a fall in the barometer reading often means that bad weather is coming
1. Mercury Barometer
A mercury barometer consists of a thick-walled glass tube, which is closed at one end.
The tube is completely filled with mercury and inverted several times to remove air bubbles. The tube is then completely filled again with mercury.
After all air has been removed, the open end of the glass tube is inverted into a container of mercury.
The mercury column drops until it reaches a height about 76 cm above the lower surface. The space between the top of the mercury and the end of the tube should contain no air; it is a complete vacuum.
The column of mercury in the tube is supported by the atmospheric pressure and its height depends on the magnitude of the atmospheric pressure
2. Fortin Barometer
A fortin barometer is a type of mercury barometer which has a higher accuracy.This barometer has a vernier scale which gives a more accurate reading of the atmospheric pressure. The mercury level in the container can be adjusted by a screw until the pointer touches the surface of the mercury. This eliminates the zero error.
The atmospheric pressure is measured in mm Hg.
3. Aneroid Barometer
An aneroid barometer does not use any liquid. It consists of a sealed metal chamber in the form of a flat cylinder with flexible walls. The chamber is partially evacuated and a spring helps prevent it from collapsing.
The chamber expands and contracts in response to changes in atmospheric pressure. The movement of the chamber walls is transmitted by a mechanical lever system which moves a pointer over a calibrated scale.
The Aneroid Barometer can be used a an altimeter (to determine altitude) by mountaineers or pilots to determine an airplane's altitude. The scale can be calibrated to give readings of altitude equivalent to a range of values of atmospheric pressure.
- an aneroid barometer is also used as a weather glass to forecast the weather.
Rain clouds form in large areas of lower pressure air, so a fall in the barometer reading often means that bad weather is coming
Instruments for measuring Gas Pressure
There are several instruments
for measuring gas pressure, I will explain only two here
1. Manometer
A manometer consists of a U-tube filled with a liquid (mercury, water or oil) with a certain density.
The manometer is used to measure the difference in pressure between the two sides of the U-tube.
When the manometer is not connected to the gas supply, i.e. when both arms are open to the atmosphere, the liquid levels in both arms are equal.
To measure the pressure of a gas, the other arm is connected to the gas pipe and the gas pressure acts on the surface of the liquid in the respective arm.
if the gas pressure is greater than the atmospheric pressure, the liquid in the respective arm (say arm B) will be pushed downwards. Under equilibrium conditions, (same pressure from both arms), the level of the liquid will be at the same level.
2. Bourdon Gauge
A Bordon gauge consists of a coil of flattened copper tube with an oval cross section connected to a lever system.
When the gas supply is connected, the pressure in the gas acts to straighten the copper coil.
1. Manometer
A manometer consists of a U-tube filled with a liquid (mercury, water or oil) with a certain density.
The manometer is used to measure the difference in pressure between the two sides of the U-tube.
When the manometer is not connected to the gas supply, i.e. when both arms are open to the atmosphere, the liquid levels in both arms are equal.
To measure the pressure of a gas, the other arm is connected to the gas pipe and the gas pressure acts on the surface of the liquid in the respective arm.
if the gas pressure is greater than the atmospheric pressure, the liquid in the respective arm (say arm B) will be pushed downwards. Under equilibrium conditions, (same pressure from both arms), the level of the liquid will be at the same level.
2. Bourdon Gauge
A Bordon gauge consists of a coil of flattened copper tube with an oval cross section connected to a lever system.
When the gas supply is connected, the pressure in the gas acts to straighten the copper coil.
The movement of the copper coil is transferred to the lever system which actuates a pointer to move across a scale which has been calibrated to give readings of pressure.
The unit of measurement used in the Bourdon gauge is Pascal. Bourdon gauges are normally connected to gas cylinders to give an indication of the quantity of gas in the cylinders.
Bourdon gauges are more robust than manometers and more suitable for measuring higher pressures. But they have to be calibrated before they can be used.
Applications of Pascal's Principle in Everyday Life
A hydraulic system is a device in which a small applied
force can give rise to a larger force.
The principle in the hydraulic system is widely used in jacks, vehicle brake systems, hydraulic presses and heavy machinery and a few more examples which you can find (for yourself)
Hyraulic Jacks
Hydraulic jacks are used to lift a heavy load such as when changing a car tyre. When the handle is pressed down, a valve closes and the small piston forces hydraulic fluid through another valve to the larger cylinder. The pressure transmitted results in a large force on the load.
When the handle is raised, valve B closes and hydraulic fluid flows from the buffer tank through valve A into the small cylinder. The handle is moved up and down repeatedly until the load is sufficiently lifted up.
The large piston can be lowered at the end by opening the release valve to allow all the hydraulic fluid to flow back into the buffer tank.
Hydraulic Brakes
Hydraulic brakes are used in cars, lorries and motorcycles.
In a hydraulic brake system, a liquid, known as brake fluid,
is used to transmit pressure from the brake pedal to all the wheels of the vehicle.
When the brake pedal is pressed, the piston of the control cylinder applies a pressure on the brake fluid and this pressure is transmitted, via a system of pipes, to each cylinder at the wheels.
The cylinder at the wheels cause a pair of pistons to push a pair of friction pads to press against the surface of the brake discs or brake drums. The frictional forces between these brake components cause the vehicle to slow down and stop.
When the brake pedal is released, a spring restores the brake discs to their original positions.
Hydraulic Pumps
Hydraulic pumps are used to raise cars in a motor workshop.
The machine is equipped with a small cylinder connected to a large cylinder. Both cylinders are filled with oil.
Compressed air is introduced into the small cylinder in which the compressed air exerts a pressure on the surface of the oil.
This pressure is transmitted by the oil to the large cylinder where the pressure acts on a large piston to produce a force which is large enough to lift a car.
The principle in the hydraulic system is widely used in jacks, vehicle brake systems, hydraulic presses and heavy machinery and a few more examples which you can find (for yourself)
Hyraulic Jacks
Hydraulic jacks are used to lift a heavy load such as when changing a car tyre. When the handle is pressed down, a valve closes and the small piston forces hydraulic fluid through another valve to the larger cylinder. The pressure transmitted results in a large force on the load.
When the handle is raised, valve B closes and hydraulic fluid flows from the buffer tank through valve A into the small cylinder. The handle is moved up and down repeatedly until the load is sufficiently lifted up.
The large piston can be lowered at the end by opening the release valve to allow all the hydraulic fluid to flow back into the buffer tank.
Hydraulic Brakes
Hydraulic brakes are used in cars, lorries and motorcycles.
In a hydraulic brake system, a liquid, known as brake fluid,
is used to transmit pressure from the brake pedal to all the wheels of the vehicle.
When the brake pedal is pressed, the piston of the control cylinder applies a pressure on the brake fluid and this pressure is transmitted, via a system of pipes, to each cylinder at the wheels.
The cylinder at the wheels cause a pair of pistons to push a pair of friction pads to press against the surface of the brake discs or brake drums. The frictional forces between these brake components cause the vehicle to slow down and stop.
When the brake pedal is released, a spring restores the brake discs to their original positions.
Hydraulic Pumps
Hydraulic pumps are used to raise cars in a motor workshop.
The machine is equipped with a small cylinder connected to a large cylinder. Both cylinders are filled with oil.
Compressed air is introduced into the small cylinder in which the compressed air exerts a pressure on the surface of the oil.
This pressure is transmitted by the oil to the large cylinder where the pressure acts on a large piston to produce a force which is large enough to lift a car.
Basic Hydraulic System
A hydraulic system operates based on Pascal's principle.
In this hydraulic system, a small force, F1 is applied to the small piston resulting in a large force , F2 at the piston K. The pressure, due to the force, F1, is transmitted by the liquid to the large piston.
Pressure, P = F1/A1
This pressure is transmitted through the liquid and acts on the base of the large piston.
Force on the large piston, F2 = P X A2.
= (F1/A1) X A2.
The large force causes the load to rise.
Also F2/F1 = A2/A1
Output force / input force = output piston area / input piston area
Because of the much larger surface area, A2 of the piston K compared to the surface area, A1 of the piston, the resultant force, F1.
This shows that a large force can be produced by a small force, using Pascal's principle.
Hydraulic systems act as a force multiplier where A2/A1 is the multiplying factor.
For example, if A2=5A1, then F2 = 5F1
since F2 = F1 X (A2/A1)
A hydraulic system must not contain any air bubbles in any portion of its hydraulic fluid system.
The presence of air bubbles in the hydraulic fluid system will reduce the efficiency of the system as part of the applied force will be used to compress the air bubbles.
In this hydraulic system, a small force, F1 is applied to the small piston resulting in a large force , F2 at the piston K. The pressure, due to the force, F1, is transmitted by the liquid to the large piston.
Pressure, P = F1/A1
This pressure is transmitted through the liquid and acts on the base of the large piston.
Force on the large piston, F2 = P X A2.
= (F1/A1) X A2.
The large force causes the load to rise.
Also F2/F1 = A2/A1
Output force / input force = output piston area / input piston area
Because of the much larger surface area, A2 of the piston K compared to the surface area, A1 of the piston, the resultant force, F1.
This shows that a large force can be produced by a small force, using Pascal's principle.
Hydraulic systems act as a force multiplier where A2/A1 is the multiplying factor.
For example, if A2=5A1, then F2 = 5F1
since F2 = F1 X (A2/A1)
A hydraulic system must not contain any air bubbles in any portion of its hydraulic fluid system.
The presence of air bubbles in the hydraulic fluid system will reduce the efficiency of the system as part of the applied force will be used to compress the air bubbles.
Law of Flotation
Have you ever wondered what causes things to float on water
or liquid?
Well, floating is caused by an upthrust force that act on the material and interestingly there's a LAW that governs whether an object floats or not it is called the LAW of Flotation.
"Law of flotation is an application of Archimedes' principle"
When a piece of wood of density more than water is placed on water, it sinks and displaces some water.
As it sinks, more and more water is displaced. This increases the buoyant force as the the buoyant force is equal to the weight of water displaced.
The wood will sink until the buoyant force equal its weight.
Therefore,
The law of flotation states that a floating object displaces its own weight of the fluid in which it floats.
i.e.
Weight of floating object= weight of fluid displaced
Mass of floating object = mass of fluid displaced
Any changes in the density of the surrounding liquid affects the level in which an object floats.
Thus, you have to remember that an object will DISPLACE the amount of water or liquid that is equal to its own mass in order to float
Well, floating is caused by an upthrust force that act on the material and interestingly there's a LAW that governs whether an object floats or not it is called the LAW of Flotation.
"Law of flotation is an application of Archimedes' principle"
When a piece of wood of density more than water is placed on water, it sinks and displaces some water.
As it sinks, more and more water is displaced. This increases the buoyant force as the the buoyant force is equal to the weight of water displaced.
The wood will sink until the buoyant force equal its weight.
Therefore,
The law of flotation states that a floating object displaces its own weight of the fluid in which it floats.
i.e.
Weight of floating object= weight of fluid displaced
Mass of floating object = mass of fluid displaced
Any changes in the density of the surrounding liquid affects the level in which an object floats.
Thus, you have to remember that an object will DISPLACE the amount of water or liquid that is equal to its own mass in order to float
Buoyant Force and Flotation
What Causes Buoyant Force?
The difference between the forces acting on the UPPER surface
and the LOWER surface is the net force acting UPWARDS.
This net force is known as the BUOYANT FORCE. (Remember! Pressure = Force / Area). If you rearrange the Force to become the subject F = P x A.
This net force is known as the BUOYANT FORCE. (Remember! Pressure = Force / Area). If you rearrange the Force to become the subject F = P x A.
Pressure, P = hpg (h = height, p = density, g = gravitational
force)
Say,
Say,
Force acting on the upper surface, F1 = P1 A
= h1pg A
Force Acting Underside, F2 = P2 A
= h2pg A
Net Force Acting Upwards = F2 - F1
= h1pg A
Force Acting Underside, F2 = P2 A
= h2pg A
Net Force Acting Upwards = F2 - F1
= Buoyant Force
= Weight of Liquid Displaced = mg
= Weight of Liquid Displaced = mg
h1 is the distance from the TOP area of the object to the surface
P1 is pressure 1
p is density of liquid
g is gravity value
A is the total surface area.
h2 is distance from surface to BOTTOM part of the object
P2 is pressure exerted on the lower side
Therefore, the buoyant force is equal to the weight of liquid
displaced, which is in accordance with Archimedes' Principle as I will explain
here.
In the above
figure, the object is lowered into the water, the following observations are
made.
i) The object experiences a reduction in weight. The object of the weight in water is less than its weight in air. The apparent loss in weight of the obejct is caused by the buoyant force of the surrounding water
on the object.
Apparent loss in weight of object
=Weight of object - weight of object in water.
ii) The object displaces a vlume of water.
Volume of water displaced
= volume of the submerged part of the stone
iii) From the figure, the apparent loss in weight is due to the buoyant force.
Therefore :
Bouyant Force = Actual weight - weight in water
= (Say) 70N - 40 N
= 30 N
i) The object experiences a reduction in weight. The object of the weight in water is less than its weight in air. The apparent loss in weight of the obejct is caused by the buoyant force of the surrounding water
on the object.
Apparent loss in weight of object
=Weight of object - weight of object in water.
ii) The object displaces a vlume of water.
Volume of water displaced
= volume of the submerged part of the stone
iii) From the figure, the apparent loss in weight is due to the buoyant force.
Therefore :
Bouyant Force = Actual weight - weight in water
= (Say) 70N - 40 N
= 30 N
Application of Archimedes' Principle
Hello guys, these are only a few examples of the application
of Archimedes' Principle. This examples only serve as a guidance and you should
try to search for other examples. :) Hope this helps.
1. Submarine:
A submarine has a large ballast tank, which is used to control its position and depth from the surface of the sea.
1. Submarine:
A submarine has a large ballast tank, which is used to control its position and depth from the surface of the sea.
A submarine submerges by letting water into the ballast tank so that its weight becomes greater than the buoyant force.
Conversely, it floats by reducing water in the ballast tank.-thus its weight is less than the buoyant force
2. Hot-air balloon
The atmosphere is filled with air that exerts buoyant force on any object.
A hot air balloon rises and floats due to the buoyant force (when the surrounding air is greater than its weight). It descends when the balloon's weight is higher than the buoyant force. It becomes stationary when the weight equals the buoyant force.
The weight of the Hot-air balloon can be controlled by varying the quantity of hot air in the balloon.
3. Hydrometer
A hydrometer is an instrument to measure the relative density of liquids.
It consists of a tube with a bulb at one end. Lead shots are placed in the bulb to weigh it down and enable the hydrometer to float vertically in the liquid.
In a liquid of lesser density, a greater volume of liquid must be displaced for the buoyant force to equal to the weight of the hydrometer so it sinks lower.
Hydrometer floats higher in a liquid of higher density.
Density is measured in the unit of g cm-3.
4. Ship
A ship floats on the surface of the sea because the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship.
A ship is constructed in a way so that the shape is hollow, to make the overall density of the ship lesser than the sea water. Therefore, the buoyant force acting on the ship is large enough to support its weight.
The density of sea water varies with location. The PLIMSOLL LINE marked on the body of the ship acts as a guideline to ensure that the ship is loaded within the safety limit.
A ship submerge lower in fresh water as fresh water density is lesser than sea water. Ships will float higher in cold water as cold water has a relatively higher density than warm water.
5. Fishes
Certain
group of fishes uses Archimedes’ principles to go up and down the water.
To go up to
the surface, the fishes will fill its swim bladder (air sacs) with gases
(clever isn't it?).
The gases
diffuse from its own body to the bladder and thus making its body lighter. This
enables the fishes to go up.
To go down,
the fishes will empty their bladder, this increases its density and therefore
the fish will sink.
6. FLIP – Floating instrument platform.
This is a
research ship that does research on waves in deep water. It can turn
horizontally or vertically. When water is pumped into stern tanks, the ship
will flip vertically.
The
principle that is used in FLIP is almost similar with the submarines. Both
ships pump water in or out of tank to rise or sink.
Understanding Bernoulli's Principle
Have you ever thought why birds can fly so efficiently in
the air?
or likewise the plane?
that movement or phenomenon can be explained by Bernoulli's Principle.
Bernoulli's Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.
Therefore, the pressure in a moving fluids depends on its flow velocity (remember fluids = water, air)
A full definition of Bernoulli's Principle is:
IN a steady flow of a fluid, the pressure of the fluid decreases when the velocity of the fluid increases.
Bernoulli's principle is very important as it is used in the design of airplanes, boat hulls, fan blades and cars.
Example of situations that involves Bernoulli' s Principle:
Ping Pong Balls and Funnels
An inverted filter funnel can hold a ping pong ball if you blow air through the funnel, it does not drop down. Thats because the air flows around the ping pong ball at high speed and creating a low-pressured area, the higher atmospheric pressure supports the ball from falling.
Curve Balls in Baseball
Now lets move on from ping pong to baseball. A pitcher occasionally tries to fool the batter into a strike by throwing a curve ball. It seems to be heading straight into the strike zone but veers off at the last minute. STRIKE! How do they do that?
OK, you can try this at home.
Try blowing through two pieces of paper. Separate the papers slightly so you can blow air through it. Instead of the paper being separated it will get nearer to each other. That's because the faster air flow in the middle of the papers creating a lower-pressure region and the higher atmospheric pressure pushes the paper so it gets nearer to each other.
There are more situations of Bernoulli's principle in daily life if you dare to think and spend a little time observing.
or likewise the plane?
that movement or phenomenon can be explained by Bernoulli's Principle.
Bernoulli's Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.
Therefore, the pressure in a moving fluids depends on its flow velocity (remember fluids = water, air)
A full definition of Bernoulli's Principle is:
IN a steady flow of a fluid, the pressure of the fluid decreases when the velocity of the fluid increases.
Bernoulli's principle is very important as it is used in the design of airplanes, boat hulls, fan blades and cars.
Example of situations that involves Bernoulli' s Principle:
Ping Pong Balls and Funnels
An inverted filter funnel can hold a ping pong ball if you blow air through the funnel, it does not drop down. Thats because the air flows around the ping pong ball at high speed and creating a low-pressured area, the higher atmospheric pressure supports the ball from falling.
Curve Balls in Baseball
Now lets move on from ping pong to baseball. A pitcher occasionally tries to fool the batter into a strike by throwing a curve ball. It seems to be heading straight into the strike zone but veers off at the last minute. STRIKE! How do they do that?
OK, you can try this at home.
Try blowing through two pieces of paper. Separate the papers slightly so you can blow air through it. Instead of the paper being separated it will get nearer to each other. That's because the faster air flow in the middle of the papers creating a lower-pressure region and the higher atmospheric pressure pushes the paper so it gets nearer to each other.
There are more situations of Bernoulli's principle in daily life if you dare to think and spend a little time observing.
Applications of Bernoulli's Principle
1. Aerofoil
The flight of an aeroplane is based on the principle regarding to the effect of the flow of air around its wings, which is, the aerofoil.
An aerofoil shape has a rounded front edge and pointed (sharp) trailing edge. The top surface is arched (curved) and the bottom is flat.
When a wing in the form of aerofoil moves through air, the flow of the air over the top has to travel faster to cover the longer distance (compares to the lower portion) and creates a region of low pressure. The flow of air below the wing is slower resulting in a region of higher pressure.
The difference between the pressures at the top and the bottom creates a NET UPWARD FORCE..(remember! bottom part higher pressure..upper part lower pressure).This is called a Lift and helps the plane to take off.
In addition to that, inverted aerofoils are used in racing cars to create a donward force and stabilize the cars at high speed.
2. Bunsen Burner
When a bunsen burner is connected to a gas supply, the gas flows at high velocity through a narrow passage in the burner, creating a region of low pressure.
The outside air, which is at atmospheric pressure, is drawn in an mixes with the gas.
The mixture of gas and air enables the gas to burn completely to produce a clean, hot fire.
Other applications that you must read on your own.
- Hydrofoil Boat
- Insecticide Spray (or whatever sprays that available)
- The shape of canvas roof ( in car) when its moving - why the roof bulges upward?
- Carburettor
- Curve Ball Spin offs
- The shape of a ski-jumper's body when he's jumping.
The flight of an aeroplane is based on the principle regarding to the effect of the flow of air around its wings, which is, the aerofoil.
An aerofoil shape has a rounded front edge and pointed (sharp) trailing edge. The top surface is arched (curved) and the bottom is flat.
When a wing in the form of aerofoil moves through air, the flow of the air over the top has to travel faster to cover the longer distance (compares to the lower portion) and creates a region of low pressure. The flow of air below the wing is slower resulting in a region of higher pressure.
The difference between the pressures at the top and the bottom creates a NET UPWARD FORCE..(remember! bottom part higher pressure..upper part lower pressure).This is called a Lift and helps the plane to take off.
In addition to that, inverted aerofoils are used in racing cars to create a donward force and stabilize the cars at high speed.
2. Bunsen Burner
When a bunsen burner is connected to a gas supply, the gas flows at high velocity through a narrow passage in the burner, creating a region of low pressure.
The outside air, which is at atmospheric pressure, is drawn in an mixes with the gas.
The mixture of gas and air enables the gas to burn completely to produce a clean, hot fire.
Other applications that you must read on your own.
- Hydrofoil Boat
- Insecticide Spray (or whatever sprays that available)
- The shape of canvas roof ( in car) when its moving - why the roof bulges upward?
- Carburettor
- Curve Ball Spin offs
- The shape of a ski-jumper's body when he's jumping.
Read full Pressure Class 10| SEE Physics Notes
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