Chapter 3: Forces and Pressure
Physics SPM 2017
CHAPTER 3: FORCES AND PRESSURE 3.1
Pressure is the perpendicular perpendicular force per unit area.
P
Pa -2 Nm -2 N cm cm Hg m water atm bar
F
A
where P = pressure [Pa] F = = force [N] 2 A = area [m ]
3.2
SIunit EquivalenttoPa
1 atm atm = 1 bar = atmos atmosphe pheri ric c pressureatsealevel
Pressure in Liquids
does not depend on the size or shape of the container, or the volume of the liquid. It is affected by factors in the following formula:
P
h g
where P = pressure [Pa] h = height of liquid [m] -3 ρ = density of liquid [kg m ] -2 g = gravitational acceleration [m s ]
3.3
Atmospheric and Gas Pressure
3.3.1 Atmospheric Pressure Atmospheric pressure is measured by barometers. Atmospheric pressure at sea level: 76 cm Hg 10 m water 100 000 Pa 1 bar 1 atm
Mercury barometer
Aneroid barometer
Atmospheric pressure measured by a simple mercury barometer can be calculated using the formula P=hρg .
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Chapter 3: Forces and Pressure
Physics SPM 2017
For example: -3 Given that h = 76 cm and the density of mercury is 13 600 kg m : Answer: Atmospheric pressure = 76 cm Hg State the answer in Pascal: P=hρg= 0.76(13 600)(10) =
3.3.2 Gas Pressure •
can be measured using
and
Manometer
.
Bourdon gauge
In manometers, pressure can be calculated based on pressure equilibrium. equilibrium. For example: Pressure at point A = Pressure at point C Gas pressure = Atmospheric pressure + hρg
Pressure at point A = Pressure at point C Gas pressure = Atmospheric pressure - hρg
A
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C
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Chapter 3: Forces and Pressure
Physics SPM 2017 3.4
Pascal’s Principle
states that pressure which is exerted onto the surface of a fluid in a closed container will be transferred uniformly throughout throughout the entire fluid.
F 1 A1
F 2 A2
2
1
F 2
F 1
All
systems are built based on Pascal’s Principle.
3.4.1 Practical Applications Hydraulic jack
Hydraulic brake
Hydraulic pump
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When the lever is pushed downwards, valve A closes while valve B opens. Pressure is transferred to the larger cylinder and creates a large force to lift the car. When the lever is pulled upwards, valve B closes while valve A opens. The hydraulic oil flows out from the tank to fill the space in the small cylinder. As every movement downwards by the lever creates a small movement upwards for the large piston, the lever needs to be pushed and pulled repeatedly before the car can be raised to a certain height required. The car can be lowered down by opening the release valve so that the hydraulic oil can flow back into the tank. When the driver steps on the brake pedal, the piston in the main cylinder transfers pressure through the hydraulic oil in the cylinders. Pressure is transferred uniformly through the hydraulic oil. The pistons in the slave cylinders are larger, therefore a large amount of force is transferred to the brake pads. The brake pads transfer the large amount of force onto the disc so that the rotation of the tire is s lowed down. A hydraulic machine uses a hydraulic pump to transfer hydraulic liquid from one vessel into a large cylinder to create a movement. The diagram shows a hydraulic pump used to move the arm of a crane.
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Chapter 3: Forces and Pressure
Physics SPM 2017 3.5
Archimedes’ Principle states that when an object is partially or fully immersed in a fluid, the object which is equal to the .
experiences a
Buoyant force = Weight of fluid displaced displaced ρg = V ρg 3
where V = = volume of fluid displaced [m ] -3 ρ = density of fluid [kg m ] -2 g = gravitational acceleration [m s ]
If the object is floating stationary: mg=Vρg mg=Vρg
Buoyant force Weight of object
3.5.1 Hydrometer A hydrometer is used to measure the
of a liquid.
When the hydrometer floats stationary on the surface of the liquid, Weight of the hydrometer = Weight of the liquid displaced mg =Vρg where m = mass of the hydrometer [kg] 3 V = = volume of liquid displaced [m ] -3 ρ = density of liquid [kg m ] -2 g = gravitational acceleration [m s ]
The higher the hydrometer floats, the denser the liquid. The lower the hydrometer floats, the less dense the liquid.
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Chapter 3: Forces and Pressure
Physics SPM 2017 3.5.2 Practical applications
The buoyant force acting on the submarine is always the same because the weight of fluid displaced is always the same.
When water is pumped into the ballast tank, the weight of the submarine submarine increases. When the weight of the submarine is greater than the buoyant force, the submarine sinks.
When water is pumped out of the ballast tank and replaced with air, the weight of the submarine decreases. When the weight of the submarine is less than the buoyant force, the submarine floats.
A hot air balloon displaces a large volume of air. When the weight of the balloon is less than the upward thrust, the hot air balloon will move upwards. Ships are marked with at the sides as a guide on the maximum load the ship can bear in different conditions.
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TF = Tropical freshwater F = Freshwater T = Tropical ocean S = Summer ocean W = Winter ocean WNA = Winter in North Atlantic Ocean
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Chapter 3: Forces and Pressure
Physics SPM 2017 3.6
Bernoulli’s Principle states that when the
of a flowing fluid
, the pressure at that point
.
Water flows from .
The cross-section at B is the smallest, therefore water travels at the fastest speed. The pressure at B is the lowest. Speed at A and C are sthe same because they have the same cross-section area. However, the pressure gradient still decreases from A to C.
3.6.1 Practical Applications Applications
As the aerofoil shape cuts through the air, the air is forced to travel at a higher speed on top of the aerofoil compared to below the aerofoil. The high speed area causes the air to have low pressure. Therefore, the higher pressure area pressures against the aerofoil towards the lower pressure area causing a lifting force.
Carburetor mixes petrol and air for burning in the engine cylinder. When air flows through the narrower part of the tube, the speed increases thus the pressure decreases. An area with low pressure is formed at X. The atmospheric pressure, which is greater, pushes the petrol out from the jet.
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Gas flows out through the narrow jet at a very high velocity. A low-pressured area is formed at X. The air outside at atmospheric pressure flows in and mixes with gas to light it. Oxygen supply which is enough from the air will ensure a complete burning to produce a hotter flame and less luminous.
CHAPTER
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