O Level Revision : Combined Science - Science in Structures and Mechanical Systems - Machines
A device that makes work easier by reducing effort required increasing distance, or changing the direction of the force.
Friction – A force that opposes motion. It will cause an object to slow down and eventually stop.
Machine work – Force acting over a distance to move an object against some resistance. If nothing is moved then no work is done.
Pressure in solids
- Pressure is defined as the force acting per unit area.
- Pressure can be expressed mathematically as follows: Pressure = force
P = F
- We are dividing Newtons by square metres in the above equation, so the unit obtained is the Newton per square metre (N/M2).
- The other commonly used unit of pressure is the Pascal (Pa).
1 Newton per square metre = 1 Pascal i.e. 1 N/m2 = 1 Pa Consider a 5 kg solid block lying on the ground.
- The block is pushing down on the earth with a force equal to the weight of the block.
- To change mass in kilograms to weight in Newtons we always multiply the mass by 10 m/s2
- Force = weight = 10 × mass
= 10 m/s2 × 5 kg (mass of block is 5 kilograms)
Area of surface of block pushing down on earth = Length × width
= 0.5 m × 0.3 m
= 0.15 m2
Therefore, pressure = Force area
= 0.15 m2
Pressure in liquids
- Particles of a liquid are constantly pushing against the walls of a container in which the liquid is thus
(i) increases with depth-Fig. 16.7 (a)
(ii)acts equally in all directions at a given depth (is the same in all directions at the same depth)
- In liquids pressure is given by the formula:
P = pgh where P is pressure in Pascals (NM-2)
p is density of liquid in kgm-3
g is gravitational acceleration =10 ms-2
h is height of liquid column in metres
Note: Pressure in liquids does not depend on area over which it acts (unlike in solids) but depends on depth.
Measurement of fluid pressure
- The instrument used to measure fluid pressure is called a manometer.
- A manometer is a U tube that is half filled with a liquid (usually water or mercury) and it is used to measure fluid pressure.
- When both arms of the manometer (diagram (a)) are open to the atmosphere, the same pressure i.e. atmospheric (equal) pressure is exerted on the water surfaces x and y thus they are at the same level.
- To measure pressure due to a gas, one can say A is connected to the gas supply (diagram (b)).
The gas exerts pressure on surface Y resulting in the X level rising until pressure at X is equal to the gas pressure.
- At this point the gas is supporting the water column X-M plus atmospheric pressure.
- Therefore pressure due to gas = atmospheric pressure + pressure due water column (pgh).
- A pump is a device that uses atmospheric pressure to raise liquids or force air into inflatable objects such as tyres.
- The lift pump, force pump and blair lift pump, which are used to draw water from a deep well, are examples of useful applications of atmospheric and liquid pressure.
Lift pump (shallow well pump)
Downstroke and upstroke of the lift pump
- Piston moves downwards.
- Valve B is closed by liquid pressure pressing it down.
- Valve A opens due to liquid pressure pushing it up.
- Water moves into space above piston.
- Piston moves up creating a region of low pressure below it.
- Valve B opens due to atmospheric pressure underground being higher than pressure inside cylinder.
- Water moves into space below piston as it is pushed by atmospheric pressure.
- Valve A closes due to its own weight and also that of water above it hence water is pushed up and out through the spout.
Disadvantages of a lift pump
(i) It lifts water to height of less than 10 m above the water level inside the well.
(ii) It only delivers water during upstroke.
(iii) lt has to contain water before it can be used (needs priming).
This is a lift pump.
- water coming out
- Piston moves up.
- Bottom valve opens.
- Piston valve closes.
- Water enters cylinder.
- Piston moves down.
- Bottom valve closes.
- Piston valve opens.
- Water enters pump raising main and comes out through spout.
Advantages of blair pump
(i) Simple to construct (comes with a do it yourself kit). (ii) Cheap to maintain.
(i) Delivers water only on down stroke.
Force pump (deep well pump)
- It is an improvement in performance compared to the lift pump.
- It lifts water to much greater heights.
- It delivers water continuously – both on upstroke and downstroke.
- The piston moves down.
- Valve B closes due pressure inside cylinder pushing down on it.
- Valve A is opened by water pressure pushing it outwards.
- Water is transferred from cylinder towards spout, compressing the air in reservoir.
- Water comes out of the spout.
- The piston moves up creating a low pressure region below it.
- Valve B is opened as higher atmospheric pressure pushes water into space below piston.
- Valve A closes due to pressure of water pushed by previously compressed air in the reservoir, which air also pushes water out through spout.
This is a force pump.
Piston moving inwards
- Piston compresses air inside cylinder.
- Compressed air pushes the rubber leather washer to give an airtight fit against wall of cylinder.
- Tyre valve is forced open by air pressure.
- Tyre valve allows air into the tyre thus inflating it.
Piston moving outwards
- Piston moves to the right.
- Pressure is reduced inside the cylinder.
- Tyre valve closes (due to higher pressure inside).
- Atmospheric pressure forces leather washer in barrel or attached to the piston to open pushing air into the cylinder.
- Fluid systems make use of pressured liquids (hydraulic systems) or gases (pneumatic systems).
- Is a tube for drawing liquids by gravity and suction to another container positioned below the container to be emptied.
- When air is drawn from the tube (by suction), a partial vacuum is created in the tube hence atmospheric pressure in upper container forces liquid into the tube. Continuous flow is achieved through molecular attraction of liquid particles and gravitational force acting on liquid inside tube.
- Hydraulic machines use the general principles that liquids:
- pass on (equally in all directions) any pressure applied to them
- cannot easily be compressed
- can flow
- Set up in Fig. 16.13 can be used to demonstrate the properties of liquids which are made use of in hydraulic machines.
- A small effort (small book) applied to smaller piston to lift a bigger load (bigger book) placed on the larger piston.
Motor car braking system
How it works
- Force on brake pedal pushes the master cylinder piston in.
- Pressure is exerted on the brake fluid.
- But because fluids are incompressible and transmit pressure equally to all wheels so pressure acts on wheel cylinder pistons hence forces are exerted on brake fluid shoes so they press against wheel drums.
- Friction between shoes and drums stops the car.
- Return spring pulls back shoes when force of foot decreases releasing the brakes.
Advantages of using brake fluid instead of water in hydraulic systems
- Brake fluid is a lubricant and water is not.
- Water causes rusting and brake fluid does not.
Effect of heavy air bubbles in hydraulic systems
Air is compressible so efficiency of machines is reduced-car brakes will fail
The hydraulic jack
Is a device used to lift equipment e.g. car jack lifts vehicles so that maintenance can be performed.
How it works
- Small piston X is moved down by a small effort (2N) exerting pressure on liquid since liquids cannot be compressed they transmit pressure equally to all directions (200Pa).
- Pressure acts on larger piston Y lifting a larger force (50N)
Note Brake fluid is used and not water for the same reasons as in the motor car brake system.