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MACHINES
A machine is a device which converts energy from one form to another. A force can be applied at one
point and it can be used to overcome a force at another point.
The force being applied is called the EFFORT (E) and the force being applied is called the LOAD (L).
A machine in which the effort is needed is less than the load is called a force multiplier.
A machine in which the distance moved by the load is much greater than the distance moved by the effort
is called a distance multiplier.
EFFICIENCY, MECHANICAL ADVANTAGE (MA) & VELOCITY RATIO (VR)
(A)
Mechanical Advantage (MA) by using the following formula
LOAD
MA =
(B)
LOAD
EFFORT
MA
EFFORT
Velocity Ratio (VR) is calculated by using
VR = distance moved by the effort
distance moved by the load
(C)
Efficiency is calculated by the following formulae:
Efficiency =
useful work done × 100%
total work done
Efficiency = useful work done ×100%
Total work done
Efficiency = useful work done ×100%
Total work done
Efficiency = useful work done ×100%
Total work done
NB: Ideally the efficiency is 100%, but all machines have an efficiency less than 100%
because energy is loss to friction and heat.
LEVERS
A lever is a simple machine which uses a pivot or a fulcrum to transfer the work done by the effort to a
load. There are three types of levers.
1st ORDER LEVERS
2nd ORDER LEVERS
3rd ORDER LEVERS
PULLEYS
A pulley is a machine which is used to change the direction of a force and gain a mechanical advantage
greater than one.
A Single Fixed Pulley
A single fixed pulley is one with a fixed support which does not move. The pulley itself turns on its axis
as freely as possible for maximum efficiency.
The diagram shows that this pulley is used to change the direction of the effort (E) from a downward pull
to an upward lift. The tension in the string applies the upward force to the load (L).
It is easier to pull down on a rope than lift a load upwards.
The velocity ratio (VR) of the single fixed pulley is one (1), since the load raise the same distance the
effort moves.
The mechanical advantage (MA) is one (1) for the ideal single fixed pulley.
In reality the MA is less than one (1) because a small amount of work is wasted against friction on the
pulley’s bearings and lifting the weight of the load.
Block and Tackle Pulley System
A block and tackle pulley system is a machine which has a single rope which passes around the pulleys
that are in two blocks. We can find the MA of a block and tackle pulley system by counting the number of
ropes that support the lower block.
The velocity ratio can be calculated by using:
VR = distance moved by effort
distance moved by load
Examples of Block and Tackle Pulley Systems
VR = 2
VR = 3
VR = 4
VR =5
Inclined Planes
An inclined plane is one of the simplest and most uses machines. It is a slope or a ramp which allows the
load to be raised more gradually by using a smaller effort than if the load was lifted vertically.
For example if you want to load a truck, you will find it easier to slide the cargo up a ramp than lifting it
up to the truck, since less effort is needed.
From the diagram we can determine the velocity ration (VR) and the mechanical advantage (MA)
MA = weight of the object
force needed to pull it up the incline plane
NB. The longer and more gradual the slope, the easier it is to move the object and the greater the VR will
be.
Wheel and Axle
The wheel and axle can be viewed as two pulleys of different diameters fixed to the same axle the effort
rope is placed around the larger pulley and the load rope is placed on the smaller pulley in the opposite
direction. When the effort rope is pulled the load rises. When the wheel turns once; the effort rope moves
through a distance equal to the circumference and the load moves through the circumference of the
smaller pulley.