A simple machine is a device which makes our work easier, quicker and convenient. A simple machine consists of very few parts, by means of which a force is overcome by another force applied at some other point and in some other direction. A machine can never do more work than the energy put into it. Simple machines help us in the following ways:
- To multiply the force or to make the work easy
- A small effort can be used to lift a large load. A lever is a good example of force multiplier. A small force on the long end will lift a much heavier weight on the short end.
- To multiply or increase the speed of doing work
- A small movement is changed into a larger movement. A long pair of scissors is an example.
- To change the direction of applied force or to apply force in a convenient direction
- A pulley system generally raises a load upward by the application of downward force on the rope, which is more convenient.
- To do unsafe works
- A pair of tongs is used to lift a piece of burning coal from a fire.
Some terms related to simple machines:
Effort (E): The force applied on a machine in order to overcome the load is known as effort.
Load (L): The resistive force which is lifted by using effort is known as load.
Fulcrum: The point of support about which the machine is rotated is known as fulcrum.
Effort arm or Effort distance (ED): The perpendicular distance from the fulcrum to the point of effort is called effort arm.
Load arm or Load distance (LD): The perpendicular distance from the fulcrum to the point of load is called load arm.
Input work: The work done on the machine is called input work. It is given by,
Input work = Effort × Effort arm (E x ED)
Output work: The work done by the machine is called output work. It is given by,
Output work = Load × load arm (L x LD)
Mechanical Advantage (MA): MA of a machine is defined as the ratio of resistance overcome (load) to the effort applied.
MA = Load/Effort (It has no unit, it is the simple ratio of two forces)
MA of a machine is highly affected by:
- Friction: Friction exists in a simple machine because of which large amount of effort is wasted.
- Weight of the simple machine: The effort applied on a simple machine has to lift a part of simple machine itself in addition to the load to be lifted. Thus, more effort is required to raise a small load and mechanical advantage becomes less.
Velocity Ratio (VR): It is defined as the ratio of velocity of the effort to the velocity of the load. The velocity ratio of a machine is also defined as the ratio of the distance moved by the effort to the distance moved by the load (resistance overcome)
VR = velocity of effort /velocity of load
VR = Effort distance/ Load distance.
VR is also a ratio and has no unit. It is fixed by the design of a machine and is not affected by friction.
Friction affects the MA of a machine but not the VR. So, MA of a machine is always less than its VR.
Efficiency (η): The efficiency of a machine is defined as the ratio of the useful work done by the machine (output work) to the total work done on the machine (input work). It is expressed in %. It is denoted by letter eta (η).
Efficiency = Output work /Input work ×100%
= (work done by load)/ (work done by effort) x100%
=(load x load distance)/(effort x effort distance) x100%
=(load/effort) x (load distance/effort distance) x100%
=MA x (1/VR) x100%
=MA/VR x100%
Therefore, Efficiency = MA/VR × 100%
Since some work done on the machine is used in overcoming friction and to move parts of the machine itself, input work is always more than the output work. Hence, efficiency of a practical machine is always less than 100%.
A machine that wastes no part of work done on it is an ideal machine or a perfect machine.
Principle of simple machine: It states that, in a perfect machine (Having no friction), the work done on the machine is equal to the work done by the machine, which is based on the principle of conservation of energy.
E × ED=L × LD (Output work =Input work)
Simple machines and some important terms related to them