Pulley System

The Pulley System makes use of a disk-shaped block that a rope wraps around to create a simple machine. The tension force that is applied on one side of the rope uses the pulley system to divert the force in a different direction. Most of the examples in our NCERT books are ideal machines in which no force is lost due to friction or any other factors. Some of these parameter are established and mentioned below:

  • The magnitude of the applied force does not change but the direction does as the pulley is considered as a perfect simple machine. The resistance that the pulley provides to the rope is zero.
  • One does not need to worry about the rope and pulley’s energy when applying the Law of Conservation of energy.
  • When the force’s effect on the body attached to the pulley is being calculated the pulley’s mass need not be considered.

Pulleys – Their Purpose

The force that needs to be exerted gets reduced when using the pulley to lift bodies. For instance, at a mass hanging from the pulley a force F is applied. You will notice that the force normally required for lifting that amount of mass will be more than the force F that was applied to the pulley system.

In a normal situation, to lift a mass m with velocity that’s constant without the use of a pulley a force is applied that is equal to the mass of the object suspended from the string. The force will be represented as mg i.e. mass multiplied by gravity. By making use of a pulley this mass will be lifted up also with a force of mg along with tension (t) in an upward direction. The force gets dispersed amongst the tension of the rope fixed to the ceiling, T, and the tension of the rope wrapped up in your hand, F.

The block of mass (m) is pulled upwards by two forces of F and T. We know that mg will be equivalent to F + T and thus

2F = mg or F = mg/2

Thus, we are left with the concept that only half the force will be required to lift up a weight of mg. This is the use of a simple machine more specifically, a pulley.


Practise This Question

Figure shows a small block of mass 'm' which is started with a speed 'v' on the horizontal part of the bigger block of mass 'M' placed on a horizontal floor. The curved part of the surface shown is semicircular. All the surfaces are frictionless. Find the speed of the bigger block when the smaller block reaches the point A of the surface.