Welcome back! This week, we’re taking a look at one of demonstrations of simple machines: the pulley, featured in demonstration B3-12.

 simple pulley system: demo B3-12

A pulley is simply a wheel and axle with a rope over it. A system like you see in the picture here, with one or more pulleys in a fixed frame used for exerting tension forces to lift or pull something, is commonly called a block and tackle. The purpose of such a system is to provide mechanical advantage, a multiplication of force, in lifting or pulling a weight.

In this case, we can use the pulleys to lift weight. The energy used to lift the weight against gravity is constant, regardless of how many pulleys are used. But by using the block and tackle, the multiple strands of rope are pulling at the same time – the energy is the same, but the force is multiplied, while we pull more rope through the system.

The pulley-rope-mass system in the image below is in equilibrium, even though there is twice as much mass hanging on one side than on the other – in fact, precisely because there is! The block and tackle in this case doubles the force of the smaller mass, so it holds the larger mass in equilibrium. If we added an extra force by pulling down on the smaller mass, it would move twice as far down as the larger mass moved up.

 pulley system with a a mass 1M on one side and a mass 2M on the other side in equilibrium

You can experiment with this at home with this pulley simulation at the Compass Project. Drag the handle in the diagram to apply a force to the system, and see how the mass moves. You can change the number and position of the pulleys, their diameter, and the mass to see how different systems react to different conditions.