A snatch block is a wheel that has grooves at its edges to hold a rope or cable. Pulleys are simple machines and are usually designed to reduce the force required to pull or lift loads.
Someone wants to carry an extra heavy weight not through the stairs but through the building window into the building.
He uses ropes and pulleys to do this. These devices cause the weight to be lifted with less force than its own weight. How is this done?
First consider a heavy object hanging from two ropes.
In this set, the upward force on the heavy object is equal to the tension of the ropes, and the total tension of the ropes, based on the balance of forces, must be equal to the weight of the object.
If the weight and its hanging system are symmetrical, the tension of each rope will be equal to half the weight of the object.
