Sunday, 30 January 2011
Can you lift up a small car?
Pull on the rope and watch what happens to the car.
When you pull on the rope, you are able to easily lift the car into the air.
The Golden Rule of Mechanics states that “whatever you lose in distance, you gain in power.” We can use a block and tackle to illustrate this principle. Using a rope and a stationary pulley attached to an overhead beam, we can turn the downward pull on the rope into the necessary force to lift a car. The force is increased manifold when we introduce several mobile pulleys to the equation.
Each mobile pulley in Heureka’s tackle is attached to the stationary pulley that is attached to the beam. Using this system, each mobile pulley doubles the actual pulling force. Therefore, the lifting force is 26 = 64 times as great as the pulling force.
The distances that the ropes travel will be in the same proportion to one another, but reversed. So, in order to lift the car 10 cm, the rope must be pulled about 6 metres.
Simple pulleys have been used since before the common era. It has been said that Archimedes used a block and tackle to lift ships onto docks. He is believed to have said, “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world”; this statement is a fine illustration of the Golden Rule of Mechanics. In theory, it is possible to use this principle to achieve a limitless amount of force, but the practical realities, such as friction or material durability will cause problems long before such possibilities could be tested.