Kinetic and Potential Energy
Kinetic energy is the energy possessed by an object because of its motion. All moving objects have kinetic energy. The amount of kinetic energy depends upon the mass and speed of the object. Potential energy is the energy possessed by an object because of its height above the ground. The amount of potential energy possessed by an object depends on its mass and its height. When the elevator is at its highest point, it has potential energy. When the elevator actually starts to move downward, it has kinetic energy.
Balanced and Unbalanced Forces
A balanced force results whenever two or more forces act upon an object in such a way as to exactly counteract each other. The counterweight used for the elevator and the elevator itself, are balanced forces because the counterweight and the elevator are the same weight. These two forces are said to balance each other, causing the elevator to remain at rest. If you take out the counterweight, the elevator would fall making an unbalanced force. There is no longer a counterweight for the elevator so it accelerates to the ground.
Pulleys
A
pulley is a wheel with a groove along its edge, for holding a rope or a
cable. Pulleys are usually used in sets designed to reduce the amount
of force needed to lift a load.
Single pulleys are used in machines where the direction of movement must be changed. An example is an elevator, where the upward movement of the elevator is linked to the downward movement of a counterweight. This change of direction can be arranged with a wheel and a rope. The wheel is fixed to a support, and the rope is run over the wheel to the load. A pull downward on the rope can lift the load as high as the support. The load moves the same distance as the rope is pulled.
By
connecting pulley wheels together to make a compound pulley, you can raise
loads many times your own weight. You can connect two pulleys together to
raise twice the load that a single pulley could raise. This is called a
double pulley. The load moves only half as far as the free end of the rope,
but in return, the force raising the load is doubled.
The block and tackle is a system of pulleys that is compact, but is able to raise large loads because it has so many wheels. The force of a block and tackle is multiplied by each pulley wheel it contains.
So, for example, the block and tackle in the diagram contains five pulleys in each set plus a guide wheel above. The load is raised by ten pulleys, so the block and tackle increases the force applied to it ten times.
Using a block and tackle with ten pulleys, you could lift an elephant weighing up to 600 pounds!
