A machine is a device that aids man in the performance of work. Machines use energy to multiply a force, change the direction of a force, transform or transfer energy, or multiply speed. For example, a car jack is a machine that multiplies force exerted on its handle so that a relatively small force can lift a car off the ground. For instance, a force of about 50 N exerted on the jack handle produces a force of about 2 000 N on the car. Notice that the jack handle moves through a greater distance than the automobile.
A machine may also used to changed the direction of a force. A single pulley at the top of a flagpole enables one end of the rope to exert an upward force on the flag as a downward force is exerted on the other end. Another use of a machine is to transform energy. A generator transforms mechanical energy into electrical energy. A steam turbine transforms heat energy into mechanical energy. We use machines to transfer energy from one place to another. The connecting rods, crankshafts, and axles transfer energy from the cylinders in the combustions engine of a car to the tires. We also use machines to multiply speed. A bicycle is a machine that gains speed by exerting a greater force on it. No machine can be used to gain both force and speed at the same time.
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