not contain a source of energy, so they cannot do more work than they receive from the input force. A simple machine with no friction or elasticity is called an ideal machine. Due to conservation of energy, in an ideal simple machine, the power output (rate of energy output) at any time is equal to the power input The power output equals the velocity of the load multiplied by the load force . Similarly the power input from the applied force is equal to the velocity of the input point multiplied by the applied force . Therefore, So the mechanical advantage of an ideal machine is equal to the velocity ratio, the ratio of input velocity to output velocity The velocity ratio is also equal to the ratio of the distances covered in any given period of time Therefore the mechanical advantage of an ideal machine is also equal to the distance ratio, the ratio of input distance moved to output distance moved This can be calculated from the geometry of the machine. For example, the mechanical advantage and distance ratio of the lever is equal to the ratio of its lever arms. The mechanical advantage can be greater or less than one: If , the output force is greater than the input, the machine acts as a force amplifier, but the distance moved by the load is less than the distance moved by the input force . If , the output force is less than the input, but the distance moved by the load is greater than the distance moved by the input force. In the screw, which uses rotational motion, the input force should be replaced by the torque, and the velocity by the angular velocity the shaft is turned. Friction and efficiency All real machines have friction, which causes some of the input power to be dissipated as heat. If is the power lost to friction, from conservation of energy The mechanical efficiency of a machine (where ) is defined as the ratio of power out to the power in, and is a measure of the frictional energy losses As above, the power is equal to the product of force and velocity, so Therefore, So in non-ideal machines, the mechanical advantage is always less than the velocity ratio by the product with the efficiency . So a machine that includes friction will not be able to move