Mechanics

Work, Energy and Power

When a force moves in the direction along which it acts, work is done.

Work is the process of converting energy.

Energy is the ability to do work.

To calculate the work done by a moving force:

the unit of work or energy is the Newton meter or Joule

If the direction of the displacement is not the same as the direction of the force, we use the component of the force which is parallel to the displacement. For example, when a body is caused to accelerate down an inclined plane by the force of gravity:

Therefore, the more general equation to calculate work done is

where is the angle between the force and the displacement.

Although force and displacement are both vectors, work (or energy) is a scalar quantity.

The Law of Conservation of Energy

Energy is never destroyed, it is just converted from one form to another.

Examples of Energy Conversions

• A battery converts chemical energy into electrical energy.

• An electric motor lifting an elevator converts electrical energy into gravitational potential energy.

• When a moving vehicle uses its brakes to stop, the kinetic energy of the vehicle is converted to heat in the brakes.

• A nuclear reactor converts nuclear energy into heat.

Power

Power is work done per second (or energy converted per second).

unit of power is the Joule per second, Js^-1
1Js^-1 is called 1Watt (1W)
Combining this equation with the definition of work we have:

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