Work, Energy, and Power

Propulsive forces such as tension, thrust, or a driving force promote movement, whilst resistive forces such as friction, braking, and resistance resist movement. Work is done by a force when the object a force is applied to moves. Work is measured in Joules (), with one Joule equal to one Newton applied over one metre.

For a force acting in the direction of motion:

If a force is applied at an angle to the direction of motion, the component of the force that actually does work is :

For a force acting at an angle to the direction of motion^[1]:

Kinetic energy is the energy an object has because it is moving. An object of mass (in ) and speed (in ) has kinetic energy (in ) given by:

The work–energy principle states that the net work done by all the forces acting on a particle is equal to the change in kinetic energy of the particle:

Gravitational potential energy (GPE) is the energy an object has because of its position above ground level. An object of weight (in ) and height above ground level (in ) has GPE (in ) given by:

The principle of the conservation of mechanical energy states that if there are no forces other than gravity doing work on an object during its motion, then the mechanical energy (sum of kinetic energy and GPE) is constant.

The work-energy principle and conservation of mechanical energy can be combined, by considering that any change in the mechanical energy of a system is the result of work done by external forces:

Power is the rate at which work is done. Power is measured in watts (), with one watt equal to one joule per second.