The Work-Energy Principle

The definition of Work

Work done by a force is defined as the force multiplied by the distance moved in the direction of the force. This is often expressed mathematically as W = Fd, where W is work, F is force and d is distance.
Work is measured in joules (J) in the International System of units (SI).

Energy is the capacity to do work. If an entity has energy, it has the ability to exert a force over a distance.
The unit of energy, like work, is the joule (J).

Kinetic energy is the energy possessed by a body due to its motion. It’s equal to half the mass of the body multiplied by the square of its speed.
Potential energy refers to the energy stored in an object due to its position in a force field, for example a book held above the ground has gravitational potential energy.
The energy transferred to an object when work is done raises the object’s mechanical energy - the sum of the kinetic and potential energy.

The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy.
When work is done on a body that is free to move, the work done is equal to the increase in kinetic energy.

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
When considering the motion of bodies and the forces acting upon them, the total mechanical energy remains constant if only conservative forces are acting. These are forces where the work done is independent of the path taken e.g. gravitational force.

The work-energy principle can be used to solve many physics problems, especially those involving variable forces.
You can use the work-energy principle whenever the work done is easier to compute than using Newton’s second law, F=ma.