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When a force does work, energy is transfer from one form to another. If a body A exerts a contact force on a body B and does positive work, it follows that body B does negative work. Energy is lost by body A in doing positive work while energy is gained by body B having done negative work.

The unit of energy is the [N.m] or [J].

Forms of Energy

On the microscopic scale, all forms of energy turn out to be either kinetic of potential energy

Even though energy may be transferred from one form to another the total amount of energy in the universe is constant. This is another important conservation law

The total energy in the universe is constant. This is the conservation of energy

Kinetic Energy

Consider a body of mass m with a resultant force F. The body does work by being displaced by a displacement s in the direction of F. F does work on the body and changes its velocity from u to v.

The acceleration a produced by F is uniform and is related to u and v and s by

v2=u2+2 as

Using F=ma

=m (v2/2-u2/2)
=1/2 mv2-1/2 mu2

The kinetic energy K of a body is defined by the equation, K=1/2 mv2