## Forces

When the motion of a body changes, such as in increase or decrease in velocity for example, the body is being acted upon by one or more forces.

Forces include weight, friction, normal reaction, tension, thrust, compression, resistence, etc. Forces are measured in Newtons (N) which is equivalent to 1 kilogram metre per second squared.

## Newton’s First Law of Motion

When the motion of a body changes, such as in increase or decrease in velocity for example, the body is being acted upon by one or more forces. Forces can also be acting on a stationary object. If forces are acting on a body and it doesn’t move, the forces balance and we say the body is in equilibrium.

Newton’s First Law states that:

• A stationary body will remain at rest unless a force acts upon it.
• A moving body will have constant velocity unless a force acts upon it.

In both of these cases, the acceleration is constant at zero.

We can consider the forces acting on a body in any given direction. If the body is not moving in that direction then the resultant force ( = forces in given direction – forces in opposite direction) must be zero. Mathematically, this can be written as
$sum_i {\bf F}_i = {\bf a}={\bf 0}$
Note that ${\bf F}_i$ is the ith force where bold represents vectors and so this statement represents that fact that resultant force is zero in all directions.

These three laws of motion were formulated by physicist, mathematician and astronomer who studied universal gravitation Sir Isaac Newton (1643-1727). See Newton’s Second Law and Newton’s Third Law of motion.