If resultant force is zero, i.e. the body will continue moving at a constant velocity or remain stationary.

If there is a resultant force: the object will accelerate or decelerate.

Newton's 2nd Law: F = ma

Newton’s 3rd Law: when two objects interact, they exert an equal and opposite force on each other.

W = mg

Weight is a type of force.

W: weight (N), F: force (N), m: mass (kg), a: acceleration (m/s^{2}).

An object moving in a circular orbit can have a constant speed but a changing velocity.

For motion in a circle there must be a resultant force known as a centripetal force that acts towards the centre of the circle.

Inertial mass: a measure of how difficult it is to change the velocity of an object (including from rest) and know that it is defined as the ratio of force over acceleration