Newton's First Law of Motion
Newton's first law of motion – the first of three – states that 'a body continues in a state of uniform motion unless acted on by unbalanced forces'. The word 'uniform' here means constant speed in the same direction. The converse is also true, so that if a body is acted on by unbalanced forces, either the speed of the body or the direction in which it is travelling will change.
A body will usually have at least two forces acting on it. These forces must be unbalanced if an acceleration is to result. All the bodies below have unbalanced forces acting on them.
The forces on each body are unbalanced because the force on each body acting to the right is not balanced by equal forces acting to the left. Because the forces are unbalanced, each body will accelerate in the direction of the net force. The first and second bodies will accelerate to the right and the the third body will accelerate to the right.
The forces acting on body below are balanced, so each body will continue in a state of uniform motion. Note that for the second body, a force of 6N to the right is balanced by two forces of 4N and 2N to the left and on the third body a force of 5N acting to the left is balanced by two forces of 3N and 2N acting to the right.
Common examples of balanced forces and uniform motion include:
A car travelling at constant speed. The force exerted by the car engine is balanced by athe equal and opposite force of friction acting on the car's wheels.
A plane travelling at constant speed and height. The force exerted by the plane's engines is balanced by the force of air resistance.
An body falling through a significant height will eventually reach a constant speed, where the force of gravity is balanced by the force of air resistance.
A plane travelling at constant speed on level fligh will expience balanced horizontal and vertical forces as shown below.