Newton’s Laws of Motion

The relationship between forces and motion were first fully explained by Sir Isaac Newton in the sixteenth century. Before explaining Newton’s laws of Motion it is important the term resultant force is understood.

Resultant Force

The resultant force on an object is the sum of all the individual forces acting on the object taking into account the direction in which they are acting. Therefore all the forces acting on an object may be replaced by a single force which has the same effect as all the original forces acting together.

Newton’s First Law of Motion

Newton’s first law deals with objects at rest or those moving at constant speed.

Newton stated that if the resultant force on an object is zero then an object at rest will remain at rest and an object in motion will continue its motion in the same direction at constant speed.



This means that all objects have a natural tendency to keep on doing what they are doing. All objects have a reluctance to change their state of motion and require an unbalanced force to bring about a change.

The reluctance or resistance for a mass to change its state of motion is referred to as inertia. This is why it is critical for drivers and passengers to wear seat belts. Passengers in cars possess a lot of inertia; if the car is forced to stop suddenly the passengers in the car will still move forward, the seatbelts however exert large forces on the passengers to stop them.

The animation below explains Newton’s first law of motion: