Forces: Newton's First Law

Forces: Newton’s First Law

Newton’s First Law


  • Newton’s First Law of Motion, also known as the Law of Inertia, states that an object will remain at rest or continue moving at a constant velocity unless acted upon by a resultant force.


  • Inertia is the tendency of an object to resist a change in its state of motion.

  • The inertia of an object depends solely on its mass - it is the mass of an object that resists change in motion.

  • Heavier objects (objects with more mass) have more inertia and therefore require a greater force to get them moving or to change their direction or speed.

Balanced Forces

  • When the forces acting on an object are balanced, the resultant force is zero. In this case, an object at rest stays at rest and an object in motion continues to move at a constant speed in a straight line.

  • Examples of balanced forces include a book resting on a table (force exerted by the book due to gravity is balanced by the upward force of the table), or a car moving at a constant speed (forward force of the engine is balanced by the opposing force of air resistance).

Unbalanced Forces

  • Unbalanced forces occur when the forces acting on an object are not equal and opposite. This results in a nonzero resultant force.

  • A nonzero resultant force acting on an object will cause it to accelerate or decelerate, following F = ma, where F is force, m is mass and a is acceleration.

  • Unbalanced forces can also cause changes in an object’s direction. For example, a turning car experiences unbalanced forces because the movement of the car is not in the same direction as the force of friction between the tyres and the road.


  • Understanding Newton’s First Law is useful in many real-world applications. For example, in vehicular safety: seat belts are designed to prevent the occupants from continuing to move forward (maintaining their state of motion) when the car comes to a sudden stop.

  • In sports, athletes often use principles related to Newton’s First Law to improve their performance. For instance, in football, a player will attempt to use a smaller force when touching the ball to continue its motion with minimal change, making their control over the ball better.