Force is anything that causes the velocity of an object to change.

  • Force has direction in which it acts – it is a vector quantity.
  • The symbol for force is F.
  • The unit of force is the newton (N).
  • Acceleration of a body is directly proportional to the force causing it, i.e. for a given body:


The mass (m) of a body is a measure of how difficult it is to accelerate that body.

  • The unit of mass is the kilogram (kg).
  • Acceleration of a body is
  • i.e for a given force: 

The Newton

1 newton is the force that gives a mass of 1 kilogram an acceleration of 




. where is a constant.

i.e. Force = Mass x Acceleration

Weight and Mass

The weight of an object is the force of the Earth’s gravity acting on it.

i.e. Weight = Mass x Acceleration due to gravity


The momentum of a body is equal to its mass multiplied by its velocity.

Momentum = Mass x Velocity

The unit of momentum is the kilogram metre per second 


Newton’s Laws of Motion

  • First law of motion: A body will remain in a state of rest or travelling with a constant velocity unless an unbalanced external force acts on it.
  • Second law of motion: The rate of change of a body’s momentum is proportional to the force which causes it and takes place in the direction of the force.
  • Third law of motion: If a body A exerts a force on body B, then B exerts an equal but opposite force on body A.

Newton’s second law

But acceleration is 

where k is a constant.

i.e. Force is equal to the rate of change of momentum.

The Principle of Conservation of Momentum

The Principle of Conservation of Momentum states that in any interaction between two or more bodies the total momentum of the bodies before the interaction is equal to the total momentum after the interaction provided no external forces act on the system of bodies.