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When an object moves from a starting point to another point, its average speed travelled will be the distance covered divided by the time taken. That is:

Speed = distance travelled

time taken

Time taken = distance travelled


Distance travelled = speed x time taken

It has units of metres per second (m/s), or if distance is measured in miles and time in hours, the units will be miles per hour (mph).

Although people tend to use speed and velocity interchangeably, there is a difference between them. Velocity has a direction associated with it, and is called a vector quantity.

  • Speed is the rate of change

  • Velocity is the rate of change of displacement

  • Acceleration is the rate of change of velocity

The units of speed and velocity are m/s, while those for acceleration are m/s2.

Momentum and force

Momentum is the product of mass and velocity and is measured in kg.m/s.

  • Momentum = mass x velocity

Force is the product of mass and acceleration and the units are kg.m/s2 known as the Newton (N).

The force of gravity provides an acceleration that acts on everything. At the earth's surface, anything that is dropped will accelerate, under gravity, at 9.81 m/s2 (this quantity is often referred to by the symbol g). It also determines the forces that are responsible for the movement of hot, buoyant gases in fires.

The weight of a body is a measure of how strong the force of gravity is on an object. In everyday speech, we usually use mass and weight interchangeably, but just as the words speed and velocity have different meanings, so do weight and mass.