The velocity of an object describes both speed and direction. It is important in science that speed and velocity are not used interchangeably.

We could say that our speed was 65 km/h but if we are to describe the velocity it would be 65 km/h southwards.



The specific definition for acceleration is the rate of change with time. It is normal to think of acceleration as only referring to a change of speed. So in everyday language we might say: 'the car accelerated away quickly',
in science acceleration refers to changes in speed or direction. Imagine a speedboat starting a race. It could be said to be accelerating because the speed has changed
if the race is a circular one it would be equally correct to say the boat has accelerated because it has changed direction.

This means that a boat that is constantly changing its direction is described as accelerating even though the speed is constant.

We have described a Force as a Push or a Pull. If an object has a force acting upon it can change its movement and / or its shape. The change in one or both of these will depend on two things: the direction and the magnitude of the force.

Newton’s Second Law of Motion

A Force of magnitude F causes a body of mass m to accelerate in the direction of the force with an acceleration of magnitude a.

The equation is: F = ma


Imagine I have a wall of lego bricks and a ball bearing. If I roll the ball bearing gently into the wall it may do little damage BUT if I push the ball bearing very hard or change it to one of greater mass then the brick wall may fall over. In this example we have changed the magnitude of the force by changing the push in a particular direction and also by changing the mass of the ball.

You might also think of this in terms of a car crashing at 30km/h compared with a fully laden lorry doing the same.


Measuring Force, Mass, Speed
Balanced Forces
Forces that oppose motion
What children might say or think about forces
Self assessment