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To change the way an object is moving you need a force.

This force can either:

(a) speed up the object like a sprinter at the start of a race
(b) slow down the object like a motorbike slowing down as it approaches a red traffic signal
(c) change the direction in which the object is moving like a car turning a corner

Steady speed

Let’s first think about objects that have a steady speed – one that does not change as time passes (this can include zero, no speed at all – in other words the object is not moving).

A skater moving across the ice or a space craft in deep space can travel with a steady speed although there is no force acting on them. In fact no force is needed to keep them moving.

There are actually two forces acting on the skater – one is their weight downwards and the other is an equal an opposite force of the ice acting upwards on them. However these two forces cancel each other out. This means that there is no remaining, ‘left over’ or resultant force on the skater.

The two forces cancel out and so the skater moves at a steady speed in a straight line across the ice.

Speeding up and slowing down

Now think about a train travelling at a steady speed along a track. To change its speed you need a force – this is provided by the engine. The force can either slow the train down (decelerate it) or speed it up (accelerate it).

The bigger the force the bigger the rate of change of speed and so the bigger the acceleration

The mass of the object that you are trying to move is also important. If you replaced the four huge diesel engines in a cruise liner with four car engines instead the ship wouldn’t be able to accelerate anywhere near as fast as the cars!

Or imagine pulling a an empty truck across the floor. You could accelerate the truck much faster than you could if it had two heavy people sitting on it.

Isaac Newton put these ideas into his laws of motion which you can study by looking at the 14-16/Mechanics/Forces in motion section. (See: Newton's Laws)
© Keith Gibbs 2011