Newton's First Law of Motion

Until the seventeenth century people thought that a force was needed to keep something moving - no force then no motion! However this isn't true and the explanation of this was first discussed by Galileo.

He reasoned that if a ball was rolled down a frictionless slope it would rise to the same height on the other side before stopping. (Figure 1(a). If the angle of the uphill part of the slope was lowered the ball would still get to the same height as it started from but clearly it would travel further in doing it. (Figure 1(b)). If the slope was flattened out the ball would go on for ever, always trying to reach the initial height, as there would be nothing to slow it down. (Figure 1(c)).

A truck on a linear air track or a stone sliding on ice are fairly good examples of this. The problem is getting rid of the effects of friction. When we said no force earlier on it should really have been no unbalanced force. In the example of the stone the weight of the stone is just balanced by the upward force of the ice - the two forces on the stone are equal and so it continues moving at a constant velocity. Only if there is another external force such as friction will it slow down.

Objects stay still if they are still and carry on moving in straight line at a steady speed unless a force acts on them.
This is expressed as Newton's first law of motion:

An apple hanging from a tree or a skydiver falling at their terminal velocity are also both examples of Newton's First Law.

In the case of the apple the force of gravity downwards balances the force of the stalk upwards and so there is no unbalanced force acting on the apple – it therefore stays where it is.











The skydiver is falling but because the force of gravity is balanced by the drag of the air there is once again no unbalanced force and the motion of the skydiver is unchanged. This does not mean that they stop in mid air – drag is equal to weight and the parachutist falls at constant speed – their terminal velocity.