Newton's laws are of universal and profound importance, and give excellent explanations of many types of motion over a very wide range of circumstances. These laws not only enable us to make sense of motion on a cosmic scale, but enable us to make sense of motion right down to an atomic scale, including our everyday world in between. They were enunciated in 1687 by the British scientist and mathematician Isaac Newton (1642–1727). They explain, amongst other things, the way the planets move around the sun.
Newton's first law of motion
What do you think is the 'natural' state of motion of an object? In other words, how does it move (if at all) when there is no external influence on it? You might well have answered that the object stays where it is — it remains at rest. All around us we seem to have examples of the need to disturb something to make it move.
For example, a glass on a table just sits there unless it is disturbed in some way, perhaps by being pushed, or lifted. If you push the glass it comes to rest soon after you stop pushing it. If you throw a ball it ultimately comes to rest. It would seem that being at rest is the undisturbed state of motion, and indeed this was the view of many of the philosophers of antiquity, notably the Greek philosopher Aristotle (384–322 BC). It might therefore come as a surprise to learn that this is not the viewpoint that underlies Newton's laws of motion.
To come to the Newtonian viewpoint, suppose you now place the glass on a very smooth table. You could then give it a push, and after you had finished pushing it, it would continue moving, only gradually slowing down, and possibly falling off the end of the table with unfortunate consequences.
Would the glass move across the table in a straight line, or along a curved path?
The glass would move along a straight line.
A puck on an ice rink behaves in a similar way: it slides along in a straight line and only slowly comes to rest. The difference between the ice rink, or the very smooth table, and an ordinary table is that with the ordinary table there is considerable friction between the surface and the glass. Friction opposes motion. Try continuously pushing a glass across a rough table and then across a smooth table and you can feel the greater opposition to motion in the former case.
Let's now do a thought experiment and imagine some super-smooth horizontal surface that has no friction at all. In this case, once you set the glass moving, it will not slow down but will continue moving at constant speed in a straight line. The only way to slow it down, or speed it up, or change its direction of motion whilst it is on this surface, is to disturb it, by pushing it again, in front, or from behind, or to one side. Thus the undisturbed motion in this case is constant speed in a straight line.
Friction is a disturbance, which is why objects moving on real surfaces eventually come to rest unless there is something pushing them along. Note that when friction eventually brings the glass to rest the frictional disturbance then vanishes. The glass is again undisturbed, so being at rest is merely a special case of undisturbed motion, and not, as was believed by Aristotle, the only case.
In all situations we can conclude that undisturbed motion is either being at rest, or moving in a straight line at a constant speed. The proper scientific name for the sort of disturbances that we have been considering here is force: a push is a force, friction is a force, and there are many other kinds. A disturbance that destroys the undisturbed state of motion of an object is an unbalanced force.
We can now state Newton's first law of motion:
An object remains at rest or moves in a straight line at constant speed unless it is acted on by an unbalanced force.
We need to explore this first law, to clarify what we mean by 'motion in a straight line at constant speed', and to explain what is meant by 'an unbalanced force'.
Exploring the first law of motion
Motion in a straight line at constant speed
Imagine going for a drive, during which, at different times, you travel over the four stretches of road.
Four pieces of road:
(a) straight and horizontal;
(b) straight but climbing a uniform hill;
(c) a horizontal curve;
(d) a curve over a bridge.
A plan view is the view from above.
Which of these pieces of road are straight, in that they curve neither left or right, nor up or down?The roads in a and b.
Road a is not only straight but horizontal - there is no gradient down or up. The road b is also straight, but now there is a steady gradient - a hill of uniform slope to climb or descend. Road c is clearly not straight but follows a bend, whereas road d curves up and then down as we cross the bridge.
Consider now the meaning of 'constant speed'.
Think of a word equation for speed:
speed = distance travelled divided by the time interval
Now, for the speed to be constant the distance travelled in a fixed time interval must be constant. For example, if the time interval is 10 seconds, the distance travelled must be the same during every interval of 10 seconds. Moreover the distance travelled must be the same during every interval regardless of how short we make that time interval, otherwise we could travel during one interval of, say, six seconds at the same speed at every instant, and in the next six seconds we could travel very slowly for the first three seconds and then make up for this by travelling very quickly for the remaining three.
The next image shows snapshots of a car moving at a constant speed along the straight roads a and b. These snapshots are taken at intervals of 2 seconds, and you can see that the distances covered are the same during each interval.
Snapshots every 2 seconds of a car moving at constant speed: (a) along a straight and horizontal road; (b) up a straight, uniform hill.
What is the speed of the car in each case?
On road a, the car covers 28 metres in 2 seconds, so, from our equation:
speed = 28 metres divided by 2 seconds = 14ms-1
On road b the car covers 20 metres in 2 seconds, so, from our equation:
speed = 20 metres divided by 2 seconds = 10ms-1
Now consider the car moving along the straight roads a and b, but with snapshots as in the next image.
Clearly the distances travelled change from one two-second interval to another, and so the speed is varying: in these cases the motion is in a straight line, but it is not at constant speed. Thus, from Newton's first law of motion we can deduce that an unbalanced force must be acting on the car: if an object is not at rest and not moving in a straight line at constant speed then it is being acted on by an unbalanced force.