Newton's first law

Newton's first law and the concept of inertia

Newton developed three laws which identify the relationship between force and motion. The first of these is called the "Law of Inertia" which may be stated as follows:

Every object will continue in a state of uniform motion in a straight line (or remain at rest) unless compelled to change its state of motion by a net external force acting on it.

This law indicates that an object has a natural reluctance to change its current state of motion. If it is moving in a straight line it will have a natural tendency to continue moving in a straight line at a constant speed. If it is stationary it will have a natural tendency to remain at rest. It is the reluctance to change its state of motion that is referred to as an object's INERTIA.

There are several points that need to be made:

a) as it is the reluctance to change attribute which describes inertia, an object does not lose, gain or overcome inertia. The only way that an object can change its inertia is if it loses or gains some mass. From this it is apparent that it is mass that determines the inertial characteristics of an object within the context of linear motion. If an object's mass remains the same then so does its inertia. One of the significant implications for sports performers is that any excess mass will give them a larger inertia and therefore inhibit rapid changes in speed and/or direction.

b) the law makes it clear that the natural behaviour of objects is to move in a straight line with constant speed. This implies that if an object is increasing its speed, decreasing its speed, and/or changing direction then there must be a net external force acting.

c) before Newton it was thought that if motion was present then there must be a force acting. However, Newton's First Law indicates that this is not necessarily the case. If an object is moving in a straight line at a constant speed, then according to the first law, it is not experiencing a net force. Subjective experience in observing every day motion events may seem to contradict this. For example, if a ball is kicked it does not continue to roll for ever. However, the behaviour of the ball in coming to rest does not contradict Newton's law, as it does in fact experience a net force (friction). People not trained in mechanics may not always be aware of all the forces that are present.

When observing movement in the real world there are always a number of forces acting (although in some cases they may be very small), and this can make it difficult to appreciate the principle of Newton's First Law.

The following examples illustrate how Newton's First Law may be applied within the context of sport. When reading the examples remember that the law of inertia implies the following.

That if an object is speeding up, slowing down and/or changing direction then it must be experiencing a net force.

Consider an example based on an ice hockey puck.

Phase 1 - Imagine an ice hockey puck at rest on the surface of the ice. Although it may not be possible to identify all the forces acting on the puck at this point, it should be clear that the net effect of any forces that are acting must be zero as the pucks motion is not changing.

Phase 2 - The puck now has a force applied to it when it is hit by an ice hockey stick. During the period of time that the stick is in contact with the puck, the puck changes its state of motion from one of being at rest to one of forward motion.

Phase 3 - As soon as contact between the stick and the puck is broken the force is no longer present, and the puck will then continue to travel in a straight line at a constant speed. (There will inevitably be some small friction forces between the puck and the ice, and between the puck and the surrounding air, which will slow the puck down a little, but these effects will be ignored for the present).

Phase 4 - The puck will continue to travel down the rink until another significant force is applied. This will occur when it hits the side of the ice rink. During the period of contact, the puck experiences a force which changes the puck's direction, and causes it to rebound with a new speed.

Phase 5 - Once the contact with the side wall has been broken the force ceases to be applied to the puck so it will once again continue to travel in a straight line with a constant speed.