Newton's Third Law

Newton's third law

Newton's Third Law may formally be stated as follows.

When one object exerts a force on a second object there is a force equal in magnitude but opposite in direction exerted by the second object on the first.

This frequently is called the action/reaction law, which is an easy to remember catch phrase, but for the purpose of analysing sports movements reference needs to be made to the full definition outlined above.

There are a number of important points which need to be considered when making interpretations using this law.

a) the law refers to two objects and therefore it is important when making interpretations to identify the two objects that are involved in the interaction. The action/reaction forces may be considered as a pair, but they never act as a pair on the same object. (Note that 'object' here refers to the 'system of interest' which defines forces to be external or internal.). Consider the following example.

A sprinter on the blocks experiences a force driving him/her forwards. From Newton's Third Law it can be seen that the force driving the sprinter forwards must be matched by an equal but opposite force acting on the blocks. The two objects in this example are the sprinter and the blocks, but as the blocks are attached to the surface of the planet, the second object is effectively the planet.

One of the reasons why it is important to identify the two objects can be made clear if reference is made to Newton's Second Law. The sprinter experiences a force which causes him/her to accelerate. The blocks experience exactly the same force in terms of magnitude but opposite in direction and because the effective mass of the blocks is that of the planet any acceleration would be insignificant (also the planet is experiencing millions of other interactions).

b) The action/reaction phrase can sometimes seem to imply that the action is followed by the reaction. This is not an appropriate interpretation and as such the action/reaction forces should be considered to be acting at the same time

One further example may help when interpreting situations where several forces are acting.

Consider an ice puck resting on the surface of an ice rink. The force of gravity is clearly one force that must have be acting on the puck. According to Newton's Third Law, the rink (planet) as the other object in the interaction must be experiencing an equal but opposite force.

However, this only identifies one force acting on the puck. As the puck is not accelerating, when it is at rest on the ice surface, then there must be at least one other force acting to neutralise the identified gravitational force.

Whenever two objects are in contact with one another then there is force present as a result of this interaction. In this case the puck exerts a force on the surface of the ice, and in accordance with Newton's Third Law, the puck experiences an equal but opposite force. Therefore the puck when at rest is experiencing two forces, the gravitational force (which is acting independently of any contact) and the contact reaction force.

TASK: Copy the picture below of a performer standing still prior to performing a vertical jump. (You can print out the picture if you wish or draw a representation in stick figure format). On the diagram draw vectors representing the action-reaction force pairs. Note as the performer and the planet are the two objects in this context both 'objects' will have to be represented. A solution to the problem will be shown in the lecture on Newton's Third Law.

TASK: To get an idea of how ground reaction forces vary in magnitude try standing on a set of bathroom scales and swing your arms - you should see quite a fluctuation in the scales reading to reflect the fact that you are transmitting a variable force. If the scales are experiencing a variable force then according to Newton's Third law, so must the jumper.