Forces and Motion

Newton's third law untangled

Physics Narrative for 11-14 Supporting Physics Teaching

Some simple cases: a first example – cup on table

The cup is acted on by two forces. Gravity pulls it downwards and the upward contact force from the table pushes upwards. These two forces add to zero and the result is a cup in equilibrium. Episode 01 has considered this type of situation in detail.

At the interface between the table and cup, the cup is in contact with the table. It is this contact which results in the upward force which supports the cup. But the cup also pushes down on the table. So if you are modelling the forces on the table, you need to consider the force exerted by the cup, acting on the table.

So at the interface there is a force exerted by the table acting on the cup and a force exerted by the cup acting on the table. This is the pair of forces representing the interaction.

Note that this pair of forces act on two separate objects, the table and the cup. The two forces, both equal in size but opposite in direction, act on two different objects. You can't claim that these two forces are balanced as they are acting on two separate objects.

A second example: a falling apple

On its way downwards there is only one force acting on the apple (ignoring air resistance). It is the force due to gravity. However if we consider the bigger picture we note that the apple is falling towards the Earth. A gravitational force from the Earth acts on the apple and an equivalent force from the apple acts on the Earth. Here we have a pair of forces representing an interaction. The pair of forces acts even though the Earth and apple are not in contact. One way to think of this is to see the Earth and apple attracting each other. The force of gravity acts always between two objects. In this case the Earth and the apple are the two objects. The apple and Earth are not in equilibrium. Both Earth and apple are accelerating towards each other.

A third example: a deflating balloon

In the third example the pair of forces again act on two different objects, as they must. There is a force exerted by the balloon material on the air. The air is pushed out of the balloon. There is also a force which the air exerts on the inside surface of the balloon, a force acting on the balloon. The result is that the air is being pushed in one direction and the balloon is pushed in the other. In this case there is certainly no equilibrium. The balloon and air will move apart – the balloon is about to whizz around the room. These two forces are equal in size and opposite in direction, but they act on two different objects. The interaction is described by a pair of forces.

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