Some Examples

If you step off a wall the Earth pulls you down towards the ground. According to the third law, you also pull the Earth up with an equal-sized force. So why doesn’t the Earth accelerate upwards to meet you instead of the other way round? The answer is it does, but if you apply F = ma to work out the Earth’s acceleration, it turns out to be minimal.

Planets pull moons, and according to the third law, moons pull planets with an equal-sized force. Why does the moon go round the planet instead of the other way round?

Pluto and its moon Charon are so close together in terms of their mass that they spin round like a pair of dancers, each of them orbiting a point in space between the planet and its moon.

Again, the answer is, it does. In fact, they both rotate around a common centre of mass. In the case of the Earth and its Moon, the centre of mass is so close to the centre of the Earth that to all intents and purposes the Moon orbits the Earth.

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The diagram shows another pair of forces that are equal in size and opposite in direction. This is an application of the first law – the vase is in a state of rest so there is no resultant force acting on it.

This type of situation is where the greatest misunderstanding of Newton’s third law occurs. Remember, third law pairs of forces act on different objects.