Forces and Motion

A teapot: not Newton's third law

Physics Narrative for 11-14 Supporting Physics Teaching

Two forces on one object

Two forces act on the teapot resting on the tabletop. These two forces add to zero. They appear to be equal and opposite. They are equal in size and they act in opposite directions.

However this is not the same case as described in the commonly quoted Newton's third law of motion: to every action there is an equal and opposite reaction (this is far from the most helpful statement of the law – more on that in the SPT: Force and motion topic).

In our example the two forces both act on the teapot. The forces are the gravitational pull of the Earth and the upward push of the tabletop. The two forces are both acting on the same object, the teapot.

When two forces act on one object to create a balance it is not the same as the action and reaction pairs.

This is often misunderstood, but Newton saw this distinction. His first law of motion describes equilibrium situations where forces add to nothing and act on the same object, just like on the teapot. The second screen of the interactive shows a pair of forces acting on different objects – this is an example of Newton's third law.

Newton's third law

A completely separate law, Newton's third law of motion, describes pairs of forces. These are often called action and reaction, but we don't think that this is helpful.

Newton realised that in every case, forces appear in pairs. If there is a force exerted by A acting on B, then there will be an equivalent force exerted by B acting on A. This is true, even if A and/or B are not in equilibrium. In the case of the teapot, the support force from the table acting on the teapot (one of the pair of forces) is accompanied by an equivalent force from the teapot acting downwards on the table (the second of the pair of forces). A force exerted by one object and acting on another is only half the story of the interaction between the two objects.

In starting out on developing a force description we suggest that you focus on one object at a time. So you'll be working with half the interaction, and so considering only ever one of the pair of forces described. Either describe the forces acting on the teapot, or the forces on the table. Keep it simple: avoid the complexity of trying to describe both objects at once.

If you really, really must work with both: use two diagrams, one for each object.

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