Newton's Third Law
Forces and Motion

Passive and active forces

Teaching Guidance for 14-16 Supporting Physics Teaching

Helping smooth the paths to Newton 3

All forces on an object arise as a direct result of the environment into which it is placed. Some of these forces seem more active and more like the actions the student might themselves carry out. Other seem more passive and result from local adjustments – for example, the object deforming its environment. It may help to smooth the full acceptance of the third law by explicitly discussing these differences.

The non-contact forces seem more like active forces (gravity, electrical, magnetic). They're the result of free agents, without limit to the forces they can exert.

The normal and frictional forces are a result of local interactions. These forces have evident limits (the buoyancy force is limited by the volume of water that can be pushed out of the way; the slip force by the materials of the surfaces; the compression force by the strength of the materials.)

It's worth noting that the active forces are thought to be the more fundamental ones in physics.

But, in both cases (active and passive), the forces are just forces, however close or otherwise they seem to our actions. The point of explicitly raising the issue is to point out its non-importance in the analysis.

Teacher Tip: Explore and revisit the kinds of forces to ensure that all are understood to be just forces and so all are equally described by Newton's third law.

Newton's Third Law
is used in analyses relating to Collisions
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