Forces and Motion

Identifying the forces acting on the lever

Teaching Guidance for 11-14 Supporting Physics Teaching

Describing the forces

Wrong Track: I push down this end and the lever pushes up at the other end.

Right Lines: At this end I exert a force downward on the lever, at the other end the rock exerts a force downwards on the lever. Here the pivot exerts a force upwards on the lever. The resultant force is zero.

Identifying the forces

Thinking about the learning

A central part of coming to understand levers is to correctly identify the forces; what they act on and where; what exerts them and in which direction. That is quite a lot of things to get right, all represented by drawing a few arrows on a diagram.

Thinking about the teaching

When you first introduce pupils to the forces acting on and around a simple lever:

Choose a simple example to look at, maybe a simple see-saw arrangement where the lever is initially horizontal and the forces acting are vertical. Pupils usually find the effort force straightforward to understand:

  • The downward push of your hand on the lever.

However, there is often some confusion between the pair of forces acting at the other end of the lever:

  • The downward force of the load on the lever
  • The upward force of the lever on the load

These forces provide a good example of a pair of forces, replacing an interaction, each one acting on a different object (a Newton's third law pair of forces): the force of the load on the lever is equal the force of the lever on the load.

As you are interested in the forces acting on the lever, you need to look at:

  • The effort force, acting on the lever
  • The downward force exerted by the load, acting on the lever

It's the same old story, emphasised in the SPT: Forces topic. To avoid errors and keep things simple, focus on one object at a time, isolating it from the environment and drawing only the forces acting on that object.

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