Forces and Motion

The forces acting on levers

Physics Narrative for 11-14 Supporting Physics Teaching

Forces acting on levers

The simplest lever has three forces acting on it. Think about removing the metal cap from a bottle (maybe a beer bottle) with a bottle opener. You pull up on one end of the bottle opener, the bottle opener pivots on the middle of the metal cap and the lip of the opener forces the lid off.

The three forces at work on the lever are:

  • The force of your hand as it pulls up on the handle of the lever: Sometimes called the effort applied to the lever.
  • The force of the cap as it pulls down on the lip of the lever (this is the paired force of the lever pulling up on the cap): Sometimes called the load on the lever.
  • The force of the corner of the lid as it acts downwards on the lever at the pivot.

With these three forces balanced, the lever stays still; it does not accelerate off, either upwards or downwards.

All three forces are linked and so to lift off a particularly well fastened cap, you need to pull up harder on the end of the opener (a bigger effort). As a result the lip of the opener will push up more on the lid, opening the bottle and the corner of the lid pulls down more (at the pivot).

In thinking about how levers work we are really just interested in those forces which cause turning and can therefore ignore the forces acting at the pivot.

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