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Stretching, balancing and turning forces
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Forces can stretch springs, hold things up and make things turn. These ideas lead straight into GCSE Physics.
Hooke's law
When you hang weights on a spring it stretches. Extension = stretched length − original length.
Hooke's law: the extension is directly proportional to the force, so doubling the force doubles the extension. This works up to the limit of proportionality; beyond it, the graph curves and the spring may not go back to its original shape.
The work done stretching a spring is stored as elastic energy.
Hooke's law: the extension is directly proportional to the force, so doubling the force doubles the extension. This works up to the limit of proportionality; beyond it, the graph curves and the spring may not go back to its original shape.
The work done stretching a spring is stored as elastic energy.
Balanced forces at rest
A spring holding up a weight stretches until its upward pull balances the weight.
When you stand on a floor, you squash it very slightly. The floor pushes back up with a force equal to your weight: a reaction force. The forces are balanced, so you stay still.
When you stand on a floor, you squash it very slightly. The floor pushes back up with a force equal to your weight: a reaction force. The forces are balanced, so you stay still.
Moments
A force can make an object turn about a pivot. Its turning effect is a moment.
moment (N m) = force (N) × perpendicular distance from the pivot (m)
A see-saw balances when the clockwise moment = anticlockwise moment.
That is why a long spanner makes it easier to undo a nut.
moment (N m) = force (N) × perpendicular distance from the pivot (m)
A see-saw balances when the clockwise moment = anticlockwise moment.
That is why a long spanner makes it easier to undo a nut.
A spring is 10 cm long. With a 4 N weight it is 18 cm long. What is the extension? What will the extension be with 2 N (within the limit of proportionality)?
- Extension = 18 − 10 = 8 cm
- Extension is proportional to force: half the force, half the extension.
- 8 ÷ 2
Answer: 8 cm, then 4 cm
Extension = stretched length − original length. Hooke's law: extension is proportional to force, up to the limit of proportionality. Balanced forces hold things still. Moment = force × perpendicular distance; balanced when clockwise = anticlockwise.
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