Topic 9 – Forces and their effects — Edexcel GCSE Physics
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Topic 9 – Forces and their effects explained
A lever is a rigid bar that turns about a pivot (fulcrum).
Read the full explanation
A force applied at one point produces a turning effect, the moment, equal to force × perpendicular distance from the pivot. Because the moment depends on distance, a small effort applied far from the pivot can balance a larger load placed close to it, so levers transmit and magnify rotational effects. Gears transmit rotational effect between shafts: meshing teeth make the driven gear turn in the opposite direction to the driving gear, and the ratio of teeth sets the ratio of turns and of moments. A larger driven gear turns more slowly but with greater turning effect; a smaller driven gear turns faster with less turning effect. Energy is transferred through the system, and the product of force and distance is traded, not created.
Your focus
- Describe how a lever produces a turning effect about a pivot.
- Explain how the position of a force on a lever changes its moment.
- Explain how meshing gears transmit rotational effect and how tooth numbers set the speed and turning-effect ratio.
Topic 9 – Forces and their effects exam tips
Marking Points
- States that a lever turns about a pivot and that the turning effect of a force is its moment, calculated as force multiplied by the perpendicular distance from the pivot.
- Explains that increasing the distance between the effort and the pivot increases the moment for the same force, so a lever can move a larger load with a smaller effort.
- Describes how meshing gear teeth transmit rotational effect from one shaft to another and states that the driven gear rotates in the opposite direction to the driving gear.
- Explains that the ratio of the numbers of teeth determines the ratio of rotations, so a larger driven gear turns more slowly and with a greater turning effect.
- Applies the idea of conservation of energy to argue that a gear system trades speed against turning effect rather than creating extra energy.
Examiner Tips
- 💡Define the moment precisely as force × perpendicular distance from the pivot before applying it to a lever example.
- 💡Use a labelled diagram of a lever or a pair of meshing gears to make the direction and ratio reasoning explicit.
- 💡When comparing gears, quote the tooth numbers and state clearly which gear is the driver and which is driven.
Common Mistakes
- Measures the distance along the lever rather than perpendicular to the line of action of the force; use the perpendicular distance from the pivot to the force.
- Claims that meshing gears turn in the same direction; state that the driven gear turns in the opposite direction to the driving gear.
- Asserts that gears create extra energy or extra turning effect from nothing; explain that speed is traded for turning effect with energy conserved.