OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain how levers and gears transmit the rotational effects of forces (to…
Full specification wording
Forces in action: explain how levers and gears transmit the rotational effects of forces (to include: an understanding of ratios and how this enables gears and levers to work as force multipliers) (content found only in the physics separate science qualification)
A force applied at a distance from a pivot produces a turning effect, the moment, equal to force × perpendicular distance from the pivot.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 19 of 21
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A force applied at a distance from a pivot produces a turning effect, the moment, equal to force × perpendicular distance from the pivot. A lever transmits this rotational effect: a small effort applied far from the pivot balances a large load close to it, because the moments must match. The ratio of the effort arm to the load arm sets the force multiplication, so a lever with a long effort arm and short load arm turns a small force into a large one. Gears work the same way through their teeth: a small gear driving a large gear turns more slowly but with greater turning effect, and the ratio of teeth numbers gives the force (torque) multiplication. A larger driven gear multiplies force but reduces rotational speed, so energy is conserved while force and speed trade off.
What earns the marks
- States that the moment of a force equals the force multiplied by the perpendicular distance from the pivot.
- Explains that a lever balances when the moment of the effort equals the moment of the load about the pivot.
- Identifies that a longer effort arm relative to the load arm increases the force multiplication of a lever.
- Describes that the ratio of the distances from the pivot determines how much a lever multiplies force.
- Explains that meshing gears transmit rotational effect and that the ratio of teeth numbers sets the force multiplication.
- States that a larger driven gear increases turning effect but reduces rotational speed, conserving energy.
Common mistakes and how to put them right
Uses the distance along the lever rather than the perpendicular distance from the pivot.
Put it right: Measure the perpendicular distance to the line of action of the force.
Assumes a lever or gear creates energy from nothing.
Put it right: Recognise that force is multiplied while distance or speed is reduced so energy is conserved.
Compares gears by their radius alone while ignoring teeth numbers.
Put it right: Use the ratio of teeth to find the force multiplication.
Treats the effort and load arms as adding to give force multiplication.
Put it right: Divide the effort arm by the load arm to find the ratio.
Exam tips
- Write the moment equation and substitute values before comparing effort and load.
- State the ratio explicitly, for example effort arm divided by load arm, to show the force multiplication.
- Link any force gain to a matching reduction in distance moved or rotational speed to show energy conservation.
Try the lesson
The start of the Forces in action: explain how levers and gears transmit the rotational effects of forces (to include: an understanding of ratios and how this enables gears and levers to work as force multipliers) (content found only in the physics separate science qualification) lesson, made from checked teaching notes for this specification point.
Have a go first
A lever has its effort applied 0.60 m from the pivot and its load 0.20 m from the pivot. What is the ratio of the effort arm to the load arm?
Listen
A force applied at a distance from a pivot produces a turning effect, the moment, equal to force × perpendicular distance from the pivot. A lever transmits this rotational effect: a small effort applied far from the pivot balances a large load close to it, because the moments must match. The ratio of the effort arm to the load arm sets the force multiplication, so a lever with a long effort arm and short load arm turns a small force into a large one. Gears work the same way through their teeth: a small gear driving a large gear turns more slowly but with greater turning effect, and the ratio of teeth numbers gives the force (torque) multiplication. A larger driven gear multiplies force but reduces rotational speed, so energy is conserved while force and speed trade off. A strong answer earns marks for these points. States that the moment of a force equals the force multiplied by the perpendicular distance from the pivot. Explains that a lever balances when the moment of the effort equals the moment of the load about the pivot. Identifies that a longer effort arm relative to the load arm increases the force multiplication of a lever. Describes that the ratio of the distances from the pivot determines how much a lever multiplies force. Exam tip: Write the moment equation and substitute values before comparing effort and load.
Key words
Say what each word means before you turn the card.
Moment
Try answering aloud or in your head before turning the card.
Lever
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Force multiplication
Try answering aloud or in your head before turning the card.
Gears
Try answering aloud or in your head before turning the card.
See every definition
- Moment
- A force applied at a distance from a pivot produces a turning effect equal to force multiplied by the perpendicular distance from the pivot.
- Lever
- A lever balances when the moment of the effort equals the moment of the load about the pivot, so a small effort far from the pivot balances a large load close to it.
- Force multiplication
- The ratio of the effort arm to the load arm sets the force multiplication, so a long effort arm and short load arm turn a small force into a large one.
- Gears
- Meshing gears transmit rotational effect, and the ratio of teeth numbers sets the force multiplication, with a larger driven gear increasing turning effect but reducing rotational speed.
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