Couples and torque Cambridge International AS & A Level Physics revision
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In plain words
Turn a steering wheel with two hands: one pushes up, the other pulls down. The two forces are equal and opposite, so the wheel doesn't go anywhere, but it certainly turns. A pair of forces like that is called a couple.
Three things to know
- A couple is a pair of equal and opposite forces whose lines of action do not coincide. It produces rotation only: there is no resultant force.
- Torque of a couple = one of the forces × the perpendicular distance between their lines of action. Units: N m.
- The torque of a couple is the same about any point.
Worked example
A driver applies forces of 15 N to opposite sides of a steering wheel of diameter 0.40 m, each along a tangent. Find the torque.
- The two forces are equal, opposite and not in the same line: a couple.
- Torque = one force × the perpendicular distance between them = 15 × 0.40.
- = 6.0 N m.
Tips and tricks
- Use one force, not the two added together, and the distance between the two forces, not the distance from the centre.
- A single force can never be a couple: a single force always gives a resultant force as well.
It lands in your notebook with its questions as flashcards.
Couples and torque: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is a couple?
It produces rotation without any resultant force.
Two 10 N forces form a couple, and their lines of action are 0.30 m apart. What is the torque?
One force × the perpendicular distance between them.
What is the resultant force of a couple?
The forces are equal and opposite.
What does a couple produce?
With no resultant force, the object's centre doesn't accelerate.
A couple has a torque of 8.0 N m. The distance between the forces is doubled and the forces are unchanged. What is the new torque?
Torque is proportional to the separation.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Couples and torque
Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org
Name ______________________________ Date ______________
A couple consists of two 15 N forces whose lines of action are 0.20 m apart. Find its torque.[2]
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3.0 N m.
State two features of the forces that make up a couple.[2]
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They are equal in magnitude and opposite in direction, and their lines of action are parallel but do not coincide.
A torque of 12 N m is to be produced by a couple whose forces act 0.30 m apart. Find the size of each force.[2]
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40 N.
Answers: Couples and torque
- 1. 3.0 N m.
- 2. They are equal in magnitude and opposite in direction, and their lines of action are parallel but do not coincide.
- 3. 40 N.