Exam-style question
Try this first
In which situation would you use the torque equation τ = rF instead of F = ma?.
- A.A car accelerating straight ahead on a flat road.
- B.A wheel turning when a force is applied at its rim.
- C.A mass falling under gravity.
- D.A spring compressing in a horizontal direction.
Model answer
What a good answer should say
- A wheel turning when a force is applied at its rim.
Explanation
Why this works
Define both: τ = rF is used for rotational systems where a force acts at a distance from an axis; F = ma is for linear systems. Identify key difference: τ = rF includes a lever arm (distance) and is vectorial, whereas F = ma does not.
Explain when each applies: use τ = rF for a wheel turning because the force acts at the rim creating a turning effect; use F = ma for linear acceleration of a car or falling mass. Conclude: the wheel scenario requires the torque equation.
Common mistake
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