A force of 20 newton is applied at right angles to a rod at a point 0.5 metre from the axis of rotation. The torque about the axis is
- A.4 newton metre
- B.10 newton metre
- C.40 newton metre
- D.20.5 newton metre
Correct answer
Explanation
The correct answer is B, 10 newton metre. Torque is r into F into the sine of the angle between them, and since the force here is perpendicular to the rod the sine is one, so the torque is simply 0.5 multiplied by 20, that is 10 newton metre. Option A, four newton metre, comes from dividing the force by the distance instead of multiplying, a slip that also gives the wrong unit for the quantity asked. Option C, forty newton metre, comes from dividing 20 by 0.5, the same error in the opposite direction, and it is the commonest wrong choice in this kind of question. Option D, twenty point five newton metre, comes from adding the two numbers, which has no physical meaning because a force and a length cannot be added. Always check that the distance used is the perpendicular distance from the axis.
Read the full article: Centre of Gravity, Torque and Equilibrium: Exam Notes
Practice Questions
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- A.Newton
- B.Newton metre
- C.Newton per metre
- D.Joule per second
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- A.Thrice the area of the triangle
- B.Twice the area of the triangle
- C.The area of the triangle
- D.Half the area of the triangle
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- A.On the ring itself, at any point
- B.At the centre of the ring, outside its material
- C.At one third of the radius from the centre
- D.Nowhere, because a ring has no centre of gravity
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Read the full explanation →The torque produced by a force about an axis is maximum when the angle between the force and the arm is
- A.0 degree
- B.45 degrees
- C.90 degrees
- D.180 degrees
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Read the full explanation →A long spanner loosens a tight nut more easily than a short one because it
- A.Reduces the force needed by increasing the moment arm
- B.Reduces the friction at the nut
- C.Increases the mass acting on the nut
- D.Changes the direction of the weight of the nut
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