5.8 Moments, levers and gears
9 exam questions worth 24 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the unit of a moment?
Choose one answer.
Correct Not quite: the answer is N m
Mark scheme
B: N m
Model answer
N m (B). A moment is a force multiplied by a distance.
A force of 40 N is applied at right angles to a spanner at a distance of 0.25 m from the nut. Calculate the moment of the force.
Mark scheme
M = F d = 40 × 0.25 (1); 10 N m (1)
Model answer
M = F d = 40 × 0.25 = 10 N m.
A force of 20 N creates a moment of 5.0 N m. Calculate the perpendicular distance from the pivot.
Mark scheme
d = M ÷ F (1); 5.0 ÷ 20 (1); 0.25 m (1)
Model answer
d = M / F = 5.0 / 20 = 0.25 m.
A see-saw is balanced. A child of weight 500 N sits 1.2 m from the pivot. A second child sits 1.5 m on the other side. Calculate the weight of the second child.
Mark scheme
clockwise = anticlockwise (1); 500 × 1.2 = 600 N m (1); W × 1.5 = 600 (1); 400 N (1)
Model answer
Anticlockwise moment = 500 × 1.2 = 600 N m. For balance the clockwise moment is the same, so W × 1.5 = 600 and W = 400 N.
State the principle of moments.
Mark scheme
for an object in equilibrium / balanced (1); the sum of the clockwise moments equals the sum of the anticlockwise moments (about the same pivot) (1)
Model answer
When an object is balanced, the total clockwise moment about a pivot is equal to the total anticlockwise moment about it.
Explain why it is easier to undo a tight nut with a long spanner than with a short spanner.
Mark scheme
the force is applied at a greater distance from the pivot / nut (1); so the moment (turning effect) is bigger for the same force (1)
Model answer
With a long spanner the same force acts further from the nut, so it has a bigger moment, and it turns the nut more easily.
A crowbar is used to lift a box. Explain how the crowbar allows a small force to lift a large load.
Mark scheme
the effort is applied far from the pivot (1); the load is close to the pivot (1); the moment of the small effort is equal to the moment of the large load (the lever acts as a force multiplier) (1)
Model answer
The effort is applied far from the pivot and the load is close to it. The small effort has a large moment because of its distance, which balances the moment of the larger load.
A gear with 12 teeth drives a gear with 36 teeth. Describe how the speed and the turning effect of the larger gear compare with those of the smaller gear.
Mark scheme
the larger gear turns more slowly (1); it turns three times slower / at one third of the speed (1); it has a larger moment / three times the moment (1)
Model answer
The larger gear turns more slowly, at a third of the speed, but has a bigger moment, three times as much.
A uniform metre ruler is balanced on a pivot at the 50 cm mark. A 2.0 N weight is hung at the 20 cm mark. Calculate the force needed at the 90 cm mark to balance the ruler.
Mark scheme
anticlockwise distance = 0.30 m (1); moment = 2.0 × 0.30 = 0.60 N m (1); clockwise distance = 0.40 m (1); F = 0.60 ÷ 0.40 = 1.5 N (1)
Model answer
The 2.0 N weight is 0.30 m from the pivot, so its moment is 0.60 N m. The force at 90 cm is 0.40 m from the pivot, so F = 0.60 / 0.40 = 1.5 N.
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