A bicycle mechanic checks a pedal crank before a race. Fig. 1 shows the crank viewed from the side. The rider pushes down with a force of 250 N. At position...
Question 1 Report
A bicycle mechanic checks a pedal crank before a race. Fig. 1 shows the crank viewed from the side. The rider pushes down with a force of 250 N. At position A the crank is horizontal, so the force is at right angles to the crank. At position B the crank is vertical. The distance from the centre of the axle to the pedal is 0.17 m. The mechanic explains that the turning effect changes as the pedal moves.
The same force is used at both positions.
(a) Calculate the moment at position A. [2]
(b) Which position, A or B, produces the greater moment? [1]
(c) What is the moment at position B? [1]
Answer Details
(a) At position A, the force is perpendicular to the crank:
\[250\times0.17=42.5\text{ N m}\]
The moment is 42.5 N m. [2]
(b) Position A produces the greater moment because the perpendicular distance from the axle to the force's line of action is greatest. [1]
(c) At position B, the force acts through the axle, so the perpendicular distance is zero. The moment is 0 N m. [1]
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