THE TURNING EFFECT OF FORCES
5 marks · HardAn athlete is balancing on a horizontal board that is supported at both ends (points L and M). The board has negligible mass. The athlete stands a distance d from support L on the 3.0 m long board. The athlete's weight is 630 N.
Write an expression for the upward force, F_M, at support M in terms of d.
The athlete stands 2.2 m from support L. Calculate the upward force at support M.
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(a) F_M = 630 × d / 3.0 · must show use of d in numerator, division by total length
(b) F_M = 630 × 2.2 / 3.0 = 462 N · must show substitutes d correctly, final value rounded
Common mistakes: Swapping distances; Incorrectly setting up moment equation
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Marking points
M1 (a) · 1 mark · Equates moments about point L using variable d.
A1 (a) · 1 mark · Correctly expresses F_M = 630 × d / 3.0.
M2 (b) · 1 mark · Substitutes d = 2.2 into expression.
A2 (b) · 1 mark · Calculates F_M to correct significant figures.
A3 (b) · 1 mark · Final answer with unit.
Worked solution
a) Take moments about support L. Distance between supports is 3.0 m.
b) Substituting d = 2.2 m: