A-level · Moments

Moments of forcesExam-style questions

The moment (turning effect) of a force as force × perpendicular distance from the pivot, measured clockwise or anticlockwise, in newton-metres — including forces acting at an angle to a rod.

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23 questions · A-level level · mark scheme and worked solution on each · no sign-up to try

DifficultyAll Level 2 Level 3 Level 4 Level 5

1
Figure 4 shows a non-uniform plank $AB$ of length $6$ m and mass $30$ kg resting horizontally on two supports at $C$ and $D$ , where $AC = 1$ m and $DB = 1.5$ m. The centre of mass of the plank is at the point $G$ .Figure 4 (not accurately drawn)

A child of mass $25$ kg stands on the plank at $A$ . The plank is then on the point of tilting about $C$ .
(a) Find the distance $AG$ , giving your answer to $3$ significant figures. (5 marks)

The child now stands at $B$ instead.
(b) Find the magnitude of the reaction at $C$ and the magnitude of the reaction at $D$ . (6 marks)
(c) Find the greatest mass of a child who could stand at $B$ without the plank tilting, giving your answer to $3$ significant figures. (4 marks)
Level 5 · 15 marksTry it
2
Figure 2 shows a non-uniform rod $AB$ of length $4$ m and mass $25$ kg, hanging horizontally in equilibrium from two vertical strings attached at $A$ and at the point $C$ , where $AC = 3$ m. The centre of mass of the rod is at $G$ .Figure 2 (not accurately drawn)

The tension in the string at $A$ is $98$ N. Take $g = 9.8$ m s⁻².
(a) Find the tension in the string at $C$ . (2 marks)
(b) Find the distance $AG$ . (3 marks)

A particle of mass $m$ kg is now attached to the rod at $B$ . The rod is then on the point of tilting about $C$ .
(c) Find the value of $m$ . (4 marks)
Level 5 · 9 marksTry it