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GCSE Questions: Moments and Centre of Gravity

Q7. Two children visit a playground.


(a) The diagram shows the two children, Margery and Johnny, sitting on a see-saw.

(i) The weight of each of the children creates a moment about the pivot.

What is meant by 'the moment of a force'?

A moment is the turning effect of a force

[1 mark]

(ii) Give the unit for the moment of a force.

A moment is measured in Nm

[1 mark]

(iii) The see-saw is balanced. What does that tell us about the size of the moment from Johnny, compared to the moment from Margery?

The moment from Margery is equal to the moment from Johnny.

[1 mark]

(iv) Margery's weight is 400 N. Calculate Johnny's moment about the pivot point.

Moment = force x perpendicular distance from the pivot point

Moment = 400 x 1.5

Moment = 600 Nm

[3 marks]

(b) Johnny now sits on a swing as shown below. The swing is not moving.

Which arrow, J, K or L, points to the centre of mass of the child?

K

[1 mark]

(c) The child on the swing is now moving backwards and forwards like a pendulum. The frequency of the swing is 0.5 hertz. Calculate the periodic time of the swing.

T = 1/f

T = 1/0.5

T = 2 s

[2 marks]

[9 Marks TOTAL]