Upper Secondary: Turning Effect of Forces

The diagram shows the first step in an experiment to determine the position of the centre of mass of a thin card.

What is the next step in this experiment?

• Find the mid-point of PQ.

• Hang the card from point R.

• Measure the mass of the card.

• Measure the thickness of the card.

State what is meant by the Principle of equilibrium. [2]

A uniform beam is pivoted at its midpoint. An object weighing 40 N is placed on the beam as shown.

What is the force required on the right, 40 cm away from the pivot?

A third weight is to be added to bring the system into equilibrium. What is its magnitude and where should it be placed?

• 1N, 2 m to the left of the pivot

• 1N, 3 m to the right of the pivot

• 2N, 1 m to the left of the pivot

• 2N, 1 m to the right of the pivot

A girl uses paper-clips to balance a toy bird on her finger as shown. What is the effect of the paper-clips?

• They help to raise the centre of gravity above her finger.

• They help to raise the centre of gravity to her finger.

• They help to lower the centre of gravity below her finger.

• They do not affect the centre of gravity but increase the weight.

A uniform bar of length 1.0 m is supported 30 cm from one end. In order to balance the bar, a weight of 10 N is glued on the end.

What is the weight of the bar? (Leave ans in 3s.f)

A pupil balances a non-uniform object on a knife edge. To do this, a weight is suspended from the left-hand end of the object.

AWhere is the centre of gravity of the object?

• A

• B

• C

• D

Mr Wang walks along a tightrope, carrying a long pole.

Why does he carry the pole?

• to make it easier to keep his centre of gravity over the rope

• to raise his centre of gravity

• to reduce his pressure on the rope

• to spread out his weight

The figure shows a stool. A horizontal force F keeps the stool balanced. C is the centre of gravity of the stool and the weight of the stool is 30 N.

At which point will the normal contact force be acting?

Calculate, using the distances marked on the figure, the moment, Ncm, of the weight about the point P. (Leave ans in 3s.f)