AQA A Level Physics 3.4.1.2 Moments

The principle of moments

• The sum of the clockwise moments is equal to the sum of the anticlockwise moments

• The product of the clockwise moments is equal to the sum of the anticlockwise moments

• The sum of the clockwise moments is equal to the sum of the clockwise moments

• The sum of the clockwise moments is not equal to the sum of the anticlockwise moments

Where is the centre of mass?

• A

• B

• C

• D

The beam is in equilibrium. Where is the centre of mass?

• Above the pivot

The beam is in equilibrium. What is the weight of the beam?

• Negligible

The beam is in equilibrium. If the weight of the beam is 100N, what is the reaction force acting at F?

Two forces acting to effect the same rotation.

• couple

• paired moments

• torque-pair

• moment-couple

Which pair of forces are a couple?

• A

• B

• C

• D

The moment of the couple that the hinges exert on the door is:

Answer explanation

To understand the forces or function of a fitting like a hinge, it is often helpful to visualise what would happen if it was removed.

The hinges are acting to hold the door in the frame. If the top hinge were to disconnect from the door, then the door would rotate clockwise about the lower hinge. If the lower hinge were to disconnect from the door, then the door would rotate clockwise about the top hinge. Together, the hinges are opposing this clockwise moment.

This clockwise movement is caused by the weight of the door acting at a distance from the hinges. The weight is 200N and the perpendicular distance is 0.75m, which is a moment of 150Nm. The moment of the hinges are equal and opposite.