AQA GCSE Science

Centre of mass

Aims

In this worksheet students will gain more confidence in centre of mass problems and understand how the position of the centre of mass affects the stability of an object.

Teaching notes

This activity could be used as a plenary at the end of a lesson on the centre of mass. It would be useful if the students had already done some experiments on stability and equilibrium.

Answers to questions

1Onlyc will topple over, because the centre of mass is ‘outside’ the base of the object. Once the line of action of the weight (which can be thought of as concentrated in the centre of mass) acts outside the base of the object, the object will topple over.

2a In the exact centre of the cardboard where the two axes of symmetry cross

bUse a pin to freely suspend the card, and draw a vertical line downwards from the pin. Repeat the procedure, suspending the card from a different point to give another similar line. The centre of mass will be at the point where the two lines cross.

3a A lower centre of mass will mean that the bus will be more stable and will not topple over until tilted to quite a large angle.

bThe easiest method is to measure the angle between the floor and the line joining the bottom of the left wheels of the bus (the pivot) to the centre of mass, as shown in the diagram. Then take this angle (38°) away from 90°. You would have to tilt the bus by that angle to position the centre of mass vertically above the left wheels. (Answer: 90° –38°  52°)

The second method, as shown in the next diagram, is to copy the diagram and rotate the bus about the pivot point (bottom of the left wheels) until the centre of mass is vertically above the pivot. Then measure the angle directly. (Answer  52°)