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GCSE Questions: Moments and Centre of Gravity

Q1.The diagram shows a back view of a computer monitor.

(a) In normal use, the monitor is stable.

(i) Explain the meaning, in the above sentence, of the word 'stable'.

It means the monitor will not easily fall over if the monitor is given a small push as the centre of mass will remain vertically above the base.

(2 marks)

(ii) State the relationship between the total clockwise moment and the total anticlockwise moment about any axis of the monitor when it is stable.

(total) clockwise moment = (total) anticlockwise moment

they are equal/balanced

(1 mark)

(b) The instruction booklet explains that the screen can be tilted. It also includes a warning.

Explain why the monitor will tip over if the screen is tilted too far back. Include the words: centre of mass, weight and moment in your explanation.

(3 marks)

The monitor is made up of two parts – the base and the screen.

When the screen tilts like this the centre of gravity of the whole monitor is lower - just below the screen – before it was tilted it would have been just above the lower part of the screen.

Many students thought the centre of gravity would be higher – just because they knew it was less stable. The reason for its instability is not the height of the centre of gravity but that it is now no longer positioned over the centre of the base… it would be towards the edge of the base or even outside it - causing it to topple.

You really have to include the words that the exam board tell you to include if you want full marks.

The position of the centre of mass has changed . The line of action of the weight is now outside the base producing a (resultant) moment that makes the monitor topple.

It is a resultant moment not a resultant force... care!

(Total 6 marks)