GCSE Questions: Moments and Centre of Gravity
Q13. (a) Here are four examples of a force causing an object to move.
![](../forces/graphics/Q13.png)
Which object is not likely to rotate?
The crate is not likely to rotate. ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(b) The diagram below shows a simple device that can be used as a weighing scale.
On the right you can see the device being used to measure a quantity of rice.
The weight of the device is balanced by the weight of the rice and basket.
![](Q13.png)
(i) The weight of the device acts through the point labelled X. What is point X called?
X is the centre of mass or centre of gravity of the device. ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(ii) How does the diagram show that the weight of the device is balanced by the weight of the rice and basket?
The pointer is vertical or the arms are horizonatal ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(iii) The basket can hang from different points on the device.
These are indicated by letter P, Q, R and S.
Where should the basket hang to measure the largest quantity of rice?
P ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(iv) Write down the equation which links distance, force and moment of a force.
moment of a force = force x distance from the pivot point
M = Fd ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(v) In the diagram, the weight of the device causes an anticlockwise moment of 0.15 Nm about the pivot.
The weight of the rice and basket acts 0.06 m from the pivot.
Calculate the weight of the rice and basket.
M = Fd
F = M/d ![](../../../graphics/symbols/nuclides/ticksmall.png)
F = 0.15/0.06 ![](../../../graphics/symbols/nuclides/ticksmall.png)
F = 2.5 N ![](../../../graphics/symbols/nuclides/ticksmall.png)
[3 marks]
(vi) Write down the equation which links gravitational field strength, mass and weight.
weight = mass × gravitational field strength
W = mg ![](../../../graphics/symbols/nuclides/ticksmall.png)
[1 mark]
(vii) The basket has a mass of 0.04 kg.
Given that the gravitational field strength = 9.8 N/kg, calculate the mass of rice in the basket.
Let mT = total mass
W = mTg
mT = W/g
mT = 2.5/9.8 ![](../../../graphics/symbols/nuclides/ticksmall.png)
mT = 0.255 kg ![](../../../graphics/symbols/nuclides/ticksmall.png)
Mass of basket is given to 2 decimal places so:
Mass of rice = 0.26 - 0.04 = 0.22 kg ![](../../../graphics/symbols/nuclides/ticksmall.png)
[3 marks]
[12 Marks TOTAL]