Define the moment of a force about a point.

1. (a)Define the moment of a force about a point.[2]
2. (b)Fig. 3.1 shows a type of balance that is used for measuring mass. [Figure 3.1] A rigid rod is pivoted about a point 6.2cm from the centre of a pan which is attached to one end. The object being measured is placed on the centre of this pan. A spring, attached to the rod 1.8cm from the pivot, is attached at its other end to a fixed point P. The spring obeys Hooke's law over the full range of operation of the balance. A pointer, on the other side of the pivot, is set against a millimetre scale which is a distance 52.6 cm from the pivot. When the system is in equilibrium with no mass on the pan, the rod is horizontal and the pointer indicates a reading on the scale of 86 mm. An object of mass 0.472kg is now placed on the pan. As a result, the pointer moves to indicate a reading of 123mm on the scale when the system is again in equilibrium.
3. (b)(i)Show that the increase in the length of the spring is approximately 1.3mm.[2]
4. (b)(ii)Calculate the magnitude of the moment about the pivot of the weight of the object. moment = ............................................................ Nm[2]
5. (b)(iii)Use your answer in (b)(ii) to determine the increase in the tension in the spring due to the 0.472kg mass. increase in tension = ............................................................ N[2]
6. (b)(iv)Use the information in (b)(i) and your answer in (b)(iii) to determine the spring constant k of the spring. Give a unit with your answer. k = ............................................................ unit ............[2]

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