SEC - May,  1997 - Paper 2A  Question 2 - Reno Anastasio

2.

a)      The moment of a force about a point is equal to the product of the force and the perpendicular distance between the line of action of the force and that point.

 (or simply :    By moment of a force I mean the turning effect of a force)                      

The principle of moments states that when an object is in equilibrium (or balanced), by a number of forces acting on it, the sum of the anticlockwise moments is equal to the sum of the clockwise moments.                                                                                         (4)

 b)

  i)                    Set up the apparatus as shown in the diagram.

ii)                   Balance the metre ruler using the masses.

iii)                 Measure the distances d1 and d2.

iv)                 If possible use different values for the masses and repeat the experiment.

 Then calculate the anticlockwise moment  =  W1  x  d1

                   and the clockwise moment       =  W2  x  d2

  They will be found to be approximately equal proving that the principle of moments is true.                                                                                                                        (6)

c) (i)                                                                                                                             (2)

ii)  Tension forces in rope = 500 N each as sum of downward forces = sum of upward forces and the weight of the plank and Tony act at the centre of the plank.                                                                                                                                           (4)

iii)


Taking moments about A

Anticlockwise moments  =  clockwise moments

            FB   x   2 m         =  (300 N  x 1 m)   +  ( 700 N  x  1.5 m)

                            FB     =         300 Nm   +  1050 Nm  

                                                               2 m

                                                                =     675 N  Answer  c  (iii)

                              therefore  FA   =  1000 N -  675 N    =  325 N   Answer c   (iii)                                                                                                                                                (4)