The following diagram (Fig. 1.20) shows two parallel and opposite forces `F_1 and F_2` each of magnitude 5N, with their lines of action separated by a distance of 2m. A point X is pivoted midway between `F_1 and F_2` while a point Y is pivoted on `F_2`
(a) calculate the total moment of the two forces about the points (i) X, and (ii) Y
(b) State the effect produced by the two forces about the points (i) X and (ii) Y

Explanation

(a) (i) Perpendicular distance of point X from either of the forces `F_1 or F_2` is `1/2 times 2m=1m`
`therefore` Moment of force `F_1` about X `=5N times 1m`
=5 Nm (clockwise)
and moment of force `F_2` about `X= 5N times 1m`
=5 N m (clockwise)
Hence total moment of the two forces about X
=5+5=10 N m (clockwise)
(ii) Perpendicular distance of point Y from the force `F_1` is 2m while it is zero from the force `F_2`
`therefore` Moment of force `F_1` about Y =`5 N times 2m`
10 Nm (clockwise)
and moment of force `F_2` about Y=0
Hence total amount of the two forces about Y
=10 Nm (clockwise)
(b) (i) The effect of the two forces about the point X is to produce clockwise rotation.
(ii) The effect of the two forces about the point Y is to produce clockwise rotation.