f4 phy

8. Torque = force x perpendicular distance between the force and pivot.
Option A:
Torque = 2 x 3 + (-2 x 3) Nm = 0 Nm
(The perpendicular distance between the force 2N and the pivot is 3 units)
Option C:
Torque = (2x1 + 2x4) Nm = 10 Nm
Option D:
Torque = (2x4 + (-2x3)) Nm = 2 Nm

10. Just imagine if the mass is too heavy, the stand would topple about the point A.
That is to say, A is the centre of rotation of the whole system.

11. The motion of the block follows the equation,
F - Ff = ma
where Ff is the max frictional force
m is the mass of the block
a is the acceleration
hence, a = (F - Ff)/m

Using equation of motion: s = ut + (1/2)at^2
with u = 0 m/s, a = (F-Ff)/m, t = 2 s
hence, s = (1/2).[(F-Ff)/m].(2^2)
i.e. s = 2(F-Ff)/m
or s = (2/m).F - 2Ff/m
This is an equation of a straight line with slope (2/m) and x-intercept Ff, which is just the graph shown in option B.