THE TURNING EFFECT OF FORCES
3 marks · EasyA uniform plank of 2.0 m has negligible mass. It rests on two supports, one at each end. A heavy box weighing 120 N is placed 0.5 m from the left end.
What is the total downward force acting on the supports?
Calculate the upward force provided by the right-hand support.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 120 N
(b) 90 N
Common mistakes: Forgetting the position of the box
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States total downward force is 120 N.
M1 (b) · 1 mark · Applies principle of moments about left end (or right end). (follow-through)
A1 (b) · 1 mark · Obtains upward force = 90 N at the right support. (follow-through)
Worked solution
a) The downward force is just the weight of the box: 120 N.
b) Take moments about the left end (let forces up at left = F1, right = F2):
But total upward force = 120 N, so left support = 120 - 30 = 90 N. The right-hand support provides 90 N.