THE TURNING EFFECT OF FORCES
5 marks · HardA carpenter tries to lift a heavy lid off a crate using a crowbar. The crowbar is 1.2 m long. She places the fulcrum (pivot point) 0.2 m from one end. She applies a vertical downward force of 250 N at the far end of the bar (opposite the pivot).
Calculate the moment produced by the carpenter's force about the fulcrum.
If the lid is just lifted (i.e., the system is in balance), calculate the minimum upward force at the lid end needed to balance the moment.
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) 250 N × 1.0 m = 250 Nm · must show correct use of distance from pivot
(b) Upward force = 250 Nm / 0.2 m = 1250 N · must show moment balance equation, distance from fulcrum for upward force
Common mistakes: Using wrong distance from fulcrum; Not balancing moments correctly
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses perpendicular distance from pivot to point of force.
A1 (a) · 1 mark · Correct moment (250 Nm).
M2 (b) · 1 mark · Equates moments at fulcrum for balance.
M3 (b) · 1 mark · Uses correct distance (0.2 m) for lid end.
A2 (b) · 1 mark · Accurate value for upward force (1250 N).
Worked solution
a) The carpenter applies her force 1.0 m from the fulcrum (1.2 m - 0.2 m = 1.0 m):
b) The upward force F at the lid end acts at 0.2 m from the fulcrum. For balance: