FORCES AND SHAPE
4 marks · HardA heavy box is pulled to the right along a rough surface by two ropes. Rope X pulls with 120 N and rope Y pulls with 80 N, both horizontally to the right. Friction acts to the left with a force of 150 N. The box experiences no other horizontal forces.
Calculate the resultant horizontal force on the box. State its size and direction.
Explain what will happen to the speed of the box as a result of this force.
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) 50 N to the right N · must show Addition of both pulling forces, Subtraction of friction, Direction (right)
(b) The box will accelerate to the right (its speed increases in that direction). · must show Reference to acceleration/increasing speed, Reference to direction
Common mistakes: Not adding forces in the same direction; Not subtracting friction; Ignoring direction
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Adds the two pulling forces correctly: 120 N + 80 N = 200 N
M2 (a) · 1 mark · Subtracts the friction: 200 N − 150 N = 50 N or equivalent calculation (follow-through)
A1 (a) · 1 mark · Gives final size and correct direction ('to the right') (follow-through)
B1 (b) · 1 mark · States the box will accelerate to the right or that its speed increases
Worked solution
a) The total pull to the right is the sum of the two ropes: 120 N + 80 N = 200 N (to the right).
The resultant force is 50 N to the right.
b) Since the resultant force is not zero and acts to the right, the box will accelerate to the right—its speed increases in that direction.