Revision

Question 1

A man exerts a force of 50N and can keep a 3.0kg box moving across a horizontal floor at constant velocity of 2.0ms^-1. Two such boxes are stacked over one another. The man then doubles the applied force on the stack that is at rest on the same floor. Which of the following describes the motion of the stack of boxes?
A: The box will accelerate at 15ms^-2.
B: The box will move at a constant velocity of 1.0ms^-1.
C: The box will not move.
D: The box will move at a constant acceleration of 17ms^-2.

Answer 1

Answer: C
Friction = 50N
“A man exerts a force of 50N and can keep a 3.0kg box moving across a horizontal floor at constant velocity…” This does not show whether the man exerts 50N at the start. The man might have exerted 100N initially before decreasing to 50N. The box will continue to move at the constant velocity when 100N was applied.

However, “Two such boxes are stacked over one another” shows that the boxes are stationary before the man exerts 100N. Since mass is doubled, friction is doubled and =100N. No acceleration and since the box was originally 0ms^-1 (stationary), its velocity will remain at 0ms^-1

Question 2

What is the order of magnitude of the diameter of a human hair?
A: 10^-2 m
B: 10^-4 m
C: 10^-6 m
D: 10^-8 m

Answer 2

Answer: B

Question 3

A second ball is dropped from the same height 1.0s after the first ball is released. Assuming air resistance is negligible, which of the following statements is true?
A: The two balls maintain a same distance as they descend.
B: The two balls dropped with constant speed.
C: The two balls get further when they descend.
D: The two balls get nearer when they descend.

Answer 3

Answer: C
Object at free fall, constant acceleration of 10ms^-2. With acceleration, velocity increases.