PHY 2053, Section 3794, Fall 2009, Quiz 7
1. A 0.450 kg steel ball strikes a wall with a speed of 12.0
m/s at an angle of 60.0◦ with the surface. It remains in
contact with the wall for 0.150 s and bounces off with the
same speed and angle.
a) What is the average force exerted on the ball by the
wall (magnitude and direction)?
Momentum changes only in the x-direction. ∆px = −2mv cos(θ) = F̄ ∆t. We find:
F̄ = −
2mv sin(θ)
2 × 0.450 × 12.0 × sin(60◦ )
=−
= −62.4 N.
∆t
0.150
So the average force is of magnitude 62.4 N in the negative x-direction.
b) What is the total work done by the ball on the wall?
Since the speed doesn’t change, ∆K.E. = 0, so no net work is done on the wall; i.e., Wnet = 0.
2. A 63.0 kg ice skater moving at 12.0 m/s crashes into a stationary skater of unknown mass.
After the collision the two skaters move as a unit at 4.0 m/s. The impact time is 0.140 s.
a) What is the magnitude of the average force experienced by each skater?
Using F̄ ∆t = m∆v, we find for the first skater:
F̄ =
m∆v
63.0 × −8.0
=
= −3600 N.
∆t
0.140
The forces on the two skaters are equal and opposite. They are both of magnitude 3600 N.
b) What is the mass of the second skater?
Using F̄ ∆t = m∆v for the second skater, we find:
m=
F̄ ∆t
3600 × 0.14
=
= 126 kg.
∆v
4.0