Lesson 12 Homework Solutions
1.
After jumping vertically upward to make a slam dunk, a 70kg
basketball player is essentially in free fall as he returns to the floor. If
he falls through a distance of 1.5 m, what is his rate of change of
momentum as he returns to the floor?
The force on a 70kg player in free fall is his weight.
Weight = 70 kg
×
9.8 ms
2
= 686 N
Since rate of change of momentum is a measure of the force that
causes the change in momentum, the rate of change of momentum =
686 kgms
1
.
2.
If you are riding in a car traveling at 72 kmh
1
, what is your
momentum relative to (i) the ground? (ii) the car?
(i)
Your speed relative to the ground = 72 kmh
1
= 20 ms
1
If m is your mass, then the momentum relative to the ground is:
P = mv =
20v kgms
1
(ii)
Your speed relative to the car is zero. Therefore, momentum
relative to the car is
zero.
3.
A 6.0g bullet traveling at 200 ms
1
strikes a bullet proof vest and
comes to a stop in 500
µ
s. What is the average force exerted by the
bullet on the vest?
Initial velocity of the bullet is:
v
i
= 200 ms
1
Initial momentum P
i
= mv
i
= 0.006 kg
×
200 ms
1
= 1.2 kg ms
1
Final momentum P
f
= 0
The change in momentum
∆
P = P
i
– P
f
= 1.2 kg ms
1
6
P
1.2
The rate of change of momentum =
2400 N
t
500
10
−
∆
=
=
∆
×
The average force exerted by the bullet is a measure of its rate of
change of momentum.
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 Fall '10
 George
 Physics, Mass, Momentum, Work, initial momentum

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