Momentum and Impulse Answer Key Pages 20 – 21
1. a) Two cars crashing
b) Sliding curling rock
c) Train or boat speeding up or slowing down
d) Moving pen or pencil while writing
2. a) Large impulse obtained by swinging harder to increase force, or using proper
technique to increase time of contact (i.e., follow-through). Both provide a large
momentum to the ball. For putting the ball, a small impulse obtained by exerting a small
force would be better. This decreases the momentum change the ball undergoes. Follow
through for direction.
b) Before dismount, a gymnast swings around the bar. As she falls, the force of gravity
applies an impulse to her. Increasing her speed and momentum, especially in the legs.
Gymnast rises upwards on the other side. Once her hands release from the bar, the
momentum is in the gymnast’s body as she flies through the air.
c) Setter must apply a force to the ball for a given length of time. Lengthen time of
contact with the ball, force applied to ball can be reduced (more control). The ball will
have a momentum that will carry it in the correct direction and to the correct spot to be
spiked. Cushion ball and guide with right force in right direction.
d) Swing bat with a large force. Accelerates bat to high velocity. Requires strength.
During swing, follow through to lengthen time of contact between bat and ball. Large
force and long contact time increase impulse applied and creates larger change in
momentum. Ball propelled with larger velocity and travels farther out of the park.
e) Car must slow down and stop, decrease momentum to zero. Braking force can be small
if time to stop is large. If braking time is small, braking force is large. Abrupt stop.
f) Ball must undergo a change in momentum to bring to zero. Catcher should attempt to
catch the ball in the webbing of the catcher’s mitt. If the ball is caught in the palm of the
hand, the ball stops in a short time. To achieve change in momentum, need a large force
to stop the ball. Reaction force of ball on hand can damage the catcher’s hand.
If the ball is caught in the webbing, the length of time to stop the ball increases in turn
decreasing the stopping force on the ball. Damage to hand is avoided.
Catcher can lengthen the time of the catch by allowing her glove to move in the direction
of motion of the ball as the catch is made. This also decreases the stopping force and
reduces changes of injury.
3. The opposing lineman has greater momentum, so it would be more difficult to stop
him. The lineman pushes the halfback backwards.
4. Both would be equally difficult to bring to rest. The boulder has a larger mass, but the
boy runs faster than the boulder. Boy cannot run any faster. However, the force of gravity
applies an impulse to the boulder. AS time goes on, boulder gains momentum and rolls
more quickly. Boulder will roll faster than the boy and will catch up. Boy would be
crushed.
Or
The boy could stop to the side out of the boulder’s path.
5.
Object
Mass (kg)
Transit bus
8 000
Velocity
(km/h)
50
Momentum
(kg-km/h)
400 000
Football pass
0.5
35
17.5
Sprinter
75
35
2700
Golden Boy statue
1 650
0
0
NASCAR Stock car
1 545
300
463 500
Marathon runner
65
12
780
Slapshot
0.15
150
22.5
Building
1 000 000
0
0
Skateboarder
68
20
1360
Comments
Large mass,
small velocity
Large velocity,
little mass
Same mass as
runner, but
large velocity
Mass but no
velocity (no
momentum)
Small mass,
large velocity
Has mass and
velocity,
velocity is
smaller
Puck has mass
and larger
velocity than
the football;
moves faster
than the
football
Not moving
therefore no
momentum
Same mass as
runner larger
velocity