Assignment #3 HK 376 – Biomechanics
Thursday September 27th, 2012
1. Eric Chisholm, the very first ever Low Organized Games MVP was involved in a very intense
game of rocks (like capture the flag). While trying to defend his rocks, Eric dove at Jeff with
a horizontal velocity of 4 m/s to try and tag him out. Jeff jumped out of the way at the last
second and Eric went flying right through the gym doors. If Eric weighs 155 lbs:
a. What was the momentum of Eric flying through the air in the horizontal direction? (1)
b. What impulse must have acted on Eric when he hit the gym doors to cut his velocity by ¼
(1)
2. What impulse is necessary to accelerate a 15 kg boulder from 5 m/s to -5 m/s? (1)
3. A 90 kg man is gliding on his skates with a velocity of 15 m/s.
a. What is the momentum of the man? (1)
b. What impulse must act on the man to change his velocity by -20 m/s? (1)
c. After the impulse in b. acts on the man what will his momentum be? (1)
4. People driving in cars with airbags are less likely to be injured because (1)
a.
b.
c.
d.
e.
Airbags reduce the impulse imparted to the driver
Airbags increase the time of contact thereby reducing the peak force
Passengers undergo less change in momentum when they hit an airbag
They feel safer
None of the above
5. An athlete applies a 215 N force on a curling rock for 1.5 seconds that was initially at rest.
Given that the coefficient of friction between the ice and the rock is 0.1 and the curling rock
has a average mass of 19 kg, answer the following:
a. What will the momentum of the rock be immediately after 1.5 seconds? (1)
b. What impulse was exerted on the rock by the athlete? (1)
c. What impulse was exerted on the rock by the force of friction? (1)
d. After 1.5 seconds, the 215 N horizontal force no longer acts on the rock. How far (in
meters) will the weight continue to slide before coming to a rest (2)
6. In an attempt to give the gym a fresh look, Gerry and Joe re-arranged the free weight and
cable machine equipment. Assume that Gerry applied a horizontal force of 400 N, directed
to the right for 1.2 seconds on a 45 kg iron plate weight (lying on the floor). The coefficient of
friction between the weight and the floor on which it is sliding is 0.8.
a. What will the momentum of the weight be immediately after 1.2 seconds? (1)
b. What impulse was exerted on the weight by the gym employee? (1)
c. What impulse was exerted on the weight by the force of friction? (1)
d. After 1.2 seconds, the 400 N horizontal force no longer acts on the weight. How far (in
meters) will the weight continue to slide before coming to a rest (2)
7. A maximum net vertical force of 3000 N and a maximum net horizontal force of 1100 N act at
the same time on the foot of a 68 kg football player. Assume the football player is
contacting the ground with one foot. Ignoring air resistance,
a. What is the resultant force from the ground acting on the football player? (2)
b. What is the football player’s resultant acceleration? (1)
c. Assume that the average vertical and horizontal forces applied by the ground over the
entire time of foot contact were 55% of their peak magnitudes. If these average forces
acted for 0.65 seconds. What impulse did the ground exert on the football player? (1)
8. How long (time?) must a 300 N force from a catcher’s mitt act to decelerate a fast ball to a
complete stop if it was traveling at a velocity of 70 m/s? (Baseball = 142 grams) (1)
9. Before the impact of two pool balls, their combined linear momentum was 3 kg*m/s. If the
cue ball comes to a complete stop after the collision, with what velocity does the .2 kg 8 ball
leave the collision? (1)
10. An average vertical ground reaction force of 1800 N acts on a high jumper's foot for 0.22 s
during the upward phase of their jump. If the jumper's mass is 70 kg, what is their take-off
velocity? (2)
11. Two football players collide in the air. Ouch! If player B undergoes a 25 kg*m/s change in
momentum, what is player C's change in velocity if has a 125 kg mass? (1)
12. I have a brutal golf game; therefore, I have a brutal slice. On the 17th hole at the Antigonish
Golf Course there is a set of train tracks that runs parallel to the length of the hole. My golf
ball is travelling with a velocity of + 42 m/s and the train is travelling in the opposite direction
at - 25 m/s.
a. What is the velocity of the ball relative to the train? (1)
b. What is the velocity of the train relative to the ball? (1)
13. A 1.4 kg squash racquet swung at 28 m/s contacts a ball moving at -30 m/s. After impact,
the racquet has a velocity of 24 m/s, and the ball has a velocity of 45 m/s. What was the
coefficient of restitution associated with the impact? (1)
_____
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This assignment is due in class on Thursday October 4th 2012. Please show all your work
and
BOX your answers. Be sure to include units.
Late assignments will be deducted by 25% per day and will not be accepted once
solutions are posted.