Download KEY

Name________________________________________________________Date____________
ESPN Movie – Relaxing with Impulse
1. How do you change an object’s momentum?
Give it a force - impulse
2. What is a force over a finite period of time called?
impulse
3. How does a football player lesson the force of a tackle?
With pads and relaxing during the tackle
4. Does an impulse have to stop an object? What else can it do?
No! It can change the direction of motion
5. If a watermelon thrown off a diving board has the same momentum when hitting the
water as one that hits the concrete deck why does the melon burst open on the deck but
not on the water? What does this have to do with time?
It took longer for the momentum to change in the water. The longer the time for the change in momentum
the smaller the force needed.
6. Write an equation showing how impulse is related to the change in momentum of an
object
Force x time = Change in momentum
(F t = pf – pi)
7. If the change in momentum of a football when kicked is 6 kgm/s and the force of the kick
acts for 0.008s, determine the force applied by the foot.
F t = pf – pi = 6kg m/s – 0 kg m/s = 6 kg m/s
F (0.008s) = 6 kg m/s
F = 6 kg m/s / (0.008s) = 750 kg m/s2 = 750 N
8. If the football is thrown with the same momentum as it was kicked (#7) would the force of
the throw be the same as the force of the kick? Explain!
It would be less! Less force would be applied but over a longer period of time
9. Why in sports do they tell you to follow through when you hit the ball?
Increases the time for which the force is acting on the ball giving a greater change in momentum
10. What does foam in football pads have to do with time and force? Explain.
Increase the time of the collision and so reduce the force acting on the player
11. What does an airbag in a car have to do with time and force? Explain.
Increase the time for the driver/passenger to change momentum and so reduce the force acting on them
12. What else in a car can reduce impact forces? (Think of at least three)
i.
shocks
ii.
foam padding inside the car
iii.
front end of car designed to cave in during a collison
13. A 0.2 kg baseball is traveling at 40m/s. After being hit by the bat, the ball’s velocity is 50
m/s in the opposite direction. Determine a) the change in the ball’s momentum and b)
the average force on the bat if the ball and bat were in contact for 0.002s.
M = 0.2 kg
a)
vi = 40 m/s
vf = -50 m/s
t = 0.002 s
p = pf – pi = mvf – mvi = m(vf – vi)
p = (0.2 kg) (-50m/s – (40m/s)) = -18 kgm/s
b)
Fav t = p
Fav = p / t
Fav = -18 kgm/s / 0.002 s = - 9000 kgn/s2 = -9000 N
(-ve means Fav acts against the ball’s initial motion)