1. UNIT 5 – Momentum and Impulse Chapter 8
Inertia in Motion
Need Eggs – several hard boiled

2. momentum = mass x velocity
p = mv
momentum = mass x velocity
vector
units: kgm/s
car truck supertanker bullet
What’s the difference between a Mack truck and a skateboarder? (mass)
Would you rather stand in front of a bullet some one throws at you or one someone shoots at you? (velocity)
We combine these two idea into mass in motion – inertia in motion … momentum.
car = m v
truck = 20m v
supertanker = 100,000m (1/6v)
bullet = (1/100,000) m x 10v
no – truck not moving
check question: Will a truck always have more momentum than a skateboarder?

3. What is the momentum of a 0.5 kg baseball thrown at 40 m/s?
What is the mass of a car that has a momentum of 10,000 kgm/s when moving at 10 m/s?
20 kgm/s
1000 kg
4 m/s 5x
A canoe has a momentum of 1,200 kgm/s when loaded with 300 kg of gear. What is its velocity?
check question: A truck has 10x the mass and half the speed of a car – compare p.

4. time during which the force acts
Impulse = force x time
I= Ft
time during which the force acts
units = Ns F t 10 5 F t 5 10 F t 20 10
video: impulse - tennis ball
video: impulse - baseball ball
video: impulse – golf ball
Video: impulse – real golf ball 150 mph
Photo: impulse – bat & ball
Impulse = 50 Ns
Impulse ~ 160 Ns (Rectangle = 200, Triangle = 100)
DEMO: force plate
answer: longer time -> lower force
check question: Why do you aim for the haystack rather than the brick wall?

5. 𝐹𝑡=𝑚∆𝑣 𝐽= ∆𝑝 Δp = change in momentum = Δ(mv) = m(Δv)
an ________ leads to a __________________________
𝐹𝑡=𝑚∆𝑣
𝐽= ∆𝑝
Increase v (m const):
Decrease F (m and v const):
Increase F:
Impulse Change in Momentum
Write: “Impulse Momentum Equation” in front of boxed equation
Optional: derive Ft = mv from Newton’s 2nd Law
Increase v: cannon, archery
Write exaggerated eqn with big F and t and Δv
Video: increase speed - carrier launch
Decrease Force: pulling hand back when catching, boxing, air bags, traffic safety, egg toss
Write exaggerated eqn with big F and small t
Video: decrease force – traffic barrels, skateboarder, carrier landing
DEMO: egg toss into sheet
Increase F: hammering, self defense
Write exaggerated eqn with big m and Δv and small t
Video: increase force - ????

6. What is the force exerted on the pole?
A 4,500 kg truck moving east at 5 m/s hits a utility pole and is brought to rest in 0.90 sec. Find the force exerted on the truck.
What is the force exerted on the pole?
F(0.9) = 4500(-5)
F = - 25,000 N or 25,000 N West
F = +25,000 N or 25,000 N East (Newton’s 3rd Law!)
+20 m/s
-25 m/s
Check questions:
Will pushing on dash of car make it go faster? (no)
Will pulling up on shoe laces make you weigh less? (no)
A car speeds up from 5 to 25 m/s. What’s the Δv?
A car accelerates from 30 to 5 m/s. What’s the Δv?

7. Bouncing The ball hits the ground at 10 m/s and splats. Δv?
The ball hits the ground at 10 m/s and bounces up at 8 m/s. Δv?
The ball is pitched at 30 m/s and is hit back toward the pitcher at 40 m/s. Δv?
Demo: bounce vs. no bounce balls
Δv = 10 m/s
Δv = 18 m/s
Δv = 70 m/s
Video: bounce - football helmet tech (go to 1:20)
Ans: splat (delta v = v, not 2v, so lower force)
check question: When designing a container to protect eggs from breaking, do you want it to splat or bounce off the floor?

8. What is the change in momentum of the car during this time?
2000 kg car traveling to the east slows down uniformly from 20 m/s to 5 m/s. How long does it take the car to decelerate if the force on the car is 3000 N to the west?
What is the change in momentum of the car during this time?
What is the impulse on the car during this time?
How far does the car travel during the deceleration?
Δv = -15 m/s
Ft = mΔv
-3000 (t) = 2000(-15)
t = 10 sec
Δp= mΔv = 2000 (-15) = -30,000 kg m/s
Impulse = Ft = (10) = -30,000 Ns (Impulse = Change in Momentum!)
d = ? d = 1/2 (vi +vf) t
vi = 20 m/s d = 1/2 (20 + 5) (10)
vf = 5 m/s d = 125 m
t = 10 s

9. Short Barrel: Long Barrel:
Water Balloon Catch
Good Catch: Bad Catch:
Paint Ball Gun
Short Barrel: Long Barrel:
Check Question: Why are cars designed with “crumple zones” (increases time and decreases force)
Note: Avoid saying “it absorbs force or energy” – neither are true
Falling on:
Concrete: Gymnastics Mat:

10. Conservation of Momentum
Collisions
Elastic
Inelastic
Explosions
Conservation of Momentum
pi = pf
Elastic: 10 kg ball moving at 20 m/s collides elastically with a 5 kg ball moving in the same direction at 5 m/s. After collision 10 kg ball continues moving forward at 10 m/s, what is the velocity of the 5 kg ball?
Demo: tennis ball (elastic) vs. ball of clay (inelastic)
Video: elastic – football hits
Video: inelastic – train vs car
Video: expolsion – pumpkin
10(20) + 5(5) = 10(10) + 5(v)
v = 25 m/s
DEMO: air track or cart on track

11. What is their momentum after? How do their two momenta compare?
Inelastic: 5 kg fish swimming at 4 m/s eats a 1 kg fish swimming at 2 m/s . What their velocity after lunch?
Explosion: The cannon has a mass of 100 kg and the cannon ball has a mass of 5 kg. What is their momentum before the explosion?
What is their momentum after?
How do their two momenta compare?
If the cannon ball fires at 100 m/s, what is the velocity of the cannon?
5(4) + 1(-2) = (5+1) v
20 – 2 = 6v
v = 3 m/s
DEMO: air track or cart on track
same
100(v) = 5(100) v = 5 m/s

12. 0 m/s 0 m/s 4 m/s _____ m/s 0 m/s 12 m/s _____ m/s 0 m/s 0 m/s

13. More complicated collisions:
Do NOT print this slide
If p = mv [down] before, what is p after explosion?

14. Do not print
Ans: 200/80 = 2.5 kgm/s

15. Ft=mΔv
more t = more change in velocity