1. Work, Energy, Power, Momentum
Impulse and
Momentum

2. p50 Egg drop Drop an egg in a beaker
Drop an egg in a beaker with a sponge in the bottom .
What do you observe?
Similarities Differences________ .

3. Impulse
Impulse is a force applied over time
To stop such an object, it is necessary to apply a force against its motion for a given period of time
Impulse = F (t)
In terms of impact and impulse, why are airbags in car a great invention?

4. Impulse What can we conclude about IMPULSE?
Which activity would require more impulse
Accelerating a soccer ball from rest to 10m/s OR accelerating a medicine ball from rest to 10 m/s?
Slowing a car from 60mph to 40 mph OR slowing the same car from 40mph to 10mph?
Landing from a jump while flexing the legs (bending at the knees) OR landing from a jump while keeping the legs straight (locking knees)?
What can we conclude about IMPULSE?

5. Bowling ball What happens if…
I swing a bowling ball at you?
Possibility #1
Possibility #2
Possibility #3

6. Newton’s Cradle What happens if
I lift and release one ball
Possibility#1
Possibility #2
Possibility #3
What if I lift and release more than 1 ball?

7. Newton’s Cradle Physics There’s an app for that!
The same principle applies to the suspended-ball desk toy, which eerily “knows” how many balls you let go…
Only way to simultaneously satisfy energy and momentum conservation
Relies on balls to all have same mass
Momentum depends on speed/velocity and mass
Giant Newton’s Cradle video

8. Discover for yourself. Record in your notebook and on a whiteboard to share out with the class.
Place 5 marbles in the center groove of a ruler. Launch a sixth marble toward the 5 stationary marbles. Note and record what happens.
Now launch two marbles at four stationary marbles. Then launch three marbles at three stationary marbles and so on. Note and record what happens each time.
Remove all but two marbles from the groove. Roll these two marbles at each other with equal speeds. Note and record what happens.
1.) How did the approximate speed of the marbles before each collision compare to after each collision?
2.) What factors determine how the speed of the marbles changes in a collision?
3.) What do you think would happen if three marbles rolling to the right and two marbles rolling to the left with the same speed were to collide?
4.) What factors affect an object’s momentum?

9. What is momentum???
Discuss with your partner and come up with an example to share with the class

10. When is a collision elastic or inelastic?
Phet: Collision Lab
Collisions and conservation of momentum
Click on advanced tab for more settings
Green arrows = velocity
Yellow arrows = momentum
Total momentum displayed in the chart
Finish both sides and get a stamp before you leave today
p53

11. To finish…
p51
CDP 8-1
CU (p307) 1-3
PtoGo (p309) 1-2

12. What did we learn about collisions and conservation of momentum and impulse from this week? (at least 5 required)
This is our MODEL for MOMENTUM
p54

13. Momentum model-driving question: what can objects do with momentum?
the impulse experienced by an object is the force*time
the momentum change of an object is the mass*velocity change
the impulse equals the momentum change
Momentum is mass x velocity p=mv
Momentum can be transferred to other objects when they collide
Momentum is conserved (none is lost or gained during collisions)
Momentum video

14. What happens if I drop one super ball? Slow motion ball bounce
I drop two balls stacked on each other?
Basketball and tennis ball

15. Elastic and inelastic Collisions
When a Ball hits the ground and sticks, the collision would be totally inelastic
When a Ball hits the ground and bounces to the same height, the collision is elastic
All other collisions are partially elastic collision

16. Superball Physics
During bounce, if force on/from floor is purely vertical, expect constant horizontal velocity
constant velocity in absence of forces
like in picture to upper right
BUT, superballs often behave contrary to intuition
back-and-forth motion
boomerang effect

17. Real-World Collisions
Is a superball elastic or inelastic?
It bounces, so it’s not completely inelastic
It doesn’t return to original height after bounce, so some energy must be lost
Superball often bounces 80% original height
Golf ball  65%
Tennis ball  55%
Baseball  30%
Depends also on surface, which can absorb some of the ball’s energy
down comforter/mattress or thick mud would absorb

18. Momentum Momentum is conserved. Momentum can pass from one
Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum
Momentum depends upon the variables mass and velocity
Momentum = mass * velocity
p = m * v
where m = mass and v=velocity
Momentum is conserved.
Momentum can pass from one
object to another (like the super balls)

19. Momentum is a vector quantity
To fully describe the momentum of a 5-kg bowling ball moving westward at 2 m/s, you must include information about both the magnitude and the direction of the bowling ball
p = m * v
p = 5 kg * 2 m/s west
p = 10 kg * m / s west

20. Check Your Understanding
Determine the momentum of a ...
1.) 60-kg halfback moving eastward at 9 m/s.
p = mv =
2.) 1000-kg car moving northward at 20 m/s.

21. Momentum and Impulse Connection
To stop such an object, it is necessary to apply a force against its motion for a given period of time
Impulse = F (t) = m D v
Impulse = Change in momentum

22. Check Your Understanding
If the halfback experienced a force of 800 N for 0.9 seconds to the north, determine the impulse
Impulse = F ( t ) = m D v

23. Impulse Question #2
A 0.10 Kg model rocket’s engine is designed to deliver an impulse of 6.0 N*s. If the rocket engine burns for 0.75 s, what is the average force does the engine produce?
Impulse = F ( t ) = m D v

24. Impulse Question # 3
A Bullet traveling at 500 m/s is brought to rest by an impulse of 50 N*s. What is the mass of the bullet?
Impulse = F ( t ) = m D v

25. Summary
the impulse experienced by an object is the force*time
the momentum change of an object is the mass*velocity change
the impulse equals the momentum change
Math review

26. Phyz springboard Introduction to Momentum notes Finish CDP 8-1
Review (song)
Review video

27. When is a collision elastic or inelastic?
Phet: Collision Lab (finish both sides and get a stamp before you leave today)
Finish Friday’s handout and get it stamped
If you have time
CU (p315) 1-3
PtoGo (p319) 1-2
p53
p52

28. Collisions Two types of collisions
Elastic: Energy not dissipated out of kinetic energy
Bouncy
Inelastic: Some energy dissipated to other forms
Sticky
Perfect elasticity unattainable (perpetual motion)

29. Things that go “bump” Record what you see
Write a possible explanation on your whiteboard
Share out
Explain the phenomena in your notebook.

30. Collisions and conservation of momentum
Number puzzles
CU (p329)1-4

31. Warm-up Questions
Twin trouble-makers rig a pair of swings to hang from the same hooks, facing each other. They get friends to pull them back (the same distance from the bottom of the swing) and let them go. When they collide in the center, which way do they swing (as a heap), if any? What if Fred was pulled higher than George before release?
A 100 kg ogre clobbers a dainty 50 kg figure skater while trying to learn to ice-skate. If the ogre is moving at 6 m/s before the collision, at what speed will the tangled pile be sliding afterwards?

32. Inelastic Collision Energy not conserved (absorbed into other paths)
Non-bouncy: hacky sack, velcro ball, ball of clay
Momentum before = m1vinitial
Momentum after = (m1 + m2)vfinal = m1vinitial (because conserved)
Energy before = ½m1v2initial
Energy after = ½ (m1 + m2)v2final + heat energy

33. Elastic Collision: Billiard Balls
Whack stationary ball with identical ball moving at velocity vcue 8
To conserve both energy and momentum, cue ball stops dead,
and 8-ball takes off with vcue 8 8
Momentum conservation: mvcue = mvcue, after + mv8-ball
Energy conservation: ½mv2cue = ½mv2cue, after + ½mv28-ball
The only way v0 = v1 + v2 and v20 = v21 + v22 is if either v1 or v2 is 0.
Since cue ball can’t move through 8-ball, cue ball gets stopped.

34. Momentum and Simple 1D Collisions
Phet: Collision Lab
CDP 8-2
Finish Conservation of Momentum Number Puzzles
Unit Test next time!
Chapter 8 workbook pages extra credit due Monday before 7:45am!
Momentum tutorial
p76

35. Momentum Quiz tomorrow
You may use your notebook on all parts of the test
Momentum tutorial

36. Angular Momentum
Another conserved quantity is angular momentum, relating to rotational inertia:
Spinning wheel wants to keep on spinning, stationary wheel wants to keep still (unless acted upon by an external rotational force, or torque)
Newton’s laws for linear (straight-line) motion have direct analogs in rotational motion

37. Angular Momentum
Angular momentum is proportional to rotation speed () times rotational inertia (I)
Rotational inertia characterized by (mass)(radius)2 distribution in object

38. Angular Momentum Conservation
Speed up rotation by tucking in
Slow down rotation by stretching out
Seen in diving all the time
Figure skaters demonstrate impressively
Effect amplified by moving large masses to vastly different radii

39. Do cats violate physical law?
Cats can quickly flip themselves to land on their feet
If not rotating before, where do they get their angular momentum?
There are ways to accomplish this, by a combination of contortion and varying rotational inertia