Work, Energy, Power, Momentum

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

2. Egg drop p50 • 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 p50 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 ballWhat happens if… • I swing a bowling ball at you? Possibility #1 Possibility #2 Possibility #3

6. Newton’s CradleWhat 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 PhysicsThere’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 http://phet.colorado.edu/en/simulation/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 p52 • 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. SuperballPhysics • 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 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. • p = mv =

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 p73 • 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. 8 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 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 p77 • Phet: Collision Lab http://phet.colorado.edu/en/simulation/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