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Stuff you asked about :

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Stuff you asked about :

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  1. I'm still having a hard time with the relation between momentum and kinetic energy, along with the concept of the system as a whole despite two separate parts (like 2 boxes). [Momentum is a vector (p = mv), while kinetic energy is not (K = 0.5mv^2). Forces acting over a time duration (impulse) change momentum while work done changes kinetic energy.] Some more check points in class are nice. I really like those because I can check my understanding....numerical examples are not needed so much. [Interesting… How many of you feel the same way?] Love the examples, would love to work some momentum in discussion and class because I'm unsure and not confident in how to use the formulas. [We’ll do an example at the end of class together.] the concept of an inelastic collision, I don't get where the energy goes. [The energy goes into vibrations, heat, sound, deformation etc. so the kinetic energy after the collision is reduced.] Combining the concept of center of mass and the concept of conserved energy took some pausing and rethinking, but it's fairly intuitive. The section on the "loss of energy" in an inelastic collision was a bit out of place though. [This just explains where the “missing” energy goes. The deformation of cars during a collision is an example: this takes energy!] Stuff you asked about:

  2. Lecture 11: Conservation of Momentum Today’s Concept: Inelastic Collisions How did the chicken cross the frictionless road? It laid an egg and threw it backwards.

  3. Announcements • Quiz 5 on Friday Unit 9 (Work and Potential Energy) and Unit 10 (Center of Mass) • Exam 2 next Friday, February 28 • Covers Units 6-11 (today’s unit is the last one on the exam) • 75 minutes: If you have a conflict, you need to email me by Monday night so I can make arrangements • Some minor changes in exam structure are likely (several T/F questions to help students earn some points quickly) • A major change to the format is also coming • Retake the exam online after normal exam: 2 hour limit • Open book, open notes, open web for online attempt • Average 1st and 2nd exam scores • No more “partial credit” on each question

  4. Momentum (Unit 11) Energy (Unit 8) Stuff you asked about: What is the different between conservation of Momentum and conservation of energy? How do I know if the Momentum is conserve or energy is conserve?

  5. ACT: Bullet and Block A) Before the collision. B) During the collision C) After the collision D) All of the above E) Only A and C above A wood block rests on a table. A bullet shot into the block stops inside, and the bullet plus block start sliding on the frictionless surface. The momentum of the bullet plus block remains constant: Before After As long as there are no external forces acting on the system, then

  6. Recap

  7. Homework Problem p1 p2 pfinal pinit p2 θ p1

  8. CheckPoint: Cart Propulsion 1 A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Suppose you are on a cart initially at rest that rides on a frictionless track. If you throw a ball off the cart towards the left, will the cart be put into motion? Left Right Conservation of momentum means the cart must move to the right since the ball moves to the left.

  9. ACT: Cart Propulsion A) Increase B) Decrease C) Will not change Suppose you are on a cart that is moving at a constant speed v toward the left on a frictionless track. If you throw a massive ball straight up (relative to the cart), how will the speed of the cart change? Left Right v The net external force in the horizontal direction is zero: Ptotalis conserved in x direction

  10. CheckPoint: Cart Propulsion 2 Suppose you are on a cart which is initially at rest that rides on a frictionless track. You throw a ball at a vertical wall that is firmly attached to the cart. If the ball bounces straight back as shown in the picture, will the cart be put into motion after the ball bounces back from the wall? Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place.

  11. ACT: Elastic vs. Inelastic Two equal-mass balls swing down and hit identical bricks while traveling at identical speeds. Ball A bounces back, but ball B just stops when it hits the brick. Which ball is more likely to knock the brick over? B A A) A B) B C) They both have the same chance.

  12. B A Momentum of ball before collision: Case A: Case B: Momentum of ball after collision: Case A: Case B: Change in ball’s momentum: Case A: Case B:

  13. Two balls of equal mass are thrown horizontally with the same initial velocity. They hit identical stationary boxes resting on a frictionless horizontal surface. The ball hitting box 1 bounces back, while the ball hitting box 2 gets stuck. Which box ends up moving faster? CheckPoint: Box and Ball Collisions A) Box 1 B) Box 2 C) same 1 2

  14. CheckPointRevote Which box ends up moving faster? A) Box 1 B) Box 2 C) same 1 2 A) The system in box 2 has the same momentum, but more mass, which means a smaller velocity. B) with box one, half the momentum from the ball will be transferred to the box and the other half will carry the ball in the opposite direction. with box two, all the momentum will be transferred to the box C) I have convinced myself that all three answers are right. I await enlightenment in the lecture. :-). Think of a 2-step “bounce”

  15. Ballistic Pendulum m v M H A projectile of mass mmoving horizontally with speed vstrikes a stationary mass Msuspended by strings of length L. Subsequently, m+Mrise to a height of H. Given H, what is the initial speed vof the projectile?

  16. M Breaking it down into steps before during after V m v H splat Which quantities are conserved before the collision? A) Momentum B) mechanical energy C) both momentum and mechanical energy

  17. M Breaking it down into steps before during after V m v H splat Which quantities are conserved during the collision? A) momentum B) mechanical energy C) both momentum and mechanical energy

  18. M Breaking it down into steps before during after V m v H splat Which quantities are conserved after the collision A) momentum B) mechanical energy C) both momentum and mechanical energy

  19. Ballistic Pendulum m v M H

  20. Ballistic Pendulum Demo L L-H d H L L L L m H v M M + m d In the demo we measure forward displacement d, not H: for d << L

  21. Optional HW problem A club-ax consists of a symmetrical 9.4 kg stone attached to the end of a uniform 3.2 kg stick. You measure the dimensions of the club-ax as shown in the figure: the stick is 98 cm long, and the 18 cm long stone is drilled through its center and mounted on the stick. How far is the center of mass of the club-ax from the handle end of the club-ax? Answer: XCM = 78.84 cm

  22. Optional HW problem Find the center of mass of the uniform sheet of plywood in the figure below. Let the coordinate origin be at the lower left corner of the sheet. Answer: XCM = 1.50 m YCM= 1.36 m

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