Mechanics Lecture 11, Slide 1 Physics 211 Lecture 11 Today’s Concept: Inelastic Collisions

Mechanics Lecture 11, Slide 2 Hour Exam 1 High Score of 92% Charles Huh Yixin Yu Mean score of 75.1%

Mechanics Lecture 11, Slide 3 Hour Exam 1 High Score of 92% Charles Huh Yixin Yu Mean score of 75.1% Repeat of an important tip: Never write an equation before making a diagram. Don’t do anything before making a diagram. Diagrams are important.

Mechanics Lecture 11, Slide 4 What is the difference between conservation of Momentum and conservation of energy? How do I know if the Momentum is conserved or energy is conserved? Momentum (Prelecture 11) Energy (Prelecture 8) Demos: Newton’s cradle, Happy/sad balls

Mechanics Lecture 11, Slide 5 Collisions Demo: Inelastic collision on air track

Mechanics Lecture 11, Slide 6 ACT A) Horizontal momentum B) Kinetic energy C) Both of the above. D) Neither. BeforeAfter As long as there are no external forces acting on the system A wood block rests at rest on a table. A bullet shot into the block stops inside, and the bullet plus block start sliding on the frictionless surface. Which of the following (for the bullet+block system) remain constant during the collision?

Mechanics Lecture 11, Slide 7 p1p1 p2p2 p2p2 p1p1 p init p final  Homework Problem

Mechanics Lecture 11, Slide 8 CheckPoint A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. LeftRight Conservation of momentum means the cart must move to the right since the ball moves to the left. 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?

Mechanics Lecture 11, Slide 9 ? ? ? ? Tell me more Why? Why not? Tell me more

Mechanics Lecture 11, Slide 10 A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. LeftRight CheckPoint Suppose you are on a cart which is initially at rest that rides on a frictionless track. You throw a ball at a vertical surface 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 surface?

Mechanics Lecture 11, Slide 11 Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. CheckPoint

Mechanics Lecture 11, Slide 12 “spot on” Great observation! “spot on” “other”? ? ? ? ! ! ! ! ! What’s the final result? Which direction is “back”? Left? Good. Great observation!

Mechanics Lecture 11, Slide 13 ACT A) Increase B) Decrease C) Will not change v As long as there are no external forces acting on the system, P total is conserved 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

Mechanics Lecture 11, Slide 14 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? A) Box 1 B) Box 2 C) same CheckPoint 1 2

Mechanics Lecture 11, Slide 15 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? A) Box 1 B) Box 2 C) same CheckPoint 1 2

Mechanics Lecture 11, Slide 16 The beginning of understanding. Momentum is also conserved in the first case. I don’t know how to dissipate momentum. Is “more conserved” like “more equal”? But the collisions are not identical. ? ? ? ? ? True. How does that affect this? ? ? ? ? ? No. No.

A) A B) B C) They both have the same chance. Mechanics Lecture 11, Slide 17 ACT A B 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?

Mechanics Lecture 11, Slide 18 A B PAPA PBPB The change in the momentum of the ball is bigger in A PAPA >  P B Demo: super/clay ball pendulum

Mechanics Lecture 11, Slide 19 H m M A projectile of mass m moving horizontally with speed v strikes a stationary mass M suspended by strings of length L. Subsequently, m + M rise to a height of H. Given H, what is the initial speed v of the projectile? v Ballistic Pendulum Show us a difficult problem that would involve momentum.

Which quantities are conserved before the collision? A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy m M v v H beforeduringafter splat Breaking it down into steps Mechanics Lecture 11, Slide 20

Which quantities are conserved during the collision? A) Momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy m M v v H beforeduringafter splat Breaking it down into steps Mechanics Lecture 11, Slide 21

Which quantities are conserved after the collision A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy m M v v H beforeduringafter splat Breaking it down into steps Mechanics Lecture 11, Slide 22

H m M v Ballistic Pendulum Mechanics Lecture 11, Slide 23

Ballistic Pendulum Demo Physics 211 Lecture 11, Slide 24 In the demo we measure forward displacement d, not H : H m M v LL L L M  m d L d H LHLH for d << L v(mph)  3d(cm)

Final thoughts Mechanics Lecture 11, Slide 25 Have we covered these?