Your Thoughts. What is the difference between elastic collision and inelastic collision?

Transcription

1 Your Thoughts This seemed pretty easy...before we got the checkpoint questions What is the difference between elastic collision and inelastic collision? The most confusing part of the pre lecture was the fact that kinetic energy is not consered. I would like to go oer that in lecture. The concept I'm struggling with is the Center of Mass Reference Frame. Wheneer I'e soled physics problems before, I neer really think, "Oh, this is in this certain reference frame! This must be how we sole this problem." I just sort of sole it, which is probably why this is a more difficult concept for me to grasp. How come the deadlines for two of the pre-lectures are a week before the corresponding lectures? [this has been fixed.] Mechanics Lecture 11, Slide 1

2 Physics 211 Lecture 11 Today s Concept: Momentum Conseration Mechanics Lecture 11, Slide 2

3 The difference when kinetic energy is consered and when momentum is consered Momentum (Prelecture 11) Energy (Prelecture 8) Mechanics Lecture 11, Slide 3

4 Recap P m tot i i i Mechanics Lecture 11, Slide 4

5 Clicker Question 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. The horizontal momentum of the bullet plus block remains constant Before After A) Before the collision. B) During the collision C) After the collision D) All of the aboe E) Only A and C aboe As long as there are no external forces acting on the system Mechanics Lecture 11, Slide 5

6 Clicker Question 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. The energy of the bullet plus block remains constant Before After A) Before the collision. B) During the collision C) After the collision D) All of the aboe E) Only A and C aboe As long as there are no nonconseratie forces acting on the system Mechanics Lecture 11, Slide 6

7 It s a ector equation dp dt p 1 tot F Net, External p init p 2 p 2 p 1 p final q Mechanics Lecture 11, Slide 7

8 CheckPoint 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 A) Yes, and it moes to the right. B) Yes, and it moes to the left. Conseration of momentum means the cart must moe to the right since the ball moes to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 8

9 CheckPoint Suppose you are on a cart which is initially at rest that rides on a frictionless track. You throw a ball at a ertical 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? Left Right A) Yes, and it moes to the right. B) Yes, and it moes to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 9

10 CheckPoint Left Right A) Yes, and it moes to the right. B) Yes, and it moes to the left. C) No, it remains in place. B) Just like the spaceship example, if a part of the system goes off in one direction, the other parts of the system will hae to moe in the opposite direction to compensate for it and ensure that momentum is consered. C) The act of throwing the ball to the left causes the cart to start to go to the right, but hitting the ertical surface will cancel it out. Mechanics Lecture 11, Slide 10

11 Your Question Say we took the same cart from the first checkpoint problem and put walls on either side of the man. If the man then threw a ball at one of the walls so that it would just bounce between the two walls, would the cart be set in motion and if so, in what direction? Left Right A) The cart would moe back & forth as the ball bounced. B) The cart would not moe C) The cart would keep moing in the direction opposite the throw Mechanics Lecture 11, Slide 11

12 Clicker Question Suppose you are on a cart that is moing at a constant speed toward the left on a frictionless track. If you throw a massie ball straight up (relatie to the cart), how will the speed of the cart change? Left Right A) Increase B) Decrease C) Will not change As long as there are no external forces acting on the system, P total is consered Mechanics Lecture 11, Slide 12

13 CheckPoint Two balls of equal mass are thrown horizontally with the same initial elocity. 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 moing faster? A) Box 1 B) Box 2 C) same 1 2 Lets think about a slightly different ersion of this Mechanics Lecture 11, Slide 13

14 Clicker Question Two equal-mass balls swing down and hit identical bricks while traeling 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 oer? A B A) A B) B C) Equally likely. Mechanics Lecture 11, Slide 14

15 demo A B DP A DP B DP A > DP B The change in the momentum of the ball is bigger in A So the block gets a bigger kick in A Mechanics Lecture 11, Slide 15

16 CheckPoint Two balls of equal mass are thrown horizontally with the same initial elocity. 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 moing faster? A) Box 1 B) Box 2 C) same 1 2 Mechanics Lecture 11, Slide 16

17 Ballistic Pendulum m M H A projectile of mass m moing horizontally with speed strikes a stationary mass M suspended by strings of length L. Subsequently, m + M rise to a height of H. Gien H, what is the initial speed of the projectile? Mechanics Lecture 11, Slide 17

18 Breaking it down into steps before during after m M H splat Which quantities are consered before the collision? A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy Mechanics Lecture 11, Slide 18

19 Breaking it down into steps before during after m M H splat Which quantities are consered during the collision? A) Momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy m ( m M) V Mechanics Lecture 11, Slide 19

20 Breaking it down into steps before during after m M H splat Which quantities are consered after the collision A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy V 2 2gH Mechanics Lecture 11, Slide 20

21 Ballistic Pendulum m M H m ( m M) V V 2 2gH V m ( m M) m M 2gH m 2gH 1 M m Mechanics Lecture 11, Slide 21

22 Ballistic Pendulum Demo L L L L m M H M m d In the demo we measure forward displacement d, not H: L-H H d L 2 2 L d L - ( H) 2 H L - L - d M m d g L (mph) 3d(cm) for d << L Physics 211 Lecture 11, Slide 22