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Lecture 11: Conservation of Momentum Physics 2210 Fall Semester 2014 Today's Concepts: a) Elastic Collisions b) Inelastic Collisions Mechanics Lecture 8, Slide 2 Unit 11: Prelecture Feedback ● ● ● ● Conservation laws give me trouble in general. Give more details. Can you explain how it is possible to find the velocity of a mass that is blown up in space? Not confident about box and ball collisions. Problems similar to homework. Mechanics Lecture 11, Slide 3 Q: How did the chicken cross the frictionless road? Q: How did the chicken cross the frictionless road? A: It laid an egg and threw it backwards What is the different 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) Mechanics Lecture 11, Slide 6 Two Kinds of Balls... Bouncy (“elastic”) at room temperature Not bouncy (“inelastic”) at room temperature Homework Problem p1 p2 pinit p2 p1 pfinal θ Mechanics Lecture 11, Slide 8 Bombs Bursting in Air m1, v1 M, V 1 m2 , v 2 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 moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 10 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? Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 11 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? Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 12 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? Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Mechanics Lecture 11, Slide 13 “Newton's Cradle” Demo CheckPoint 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 1 C) same 2 Mechanics Lecture 11, Slide 15 CheckPoint 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 1 C) same 2 Mechanics Lecture 11, Slide 16 Flashcard Question 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? A B A) A B) B C) They both have the same chance. Mechanics Lecture 11, Slide 17 Flashcard Question 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? A B A) A B) B C) They both have the same chance. Mechanics Lecture 11, Slide 18 Example: Ballistic Pendulum m v M H 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? Mechanics Lecture 11, Slide 19 Colliding Box Cars