Mechanics Lecture 11, Slide 1 Physics 211 Lecture 11 Today’s Concept: Momentum Conservation
Your Thoughts Mechanics Lecture 11, Slide 2 Some cool demos? P=mv and F=ma. Can you explain how the first is derived from the latter? i am worried about midterm :( I understood everything, but if you could put this comment up anyway that would be pretty cool. Can we hear some coldplay? I know what you're thinking... should I really be going over the new lecture material just one hour before the test... Probably not, but I did it anyway... we'll see what my test score looks like... I need clarification on the difference between inelastic and elastic collisions. *insert obligatory statement about how a prelecture was due the day after an exam here* I LOVE F=MA and I find it the MOST beautiful thing in the world. Can I get an 'A' for the course as well?!
Mechanics Lecture 11, Slide 3 Im kinda confused with the difference in when kinetic energy is conserved and when momentum is conserved Momentum (Prelecture 11) Energy (Prelecture 8)
Mechanics Lecture 11, Slide 4 Recap
Mechanics Lecture 11, Slide 5 ACT A) Before the collision. B) During the collision C) After the collision D) All of the above E) Only A and C above 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. The horizontal momentum of the bullet plus block remains constant
Mechanics Lecture 11, Slide 6 p1p1 p2p2 p2p2 p1p1 p init p final Homework Problem
Mechanics Lecture 11, Slide 7 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 8 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 9 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 10 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 11 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 Balls that bounce and balls that stick. EXPLAIN THEM.
A) A B) B C) They both have the same chance. Mechanics Lecture 11, Slide 12 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 13 A B PAPA PBPB The change in the momentum of the ball is bigger in A demo PAPA > P B
Mechanics Lecture 11, Slide 14 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
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 15
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 16
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 17
H m M v Ballistic Pendulum Mechanics Lecture 11, Slide 18
Ballistic Pendulum Demo Physics 211 Lecture 11, Slide 19 In the demo we measure forward displacement d, not H : H m M v LL L L M m d L d H LHLH for d << L v(mph) 3d(cm)