# Lect13 - relative kinetic energies in different frames of...

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Unformatted text preview: relative kinetic energies in different frames of references relative What is an elastic collision, inelastic collision, and completely elastic collision? And are there any other types of collisions I should know about? I think we should have more days off of lecture in the future." Very good prelecture, I felt like I understood all of it." i hereby declare that this will be the last attempt to get on the board for the rest of the year. I get depressed when I go without physics for a week :( I'm glad the soy bean aphids are gone. Thanks.When I die, I want to go peacefully like my Grandfather did, in his sleep die, -- not screaming, like the passengers in his car. not The pudding is a lie Why can't Juice NOT turn the ball over? Seriously...oh by the way, the last question was a little confusing, but not as confusing as our offense. I think, because I usually end up doing these later during the night before the lecture, there's usually not a chance to end up having my comments read with the slightest intention of putting them on the board. So maybe even writing this now is something of an irony, but perhaps the comments are compiled the morning of. Well... who knows. It's off to bed! And maybe just some practice with the kinetic energy of a system, thank you for asking. These are all really easy, I mean I am the second coming of Einstein. Physics 211 Lecture 13 Today's Concepts: More on elastic collisions, Average force during collisions Physics 211 Lecture 13, Slide 2 Physics Motivator: Get 80% on all of the clicker questions and I’ll play the worlds best song ☺ (and you can see a video of Mats doing a demo from this class on TV) The prelecture said that the Kinetic energy of a system of particle would be The said different for different relative observer frame. how does that make sense? More on Elastic Collisions In CM frame, the speed of an object before an elastic collision is the same as the speed of the object after. m1 v*1,i m2 v*2,i v* v*2,f v* v*1,f m1 m2 So |v*1,i - v*2,i| = |v*1,f - v*2,f| But the difference of two vectors is the same if we add the same constant to both (which is what you do if you view them in another reference frame). Rate of approach before an elastic collision is the same as the rate of separation afterward, in any reference frame! Physics 211 Lecture 13, Slide 5 Physics Consider the two elastic collisions shown below. In 1, a golf ball moving with speed V hits a stationary bowling ball head on. In 2, a bowling ball moving with the same speed V hits a stationary golf ball. In which case does the golf ball have the greater speed after the collision? A) 1 B) 2 B) C) same C) Act V 1 V 2 Physics 211 Lecture 13, Slide 6 Act A small ball is placed above a much bigger ball, and both are dropped together from a height H above the floor. Assume all collisions are elastic. What height do the balls bounce back to? H Before A After B C Physics 211 Lecture 13, Slide 7 Physics Explanation Explanation For an elastic collision, the rate of approach before is the same as the rate of separation afterward: v v M m v M m v 3v v v Physics 211 Lecture 13, Slide 8 Physics Preflight A block slides to the right with speed V on a frictionless floor and collides with a bigger block which is initially at rest. After the collision the speed of both blocks is V/3 in opposite directions. Is the collision elastic? A) Yes B) No C) Need more information V Before Collision V/3 V/3 After Collision Physics 211 Lecture 13, Slide 9 Physics Forces during Collisions Forces Ftot = ma dP Ftot = dt t2 Ftot dt = dP ∫F t1 tot dt = P(t2 ) − P(t1 ) FaveΔt ΔP F ΔP = Fave Δt Fave Δt t t2 ∫F t1 tot dt ≡ FaveΔt ti tf Impulse ΔP = Fave Δt Act Two identical blocks, A and B, initially are moving on frictionless air tracks. The initial speed of block A is twice as that of block B. You now apply the same constant force to both blocks for exactly one second. F air track A 2v F air track B v The change in momentum of block A is: A) Twice the change in momentum of block B B) The same as the change in momentum of block B C) Zero. D) Half the change in momentum of block B Physics 211 Lecture 13, Slide 11 Physics ΔP = Fave Δt Act Two boxes, one having twice the mass of the other, are initially at rest on a horizontal frictionless surface. A force F acts on the lighter box and a force 2F acts on the heavier box. Both forces act for exactly one second. Which box ends up with the biggest momentum? A) Bigger box B) Smaller box C) same F M 2F 2M Physics 211 Lecture 13, Slide 12 Preflight A constant force acts for a time Δt on a block that is initially at rest on a frictionless surface, resulting in a final velocity V. Suppose the experiment is repeated on a block with twice the mass using a force that’s half as big. For how long would the force have to act to result in the same final velocity? F A) Four times as long. B) Twice as long. C) The same length. D) Half as long. E) A quarter as long. Only 50% got this right – Lets try it again ! Physics 211 Lecture 13, Slide 13 Physics …The experiment is repeated on a block with twice the mass using a force that’s half as big. For how long would the force have to act to result in the same final velocity? F A) Four times as long. B) Twice as long. C) The same length. A) the change in momentum when the experiment is repeated is twice the amount of the first time, and since the force is half as big, when you solve for dt it will be 4 times as big as the it initial experiment. ΔP = FΔt mΔV = FΔt B) Mass does not matter, only the force. C) The forces and the mass canceled out mΔV Δt = F Physics 211 Lecture 13, Slide 14 Physics Preflight Identical balls are dropped from the same initial height and bounce back to half the initial height. In Case 1 the ball bounces off a cement floor and in case 2 the ball bounces off a piece of stretchy rubber. In which case is the average force acting on the ball during the collision the biggest? A) Case 1 B) Case 2 C) Same in both cases Only 59% got this right – Lets try it again ! Case 1 Case 2 Physics 211 Lecture 13, Slide 15 Physics …In which case is the average force acting on the ball during the collision the biggest? A) Case 1 B) Case 2 C) Same in both cases Case 1 Case 2 A) the force on the ball depends on the time it is in contact with the floor. since it is much less in case 1, then the force is much larger B) its in contact with the ball for longer Fave ΔP = Δt C) If both balls reach the same height the impulse will be the same because they both leave with the same speed. Physics 211 Lecture 13, Slide 16 Physics Egg Throwing Video The best song ever The (well – maybe not…) maybe ...
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## This note was uploaded on 09/22/2011 for the course PHYSICS 211 taught by Professor Selig during the Fall '10 term at University of Illinois, Urbana Champaign.

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