Lect11 - Physics 211 Lecture 11 Today's Concept: Inelastic...

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Unformatted text preview: Physics 211 Lecture 11 Today's Concept: Inelastic Collisions (We will talk more about CM reference frame on Wednesday…not so much today) Physics 211 Lecture 11, Slide 1 Physics Dr. Selen seemed depressed while giving the prelecture. I can relate, I'm really depressed that I never get on the powerpoint slide. I try my best to get slide. up there, but I never do and it really hurts my self-esteem, and now this thing is telling me that I'm spelling all of these words wrong and now I just don't just know what to do with my life. I'm just an old college kid who can't do anything worthwhile like get on the powerpoint, and this probably won't even get up and then I'll have no self-esteem left. examples where center of mass is useful and other formula/results using center of mass reference frame. Who is the voice in the Prelecures?? The fountain question on the exam. And stegosauruses. all of momentum please, i bet you wont put me on this because im drawing drawing attention to myself how momentum relates to kinetic energy. differences between internal and external forces Dear Students, Help! I'm trapped in the future. Please Bring food. You're my Physics 211 Lecture 11, Slide Physics only hope. Love, Jennifer 2 Exam 1 Average 78.5% Physics 211 Lecture 11, Slide 3 Physics Exam comments… I'm a little too mad about my score on the exam that I felt I did very well on to make a friendly contribution to this lecture. If I can make any suggestion, it's that I hope there is justification for why I got so many wrong. Why the exam had trick questions on it. Wasn't the law of conservation of points violated when the exam grading mistake was corrected? The fountain question on the exam. And stegosauruses. Physics 211 Lecture 11, Slide 4 Physics Should we give credit for 2.2 M? A) Yes B) No Physics 211 Lecture 11, Slide 5 Physics More exam comments… i bombed my first exam and am starting to lost motivation for this class DON’T GIVE UP This is very common ! Important advice: Figure out why you got the wrong answers - Was is silly mistakes? - Was in concepts? - Was in math? Then come to me or Tim or your TA or office hours… Physics 211 Lecture 11, Slide 6 Physics Act 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 momentum of the bullet plus block remains constant Before After A) Before the collision. B) During the collision C) After the collision As long as there are no external forces forces D) All of the above acting on the system E) Only A and C above Physics 211 Lecture 11, Slide 7 Physics 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? Preflight Left Right A) Yes, and it moves to the right. - B)onservation of momentum forces the car to Conservation moves to the left. C Yes, and it C) No, it the right. remains in place. - CM must stay at the same place. Physics 211 Lecture 11, Slide 9 Physics 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? Preflight Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. Physics 211 Lecture 11, Slide 10 Physics Left Right A) Yes, and it moves to the right. B) Yes, and it moves to the left. C) No, it remains in place. A) The initial throw of the ball will push the cart to the right. B) again, the ball is moving at the end of the scenario, so the cart must be moving in the opposite way. C) the ball hitting the wall balances out the force of the ball Physics 211 Lecture 11, Slide Physics being thrown 11 Act 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 v A) Increase B) Decrease C) Will not change As long as there are no external forces acting on the system, Ptotal is conserved is Physics 211 Lecture 11, Slide 12 Physics Preflight 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 1 2 Which box ends up moving faster? A) Box 1 1 B) Box 2 C) same 2 A) Since the momentum is initially the same in both cases, box 1 must have a greater momentum to the right to make up for the ball going to the left B) all of energy is converted into motion C) Same initial momentum Act 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. Physics 211 Lecture 11, Slide 16 Physics demo A B ΔPA ΔPB ΔPA > ΔPB The change in the momentum of the ball is bigger in A Physics 211 Lecture 11, Slide 17 Physics Ballistic Pendulum mv 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? Physics 211 Lecture 11, Slide 18 Physics Breaking it down into steps before during V M splat after mv H Which quantities are conserved before the collision the A) momentum B) mechanical energy C) both momentum and mechanical energy C) Physics 211 Lecture 11, Slide 19 Physics Breaking it down into steps before during V M splat after mv H Which quantities are conserved during the collision the A) momentum B) mechanical energy C) both momentum and mechanical energy C) mv = (m + M )V Physics 211 Lecture 11, Slide 20 Physics Breaking it down into steps before during V M splat after mv H Which quantities are conserved after the collision the A) momentum B) mechanical energy C) both momentum and mechanical energy C) V = 2 gH 2 Physics 211 Lecture 11, Slide 21 Physics Ballistic Pendulum mv M H mv = (m + M )V V 2 = 2 gH ⎛ m+M ⎞ v = 2 gH ⎜ ⎟ m⎠ ⎝ m V= v (m + M ) ⎛ M⎞ v = 2 gH ⎜1 + ⎟ m⎠ ⎝ Physics 211 Lecture 11, Slide 22 Physics Ballistic Pendulum Demo L L mv M L H M+m d L In the demo we measure forward displacement d, not H: L-H H d L L2 = d 2 + ( L − H ) 2 H = L − L2 − d 2 M⎞ g ⎛ v = ⎜1 + ⎟d m⎠ L ⎝ for d << L v(mph) = 3d(cm) Physics 211 Lecture 11, Slide 23 Physics ...
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