Physics 8 Monday, September 16, 2013

Transcription

1 Physics 8 Monday, September 16, 2013 Today: ch5 (energy). Read ch6 (relative motion) for Weds. Handing out printed HW3 now due Friday. (I put the PDF up online over the weekend.) Learning physics is both more effective and much more enjoyable if you work with your fellow students! Try it! Bill hosts HW help sessions Thursdays at 7pm in DRL 3W2. Zoey hosts HW sessions on Wednesdays at 7pm in DRL 2N36. (But Bill & Zoey will swap W/Th just for this week.) My summary of key equations ( equation sheet ): positron.hep.upenn.edu/wja/phys008_2013/equations.pdf Last 10 minutes of Wednesday s class will be a Quiz solve & hand in modified version of one problem from HW1. Closed book, 1 handwritten sheet of notes OK. Calculator OK but not necessary. This is just an incentive to make sure you understand your HW. It counts as part of your in-class work, where total semester score 80% gets full credit.

2 Zoey wants us to discuss HW2 #13 A rock dropped from the top of a building travels 30.0 m in the last second before it hits the ground. Assume that air resistance is negligible. (Homework asked, How tall is the building? ) Which of the following statements is true? (A) The rock s instantaneous velocity one second before it hits the ground is 30.0 m/s. (B) The rock s instantaneous velocity at the instant just before it hits the ground is 30.0 m/s. (C) The rock s average velocity during the last 1.0 s of its fall is 30.0 m/s. (D) Statements (A), (B), (C) are all true. (E) Statements (A), (B), (C) are all false.

3 Zoey wants us to discuss HW2 #13 A rock dropped from the top of a building travels 30.0 m in the last second before it hits the ground. Assume that air resistance is negligible. (Homework asked, How tall is the building? ) At the instant just before hitting the ground, the rock s speed is (A) 30.0 m/s (B) 30.0 m/s (C) Somewhat faster than 30.0 m/s (D) Somewhat slower than 30.0 m/s (E) We don t have enough information to decide.

4 A rock dropped from the top of a building travels 30.0 m in the last second before it hits the ground. Assume that air resistance is negligible. (Homework asked, How tall is the building? ) Let the building height be h. Let the total time the rock falls be t. Which is a true statement about the problem? (A) h 1 2 gt2 = 0 (B) 0 gt = 30 m/s (C) h 1 2 g[t 1.0 s]2 = 30 m (D) 0 g[t 1.0 s] = 30 m/s (E) (A) and (B) are both true. (F) (A) and (C) are both true. (G) (A), (B), (C), and (D) are all true. (H) (A), (B), (C), and (D) are all false.

5 I found h = 62.1 m and t = 3.56 s. Hits ground at v x = ( 9.8 m/s 2 )(3.56 s) = 34.9 m/s. One second earlier: v x = ( 9.8 m/s 2 )(2.56 s) = 25.1 m/s m (0.5)(9.8 m/s 2 )(3.56 s) 2 = m 0.0 m 62.1 m (0.5)(9.8 m/s 2 )(2.56 s) 2 = m 30.0 m Zoey s solutions will show you another way to solve this problem, which avoids the need to solve simultaneous equations. Taking t i to be the start of the final one second, he writes: 0 m = +30 m + v xi [1.0 s] 1 2 g[1.0 s]2 then he figures out how long the rock free-falls to reach that speed, and adds it to the 1.0 s.

6 This will help you with HW3 #2 The velocity-vs-time graph below shows the motion of two different objects moving across a horizontal surface. Could the change in velocity with time be attributed to friction in each case? (a) Yes for the top curve, no for the bottom curve. (b) No for the top curve, yes for the bottom curve. (c) Yes for both curves. (d) No for both curves. (e) I have no idea how friction would affect a velocity-vs-time graph!

7 HW3 question 3 Draw diagrams that show the initial and final velocity vectors when a rapidly moving golf ball hits (a) a golf ball at rest, or (b) a basketball at rest. In each case, assume that the golf ball moves along the line connecting the centers of the two balls. (No need to work on this now, but I ll try to illustrate it for you.)

8 From Friday: The speed of a bullet can be measured by firing it at a wooden cart initially at rest and measuring the speed of the cart with the bullet embedded in it. The figure shows a kg bullet fired at a 4.00 kg cart. After the collision, the cart rolls at 2.00 m/s. What is the bullet s speed before it strikes the cart? (A) 1.00 m/s (B) 2.00 m/s (C) 4.00 m/s (D) 790 m/s (E) 798 m/s (F) 800 m/s (G) 802 m/s m v xi,bullet = (m + M) (v xf,cart+bullet ) ( kg)v xi,bullet = (4.01 kg)(+2.00 m/s)

9 HW3 question 5 A load of coal is dropped vertically from a bunker into a railroad hopper car of inertia kg coasting at 0.53 m/s on a level track. The car s speed is 0.31 m/s after the coal falls. What is the inertia of the load of coal? (Since we are analyzing only the horizontal motion, we can consider the coal+car system to be isolated.) What equation does momentum conservation allow you to write down here? Consider the horizontal motion before and after the coal is dropped onto the railroad car.

10 Chapter 5: Energy Confusing: Internal energy Closed (or not); isolated (or not) system Explosive separation Coefficient of restitution

11 Checkpoint 5.13 typo Most of this answer is fine, but when he writes, Yes at the beginning, he really means to write, No. (Even Harvard professors make mistakes once in a while!) (It made me really happy that at least one person found this typo to be a source of confusion in the reading, because it shows that you re reading the book carefully. Working through the in-text questions is much more important than remembering everything that I mention in class!)

12 Kinetic energy K = 1 2 mv 2 is the energy of motion. is conserved in an elastic collision. e.g. 1 2 m 1v1i m 2v2i 2 = 1 2 m 1v1f m 2v2f 2 but it s much easier in practice to write (equivalently) v 12,i = v 12,f i.e. relative speed is the same before and after an elastic collision (v 1x,f v 2x,f ) = (v 1x,i v 2x,i ) [Eqn. 5.4]

13 Types of collisions Elastic collision: objects recoil with same relative speed as before they collided. Kinetic energy K i = K f. (v 1x,f v 2x,f ) = (v 1x,i v 2x,i ) [Eqn. 5.4] Totally inelastic collision: objects stick together. (v 1x,f v 2x,f ) = 0 Inelastic collision: objects recoil, but with a reduction in relative speed (v 1x,f v 2x,f ) = e(v 1x,i v 2x,i ) with 0 < e < 1 Explosive separation: imagine T.I.C. movie played in reverse. (v 1x,i v 2x,i ) = 0 (v 1x,f v 2x,f ) 0 Q (tricky): what value of e describes an explosive separation?!

14 If I play in reverse a movie of an elastic collision, what sort of collision would I appear to see? (a) elastic (b) inelastic (c) totally inelastic (d) explosive separation (e) it depends!

15 When we collide (on a low-friction track) two carts whose masses and initial velocities are known, conservation of momentum allows us to write m 1 v 1x,i + m 2 v 2x,i = m 1 v 1x,f + m 2 v 2x,f We have one equation, but two unknowns. Knowing something about energy gives us a second equation. Relative speed = key. elastic: (v 1x,f v 2x,f ) = (v 1x,i v 2x,i ) totally inelastic: (v 1x,f v 2x,f ) = 0 if e is given: (v 1x,f v 2x,f ) = e(v 1x,i v 2x,i ) if change in internal energy is given: 1 2 m 1v 2 1i m 2v 2 2i = 1 2 m 1v 2 1f m 2v 2 2f + E internal (or equivalently) K 1i + K 2i + E i,internal = K 1f + K 2f + E f,internal (We ll work some HW-like examples on Wednesday.)

16 Suppose you find an isolated system in which two objects about to collide have equal and opposite momenta. If the collision is totally inelastic, what can you say about the motion after the collision? (Discuss with your neighbor, and then I ll call on a few people to see what you think. If some of us disagree on the answer, it s not a problem: we will all learn by discussing.)

17 What sort of collision is illustrated by this velocity-vs-time graph? (A) elastic (B) inelastic (C) totally inelastic (D) explosive separation (E) can t tell from given information

18 What sort of collision is illustrated by this velocity-vs-time graph? (A) elastic (B) inelastic (C) totally inelastic (D) explosive separation (E) can t tell from given information

19 Imagine making two springy devices, each made up of a dozen or so metal blocks loosely connected by springs, and then colliding the two head-on. Do you expect the collision to be elastic, inelastic, or totally inelastic? (Think about what happens to the kinetic energy.) (A) elastic (B) inelastic (C) totally inelastic

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21 Newton s cradle: what do you expect to happen if I pull back two of the spheres and release them? Discuss! (No clicking required.)

22 Newton s cradle: what do you expect to happen if I pull back two of the spheres and release them? Discuss! (No clicking required.) What do you expect to happen if I put a piece of play dough between two of the spheres? Discuss! (No clicking required.)

23 If all three collisions in the figure shown here are totally inelastic, which bring(s) the car on the left to a halt? (a) I (b) II (c) III (d) I, II (e) I, III (f) II, III (g) all three

24 Which of these systems are isolated? (a) While slipping on ice, a car collides totally inelastically with another car. System: both cars (ignore friction) (b) Same situation as in (a). System: slipping car (c) A single car slips on a patch of ice. System: car (d) A car brakes to a stop on a road. System: car (e) A ball drops to Earth. System: ball (f) A billiard ball collides elastically with another billiard ball on a pool table. System: both balls (ignore friction) (g) (a) and (f) (h) (a), (c), and (f) (i) (a), (b), (c), and (f) (j) all

25 A compact car and a large truck collide head on and stick together. Which undergoes the larger momentum change? (a) car (b) truck (c) The momentum change is the same for both vehicles. (d) Can t tell without knowing the final velocity of combined mass.

26 A compact car and a large truck collide head on and stick together. Which undergoes the larger velocity change? (a) car (b) truck (c) The velocity change is the same for both vehicles. (d) Can t tell without knowing the final velocity of combined mass.

27 A compact car and a large truck collide head on and stick together. Defining the system as the car plus the truck, which of the following are unchanged (to a very good approximation)? (a) kinetic energy (b) total momentum (c) total energy (d) (a) and (c) (e) (a) and (b) (f) (b) and (c) (g) (a), (b), and (c)

28 Two 1-kg carts are about to collide; one is initially at rest, the other comes in at a speed of 4 m/s. From the information given, you (a) can (b) cannot determine the final velocities of the two carts.

29 Chapter 6: relative motion (read for Wednesday) This is the pre-einstein version of relativity. It turns out that the laws of physics do not allow you to determine whether or not you are moving with a constant velocity (unless e.g. you look out the window). Did you ever, sitting in a car at a red light and looking at the car next to you, slam on the brakes because you thought you were starting to roll, but it turned out that your car never moved?

30 Key ideas from Chapter 6 velocities add when you add the same offset to every velocity (e.g. by observing it from the perspective of a moving train) an expression like v 1 v 2 doesn t change but be careful: something like 1 2 m 1v m 2v2 2 does change Imagine a car accident, as witnessed by a stationary pedestrian and also by a passing motorist. The physics of what happened to the two colliding cars does not depend on the motion of the observer. But the two observers might write down different math corresponding to what they see. In many cases, the math is greatly simplified if you analyze the problem from the reference frame in which the center of mass is not moving. We ll work out examples later this week. If you close your eyes (and there is no wind, etc.), there is no way for you to feel the difference between being at rest and moving at a constant velocity.

31 Don t click. Just think. Suppose I m a passenger on a train that is speeding toward NYC at 40 m/s. In search of coffee, I walk toward the back of the train at 2 m/s, just as the train whizzes past Princeton Junction. To a passenger watching me from the train platform, my speed is (a) 2 m/s (b) 38 m/s (c) 40 m/s (d) 42 m/s

32 Don t click. Just think. I m driving east at 50 kph. A little kid looks out the window of a westbound car that is going 40 kph. From the kid s point of view, how fast am I moving? (a) 10 kph (b) 40 kph (c) 50 kph (d) 90 kph

33 Don t click. Just imagine. Suppose you are sitting in a soundproof, windowless room aboard Air Force One, which is flying at cruising altitude in smooth air. Which of the following can you detect from inside the room? (a) how fast the airplane is moving with respect to the ground (b) whether or not the airplane is speeding up (c) whether or not the airplane is slowing down (d) whether or not the airplane is changing direction (e) all of the above (f) (b) and (c) (g) (b), (c), and (d) We ll explore these ideas in Chapter 6, which you ll read for Wednesday. Another important Chapter 6 concept is center of mass, which is actually a big deal for architectural structures.

34 Physics 8 Monday, September 16, 2013 Read ch6 (relative motion) for Weds. Handed out printed HW3 today due Friday. (I put the PDF up online over the weekend.) Learning physics is both more effective and much more enjoyable if you work with your fellow students! Try it! Bill hosts HW help sessions Thursdays at 7pm in DRL 3W2. Zoey hosts HW sessions on Wednesdays at 7pm in DRL 2N36. (But Bill & Zoey will swap W/Th just for this week.) My summary of key equations ( equation sheet ): positron.hep.upenn.edu/wja/phys008_2013/equations.pdf Last 10 minutes of Wednesday s class will be a Quiz solve & hand in modified version of one problem from HW1. Closed book, 1 handwritten sheet of notes OK. Calculator OK but not necessary. This is just an incentive to make sure you understand your HW. It counts as part of your in-class work, where total semester score 80% gets full credit. I ll put today s slides up on Canvas this afternoon.