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final08_solutions

final08_solutions - Final Fill in Bubble A in the Test Form...

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Unformatted text preview: Physics 17 2008 Final Fill in Bubble A in the Test Form box. 1. An ice skater pulls her arms in to spin faster. Of these four quantities—angular velocity u), angular momentum L. kinetic energy K. and rotational inertia I —which quantity decreases? (c) more than one decreases 2. An ice skater pulls her arms in to spin faster. Of these four quantities—angular velocity w, angular momentum L, kinetic energy K , and rotational inertia I -which quantity increases? (a) w (b) L (C) K (d) I @more than one increases 3. Select the best answer to the warmup question, “How do athletes in extreme sports get so high in the air?" (a) They usually are hunched over or crouched when they jump. This is because you need a low-=1" "40d [“5 center of mass to get high. When athletes get ready to do a jump and get high in the air, you M ~ . do not see them on their boards or bikes in a straight position but they are usually crouching or W hunched. In skateboarding once in the air, the skater slides his front foot closer to the edge of W ._ the board using the friction between his foot and the surface of the skateboard to drag the board m . 2 up even higher. Also. the athletes build up acceleration; the greater the acceleration, the higher W the jump. (b) The skater’s weight, the force of gravity on the board, and the force that the ground exerts on theé- W {0% board are the three forces that act on a skateboarder. All of these forces add up to zero allowing - the skater to skate at a constant speed, unless another force is exerted upon the skater. 'WW ty att e @ Athletes in extreme sports like skateboarding and BMX need to achieve a large velom b . . . {46¢ . ottom of the ramp, so that they Will have laggerlginetic energy. n their way up the ramp, . . . . K . their kinetic energy converts to potential energ rand according to the formula, Pf = mgh, and 1 . . m mgh = fimvz, a larger veloc1ty on the bottom of the ramp means a greater height. the most important part of achieving great height. +Cut€ (d) Athletes in extreme sports manage to jump so high because their bikes and skateboargs are I ~ I . lightweight, so the force required to lift their bikes into the air isn’t that much. 27““; . (e) In sports like skateboarding and BMX, athletes are able to get so high because ramps enable V ‘ them to have double the termin velocity. For example. when Tony Hawk is skating off a ramp, he builds up his velo ity very h h then when he gets on the ramp, he ollies off right before the edge of the ramp. e ramp lready has a launch angle with horizontal and vertical vectors. <~ frug Because he is als jumping (0 ie), he adds another verticle vector to reach maximum height. W filelocity is M- v” / F3 'Ct‘x'C-JHOH lame F1 The diagram on the right shows the forces that act on the snowboarder when he is going straight down the hill in the picture on the left. (Note that the athlete’s boots are firmly strapped to his board so the athlete and board act as a single unit.) F1 and F3 act where the board touches the snow and F2 acts primarily on the snowboarder’s jacket. The next five questions refer to this figure. 4. Which force does not have a vertical component? (a) F1 (b) F2 (0) F3 (d) F4 @ none of these (they all have a vertical component) 5. What type of force is F3? (a) Normal ‘ .Friction W Wm W W (e) Elastic (d) Fluid drag (e) Surface tension 6. Which vector represents the force of gravity? (a) F1 ('3) F2 (C) F3 (e) none of these 7. When the snowboarder does a jump. which force does he increase? (13) F2 (C) F3 (d) F4 (e) none of these 8. If the snowboarder begins to carve a turn that will turn him to his left, the centripetal force on him is (a) downhill A (b) uphill V @0 his left A 9—- (d) to his right F c. (e) none of these 9. Why is it often easier to take a gradual route up a mountain rather than going straight up? (a) The amount ofwork is less. No PE : mzk '1 mat . £41251 ng . @The amount of force is less. Ye; M 9L" g (c) Bothofthese W: F d 1W 4) W F (d) None of these. 10. In which sport does rotation of the object make the path of the projectile more predictable? (a) discus 513;.“ W W W ——-> (b) frisbcc W W (e) football (d) none of these .all of these 1 I. If air resistance is neglected. does a projectile spend more time going upward or downward? (a) upward (b) downward @both the same 12. If air resistance is included. does a projectile spend more time going upward or downward? - . tF (a) upward HF \LW $5 D‘U‘m‘ JIVU A @ownward n21: tau IPA-A M ml r") Am a: (c) both the same :— fdj‘Q W 13. An ideal ping-pong paddle would have (5 is the coefficient of restitution and p is the coefficient of friction) ’rge e and large p Wet/ML— W jar MW LU/ filargeeand small y W Aft-act :- W e (c)smalleandlargep W t at 01 QF:'. .1? e E; ((1) small 6 and small p. g i i i 14. The figures show snapshots of a tennis ball that collides with the ground with a velocity of 90 mph at 90° (straight downward). Which quantity is relatively unimportant in this collision? (a) coefficient of restitution é~ V-Wa M“? L . oeffieient of friction é- not W (e) normal force W at W M (d) initial momentum oftennis ball Ola—um "do. (e) elastic energy wk? #4, W W M 15. Suppose you hit two balls—one spherical (like a beachball) and the other elongated (like a football)— with the side of your hand along a line that does not go through the center of the ball. Which type of force spins the ball? (a) Only normal for both balls):< _, : v L3 arm (b) Only friction for both balls. 9 2;, 1s \ Fri; (c) Friction for the football; normal for both. - ‘ ' X No W Fri; > ormal for the football. friction for both. WW N (e) None of these figc‘h‘ah has lever “NW '43? l'>‘3"’L—[L 16. What condition(s) is required for a struck ball to exit faster when it is thrown harder? Ohe mass of the “bat” must exceed the mass of the “ball." Tt‘u €— (b) The coefficient of friction must be small I“ V file Vani— (c) The coefficient of restitution must be small. La V3 e. (d) More than one of these. (e) None of these 17. Which factor helps a sailboat travel fast upwind? (a) The angle of the sail with respect to the wind can be adjusted so that the perpendicular force on the sail is not in the same direction as the wind. N (b) The hull prevents sideways motion of the boat 50. if the sail angle is properly chosen, one component of the perpendicular force on the sail is in the forward direction. N (c) The forward motion of the boat makes the apparent wind speed larger than the true wind speeg. (d) Two of these. Yes . @l of these. ‘ TORQUE/RADIUS (or mite A pool player hits the cue ball above its center to give it follow (topspin). Immediately after the cue ball strikes another ball, it is spinning but is not translating. The graphs represent what happens to the cue ball after that. The next four questions refer to these graphs. 18. In the time mt =rval from 0-2 first haltof ra hs . J. L ( K 22%. \)I 7" I_/ 2.1 us (a) he translational kinetic energy is increasing but the rotational kinetic energy is decreasing. (b) The translational kinetic energy is decreasing but the rotational kinetic energy is increasing. (c) Both types of kinetic energy are increasing. V ’l‘ to J} (d) Both types of kinetic energy are decreasing. 19. In the time interval from 0-2 (first halfofgraphs). $+ra~§3kT lines L, r V ? LOT z) (a) The acceleration a is increasing but the angular acceleration a is decreasing. COW 0* € (b) The acceleration a is decreasing but the angular acceleration a is increasing. 6° ”Sad: °< (c) Both a and a are increasing. (d) Both a and a are decreasing. @om of these 20. Which quantity is not constant in the time interval from 2-4 (second half of graphs)? (a) Translational kinetic energy ‘V M (b) Rotational kinetic energy M.) M (c) Acceleration (1 c. 0 wt (d) Angular acceleration a = O W ,one of these (they are all constant) 2]. In the time interval from 24 (second half of graphs), there is no net force or torque on the ball so the velocity and angular velocity are constant. as predicted by Newton's @st law. (b) 2nd law. (c) 3rd law. (d) none of these The next two questions refer to the diagram of the pool table. 22. The diagram illustrates the collision of a pool ball with a side cushion. Which statement about this collision is correct? . ‘ f . M M V5 4 V“ Ac WW (a) The ball does not lose any kinetic energy in this collision. (b) The ball does not change its moment in this collision. £¢QM WW (c) This is a high coeffici nt of fricl‘tfi, owlcoefficient of restitution. collision. d: ‘ it . (d) The W: this collision. N W / _ (e) None ofthcsc. ’\ Ag}, W V4: 1" Vt . ha :3 mung/Qingd— 23. When the cue ball (open circle) strikes the shaded balbthe cue ball initially stops because (choose thg best answer) (a) momentum is conserved. + r 9‘ e. / a l $0 +1“ “L e' @mmentum is conserved and the collision is nearly perfectly elastic. (c) momentum is conserved and the collision is nearly perfectly elastic and the friction between the balls is nearly zero. & +ru.e bed- Irto‘lL (d) None of these (one of the assertions in “c" is false). needed '50? +k¢ (e) None of these (whether it stops or not depends on the spin on the cue ball). CAQ b4! l +° 5+4? 24. Which statement about work is incorrect“? (a) Your muscles don‘t get as hot when you hike downhill (compared to hiking uphill) because your muscles are doing negative work. + ”L e, (b) A baseball pitcher gives the baseball kinetic energy by doing positive work on the ball. '1“. W 2' (c) A weightlifter is not doing any work when he is holding the barbell above his head. +rtL e (d) If you apply a torque to a basketball to spin it, you do work on the ball and give it rotational kinetic energy. 4- (‘UL 8— '@They are all true. 25. In translational motion. the momentum is Mass times Velocity. What is angular momentum? (a) Lever arm times Force (b) Rotational inertia times Angular acceleration c Lever arm times Angular acceleration fi' otational inertia times Angular velocity (e) Torque times Angle 26. In the figure on the left. the boxing glove protects the boxer’s hand. Which factor is not altered by the glove? (a) the maximum force on the hand FJ/ W At ’1‘ (b) the duration ofthe collision with the punching bag At ’l‘ (c) the area of the hand that experiences the collision A ’l‘ m M .he impulse of the collision 'l' Kn. 502A A? M M W (e) They are all altered. w/Ot‘w' 3’ 27 In the figure on the right. which condition for equilibrium 15 it difficult for the gymnast to maintain? (a) No net force Wm WWW -0 net torque Mg (c) Both of these (d) Neither ofthcse 28. In the figure on the left. the punching bag is suspended by a rope. What type of force does the rope exert on the bag? (a) gravity (b) buoyancy (c) normal ‘clastic (e) none of these 29. This pair of figures is from the textbook. What physical concept does Hay use them to explain? (a) Newton’s 2nd law (b) Conservation of momentum @Stability (d) Newton’s 3rd law (e) Conservation of angular momentum The next two questions refer to the figures of the soccer ball and bicycle helmet on the last page. 30. Air drag is smaller for the bicycle helmet than for the soccer ball primarily because @e factor C in the air flow equation is smaller. ' ' I Glade. (b) the cross-sectional area 18 smaller. €- thug/m. \ (c) the soccer ball is lighter. é— (d) the bicycle travels faster. é‘ fiat W?/ W W V Wot”? (e) the ball has a higher coefficient of friction. <—' 31. If the soccer ball was spinning counter-clockwise. which way would the air in the figure shift and which way would the lift force FL point? . - / E! (a) Bothup A (2%“ “‘6’“ a? (New‘ioms 3 ) (b) Both down = VCM V l FL (c) FL up; air down -FL down: air up (e) none of these 32. The optimal launch angle for a home run in baseball is ~ 35°, less than the optimal angle of 45" we discussed in the second week of class. Which factor does not lower the launch angle relative to 45°? ’The fence is higher than the batter. W a (b) The ball has backspin that causes lift. increasing the hang time. W 6 Wm “QM (c) Air drag is appreciable and drag has a greater effect on the horizontal motion than on the vertical motion. W M W => W 9 (d) two of these (e) none of these 33. Select the best answer to the wannup question, “If a weightlifter jerks a barbell, is it best to have the weight close to the body or out at arm’s length?“ Womx wei5k+ Clo 58 (a) At arm's lengtlzgas strength and stability are maximized at this position. (b) If a weightlifter jerks a barbell, it is best to have the weight close to the body than out at arm’s length because it offers an advantageous leverage as compared to when the weight is out at arm’s length. where the length of the arm increases the force r‘iceeqzd to lift the sa e amount 'weigh l. , 59. M9 fee ‘i‘O or a (c) Close to the body. It takes more energy to keep it further away from the body. 4‘ “an I 5e . . M . . . JNV’ ei- ‘kinda think you’d drop the barbells before thls happened...I hope...) it [5 better to have t e ‘P Quin-d barbell close to his body. because the torque on _t_h_e lifter’s arms if the barbell was straight out chef‘s; V would rip his arms from their sockets. [qrzer ieV¢V arm :7 more int-7:4: (e) It is easier to have the weight at arm’s length<because there is more leverage and the radius of rotational motion increases; additionally the coefficient for torque is increased as well. The next two questions refer to the illustrations of a swimmer on the last page. 34. When is” her velocity and acceleration in opposite directions? (ainm. Vgaat. Mém. ==> C110 @At#2. V Mime. ' W => GL<O (C)Al#3. VWfMMWZ') Q40 ((1) Two of these (e) None ofthese 35. After her flip turn. her body points in the opposite direction than it did before the turn. What must have been true during the turn? (Select the best answer.) To W 2 e I (a) The normal force was nonzééT-f-rug, ihc net torque was nonzero. k (c) The buoyancy force was nonzero. (d) Her center of gravity was outside her body. K” ME Mi . .. . ‘ I ‘ (e) She was in equ111brtum. 4-" M) AA“ MMS’ fi‘ ’t‘ 75 W . 36. In watts, the maximum power a fit athlete can produce is roughly r (a) 1 grown \N°M‘="0° (L (b) 10 +me. (c) 10000 37. In the assigned reading, Hay uses the term “moment“ as a synonym for (a) rotational inertia ‘ (c) lever arm (d) momentum (0) none of these 38. Which statement about rolling motion isfalse? ('a) Rolling is a special combination of translation and rotation. + l' L‘L e (b) In rolling. the point in contact with the ground barely moves. +rut e (c) The motion of the center of the wheel is pure translational motion. +rq g @All are true (c) More than one is false This equation applies to the next two questions. Changes in energy for the human body can be described by the equation: Change in Internal Energy = Chemical Energy — Mechanical Work — Dissipated Heat. (1) 39. Eating corresponds to which term in the equation? (a) Change in internal energy Chemical energy (c) Mechanical work (d) Dissipated heat (e) none of these 40. Sweating corresponds to which term in the equation? (a) Change in internal energy (b) Chemical energy (c) Mechanical work aissipated heat (e) none of these The next two questions refer to the picture of the bowler on the last page. 41. At the instant depicted. the bowler is running forward and has just completed his backswing. What type of energy does the ball have? (a) Gravitational potential energy I+s (a? (b) Translational kinetic energy It; V‘MS-gé Nl‘tk Him (c) Rotational kinetic energy evxoi 0“ ck swim: —9 “0+ ”+1“ ”j _/’WO of these (e) Three of these 42. Bowling balls have their mass concentrated near the center. This is so the bowler can give the ball (a) a lot of spin without slowing it down very much. ‘h‘uue. (b) much more translational kinetic energy than rotational kinetic energy. + \‘ U~ 3- [email protected])th of these (d) neither of these 10 The next four questions refer to the picture of the tennis player. 43. The labels “Action" and "Reaction” on the figure refer primarily to which of Newton's laws? (a) lst (b) 2nd (d) They don‘t refer to Newton’s laws 44. Two formulas that relate to the motion of the racket are v = no and T = Ia. A longer are on the swing is [onset at c. [net-case}; F . ‘better because L‘ T but worse because a- 1. LOW W V i}, («at W W . (b) better because i; T and better because a T. Y‘ 1. I V T {5 3°C, 4 (0) better because 1‘ T and better because a L. _ 4‘ Ion. er arc moves sum-mass POM Po Tievx «9.fo =7 [qr-Set- .I I Wcuut Uiqick F6?OVL§€ => lqrje °< 1e I/tjx \l, Hi; .1: 1944 (d) worse because ‘L' 1 and worse because a 1. (e) worse because '1' 1 but worse because a T. 45. The pictured stance is good for balance because (a) the base is wide (b) the cg is low @both of these ((1) neither of these 46. The picture shows the ball hitting the sweet spot of the racket. Hitting the sweet spot is good because (a) the coefficient of restitution is largest. Tt‘ute. ‘ (b) the handle doesn't jerk forward or backward on contact. Trove (Lexi: “ 01c Penn‘s” ° '\ (c) the main vibration that is excited has a low frequency. Fox-IS?- t Low‘ggiqemcie S 503:5 awe of these 8 YGIT‘eol @ squci S Fc‘f‘ . ’l / (e) Allot these ‘mei / us. (/6 Pack ’/ <5“ Baseball Bat? , ll 47. A force is a vector quantity because it has both 'aagnitude and direction (b) mass and acceleration (c) action and reaction counterparts 48. In the shot put (air resistance is negligible in the shot put). the shot maintains its horizontal component of velocitw‘bgcausfieke {fir (a) is not acted on by any forces. 3F “Vial"; QC+$ @is not acted on by any horizontal forces. +rke— (c) has no vertical component of speed to begin with. -C QoeA—Q. (d) the net force acting on it is zero. .CcJAQ, \b (e) none of these. 49. An oar pushes against the water with a force of 20 N. What force does the water exert on the oar? ’ 1A (a) Less than 20 N. NW '3 r O N. (c) More than 20 N. ((1) Not enough information. 50. In soccer, a proper “shooting" kick is struck on the laces but a “passing" kick is normally struck on the inside of the foot. A passing kick gives better accuracy than a shooting kick because @s easier to control the recoil angle when the surface is flatter. + l" “Q ( ) the rotational inertia is larger so a misaligned kick twists the leg more slowly. +VV‘ 1 @0th of these. (d) none of these. 12 #2 +V decreasing M #1 +V constant ——> W #3 -V increasing ...
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