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DOC007 - EXAM#2 PHYS 125 25 OCTOBER 2005 Name aw “:4,er...

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Unformatted text preview: EXAM #2 PHYS 125 25 OCTOBER 2005 Name: aw “:4,er v’h’mr‘fi f (please print legibly) Student ID #: goufir 2 Q53»; Part Score ‘ %\ ,Fg\‘; (s) ‘ Short answer 7’6 W *5 Problem #1 ”2 W Problem #2 Z4 W ‘ Problem #3 , , Honor Code Exams must be completed Within the allotted time and with no outside assistance. Instructors will periodically enter the classroom, and you are permitted to ask questions of them. However, remember that your instructor is limited in the amount of information he can give. There should be no communication with any other persons (verbal, electronic, etc.) other than the instructor. The use of calculators on the exam is permitted, but is limited to calculation. Any other uses (unit conversion, formulas, graphing, etc.) are not allowed, Please sign the honor code below as confirmation that you'have complied with this policy: “On my honor, I have neither given nor received any unauthorized aid on this exam.” msjag {05; (date) PHYS 125 Exam #2 25 October 2005 Short Answer (40 points) Each of the ten questions below is worth 4 points. 1. Ten years from now you find yourself in space some 600 km above the earth repairing a satellite observatory. You are carrying a bag of tools around your neck to help with the repairs. After a minor mistake you find yourself drifting away from the satellite, but worse still, away from your crew in the shuttle. Stranded in space with no means to have the shuttle pick you up, how do you get back to the ship? ’7“ ‘0'” “3”” "km“ i‘“ *9” ”W was ae‘vcvia‘y 17$ Jig/X‘— ( Wig” J}: 133' PL” V5314. .1: ‘g" v" CE» 3 $lx’i\ 41V: 9%: {ff Wt’u‘t dram! “(gig + if” { gflflififiiw . @ppose a ping—pong ball and a bowling ball are rolling toward you. Both , have the same momentum, and you exert the same force to stop each. How do the time intervals to stop them compare? (a) It takes less time to stop the ping-pong ball. /' ”7/53? ‘/(B") Both take the same time. m '3‘ V9 (c) It takes more time to stop the ping—pong ball. W 9 V90 , ngz How do the distances needed to stop them compare? \A (”3 W V An \’\ 9 J A i . . (a) It takes a shorter distance to stop the ping-pong ball. )(fby Both take the same distance. 4 @It takes a longer distance to stop the pingpong ball, L 3. The standard office toy and wonderful physics demonstration, Newton’s cradle, is an illustration of (a) conservation of momentum. (b) conservation of energy. @both conservation of momentum and energy. (d) neither conservation of momentum nor energy. PHYS 125 Exam #2 25 October 2005 4. Two students of equal mass are entering a building. Student A uses a wheelchair ramp to get access to the door, while student B uses the stairs. You may ignore friction. Which student does more work? _ , A B m '/ ‘x. 5. Below is a schematic of Dick Fosbury performing his “Fosbury Flop77 in the 1968 Olympics Mark his center of mass with an “x.” There are three figures just in case you mess one up. Make sure you indicate which you waane grade! 6. Sandra is driving a bus at 50 mph and it collides head-on with a Texas mosquito. The impulse delivered to the mosquito is sufficient to end its life. The magnitude of the impulse delivered to the bus is (a) greater than (b) less than (c) equal \/ / to the magnitude of the impulse experienced by the mosquito? ‘ r. u are creating a bullet proof vest. Would it be safest for the Wearer R 1f the vest stopped the bullet or if it ricocheted the bullet back into the direction from where it came? (\Ml'ra‘ w as“ 7c \3 w h“ c PHYS 125 Exam #2 25 October 2005 /8.jA “spud launcher,” or “potato cannon,” is a mechanism that fires a potato over long distances much to the delight of many juvenile Arkansans. An igniter starts an explosion. The explosion expands the gas, and the gas forces a potato out of a barrel. The figure below demonstrates is a rough schematic. When a spud launcher with a longer barrel is used, the force of expanding gases acts on the potato for a longer distance. If the barrel is cut to 1/4 of its original value, what is the m Beet/of the potato in terms of its original speed 1107 Assume that there IS faficonstant force. 1 z , :9 EM VD , T ”5/; ii VJ ~21“ \1, ;~ 9. \ M l a; (f: \V I http : //www . howstuffworks . com/enginel . htm Y \1 f, ,- V’ . A 10-kg cannonball is dropped from the top of a IO—m tall building here on the surface of Earth Which option below best represents the force with which the cannonball hits the ground? Q 7 (a) 1N WM W99,” 1' }{ w\l\ (b)10N ‘ r‘ v 2‘, w!\)"1’1 @IOON , tom/o“) ., , xxJ’M (d)1000N ‘_ V’ rte/Y: ® Need more information Q i , , x] ‘k 2 A / L) \Jr 4 ‘5 \i'? i! 1 3 L ’>z ‘ L w m "10 A hydrogen atom (a proton,an’d electron) is initially at rest when, by a \‘ /freak occurrence, the atom spontaneously breaks apart with the electron (me 2 10—30 kg) moving to the right and the proton (mp 2 2 x 10‘27 kg) moving to the left. After breaking apart, Where is the center of mass of the system relative to the atom’s original position, eg. above, to the right, i . . ' K 042, y w x )0 SQW" )) ‘ _, r 1' _» ' ‘5 V2132: N‘ .... ' L Ch" 3 {any 1‘” {A} \,/’ 1 \. 1‘ Y \/ \f \I X ”/ W \ ,M t We \1 \ e \ d ‘K ,, PHYS 125 Exam #2 25 October 2005 U0w qo Problems (80 points, 40 each) You must work problem #1, but you may choose to work either problem #2 or problem #3. Please circle the number of the problem you want graded here and on the front of the test. 1. As you may know, the Six Flags Astroworld theme park is closing. To close out its final night in style, they are creating a very dangerous, yet thrilling ride called the “Galilean Flinger.” It is formed from a small car of mass m on a cable of length L. As the figure below shows, a peg is height h = L/ 4 above the car’s lowest point. From what minimum angle 49 must the car be released in order for the ball to go over the top of the peg without the string going slack? USC— WN;lLI‘-o\ {One «\5 F r h = L 4 4 u , AK “2‘, .5. a ;&:€‘E¢‘VA 5 \ / ,x‘.’§w- A r F ? S \ / M 3425:“. K9 tMgt emf» w. m ,, . ‘ , 9" , » ~43» ' 4‘“: (ataxia, N, WAl‘h‘n 1’35 («fiaavih’fl'J’ Lanai?» i 5’0 ”(95 ‘L 6455, E7} 1' ‘1! f“ is s'lVlP‘X'Nl' meg/3 x://"’;§J{-mse S‘QLV‘, Ll. T30 ”in/Z, )\ Tel-(Asian {a SMIK: J" \ ,5 “° : i“: if” _ j , u, M “r O kg: Lx Lease PHYS 125 Exam #2 25 October 2005 7’b/Lio raction of nitrogen with the three hydrogens in the ammonia ule (NHg) can be modelled by the the “double—well” potential energy unction A I 4 :r 2 U<m)=;@<(,;§) 4C???) ), where U0 and to are constents, and as is the position of the nitrogen atom above Wen ato lS. / a; [33 r - \L {>9} UV? W a 4 its,“ 533 Kl‘fi S+¢~§i£ L Sfyelwkx‘ “I (a) (15 pts) What is the force on the nitrogen atom as a function of its \S distance above the three hydrogens m? . E '_ ' c— [‘1- \ f E); ‘ ‘3 ‘X l: M \ Mefié‘kxo‘flg 4' lie mg) '/(b) 15 pts) Where are the equilibrium points? (Your answer could de— A l"/|§ end on $0 and My) Indicate whether they are stable or unstable. (Dex w’w we = «an as We.) spl’ F—co to 1‘“ M {’2 m um um: -uo (Sam) -5 G) x0 , w (New) (c) (10 pts) If the nitrogen atom is at a: : 1‘0 with kinetic energy K 2 9 U0/3, how much work must be done on the atom such that it is able to freely bounce back and forth from above the hydrogens to below \0 the hydrogens? AK 3 N M ‘/ PHYS 125 Exam #2 25 October 2005 3. An elevator system (at right) consists of a 1200-kg car for passengers and a SOD—kg counterweight connected by a massless cable that passes over a massless and frictionless pulley. A motor in the bottom of the elevator shaft delivers the force 7 such that the elevator begins from rest with an acceleration of 2 m/s2 for the first meter of its journey, then cruises at a constant speed, and finally slows down to a stop with the same magnitude of acceleration. (a) (10 pts) Draw free-body diagrams for both objects. 3 —‘ a.» T‘ r? m ,,,,,,,,,, c WWMW; y _., x u) _ h g .2 id's! I} N 3 MAW 035% e aw (b) (10 pts) What is the cruising speed of the elevator? (c) (10 pts) What force is delivered by the motor while accelerating the elevator from rest to its cruising speed in the upward direction? (d) (10 pts) How much work is delivered by the motor if the elevator leaves from rest on the first floor and climbs 27 in to the tenth floor to stop and pick 11p passengers? Exam #2 PHYS 125 25 October 2005 Namamm ID: QOIQlZQ 50 Exercise 1 10 Steps Throw something away from the shuttle. Ping-Pong vs. Bowling Ball (b) Both take the same time. (c) It takes a longer distance to stop the ping—pong ball. (c) Conserves momentum and en— ergy Students do “same work” Center of mass needs to be below the center of the hips, but not below the feet. (0) Impulses are “equal” in magni- tude. Safest to “stop” the bullet. Speed is cut in half 1) = 110/2 (e) Need mOre information Center of mass is in the “same place” as original position of atom. Total 4 [\D 40 Max Score :1. :23 :L :L it i .0— __0__ _0;_ A I ll" Exam #2 Problem steps; x. ** ‘ «5M Score 15 1 Use centripetal foréé Apply Newton’s 2'“ Law and centripetal force at some point in the problem. If applied when mass is above. peg. Solve for minimum speed of mass above peg. 1131]." = gL/4 Use conservation of energy Apply conservation of energy between two points in the problemi If applied from starting point to when mass is above peg. Arrive at v2 = gL(1 — 200s 6) or equiva- lent. Solve for 0min Use result from conservation of energy Use result from centripetal force Mathematics and substitution cos 49min 2 3/8 —’ 49mm :2 68° Total [0 5 Ci! 40 PHYS 125 25 October 2005 -5; o o ION» PHYS 125 Exam #2 25 October 2005 Problem Steps Max Score 2a Find Fm 15 ; Try to take derivative 5 Multiply by minus sign 2 Q Result has one term depending on cc and 4 % one on m3 _ 4U a: 3 F: — T02 (; “ 53) 4 _2"'_ 2b Find equilibrium points 15 Set Fr : 0 5 O Solve for are’s (math and substitution) 4 fl 339 z {—5307 07$0} 3 .L me = {stable, unstable,stable} 3 1! 2c Find work done 10 Apply Work-Energy theorem 5 S Mathematics/Substitution 3 a WzAKzUO—UO/3:2UO/3 2 Q Total 40 25 M PHYS 125 25 October 2005 Exam #2 Problem Steps Nlax Score 321 Free Body Diagrams 10 Two diagrams 1 __ Elevator car Tension drawn correctly 1 % Weight drawn correctly 1 W ‘ No other forces drawn 1 ____ “l" Axes labelled 1 H Counterweight Tension drawn correctly 1 ____ " Weight drawn correctly 1 __ Applied force drawn correctly 1 ____ No other forces drawn 1 __ Axes labelled 1 __~_ 3b Solve for cruising speed 10 Apply kinematics to find 1) 5 __.. Mathematics/Substitution 3 h v = x/QaAz = 2 m/s 1 __ Correct units l __ 1 ‘ 3cm Solve for T“ 10 Apply Newton’s 2’“ Law 2 * Can be applied to individual objects F+mcwg—T=mcwa 1 ____ T ‘ mcarg : mcara 1 ..__ Same tensions 1 __~ Same accelerations 1 5‘ or Can be applied to entire system F + mcwg ‘ mcarg : (mcar + mew) (I. 4 —__. Mathematics/Substitution 2 _ F 2 (mcar _ mcw)g + (mcar + mew) 0. I 1 ~__ 8000 N Correct units 1 _____ 3d Solve for Wp 10 Apply Work-Energy theorem 5 __ Mathematics/Substitution 3 __ Wp : AK — W9 : AK + AU” : l N (mmr — 771cw)gA7 : 1.08 X 105 J Correct units 1 w Total 40 m ...
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