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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 pingpong 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 pingpong 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 headon 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
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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“
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a; (f: \V I http : //www . howstuffworks . com/enginel . htm Y \1 f, , V’ . A 10kg 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
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(d)1000N ‘_ V’ rte/Y:
® Need more information Q i
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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‘ ....
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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, )\
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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
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“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 1200kg 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 freebody 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. PingPong 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
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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 WorkEnergy 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 WorkEnergy 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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 Spring '08
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