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Unformatted text preview: EXAM #1
PHYS 125
26 SEPTEMBER 2006 Name: l4 y’le Ho (r ma rd
(please print legibly) Student ID #: ‘10 r {j a 3 Part Score Short answer L W
Problem #1 33 L W i r! 457 '2
3,335 tit!) » , Prfblem #2\@
I ’ _ O Problem #3 Tota Honor Code Exams must be completed W1 1 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 0n the exam is permitted, but is limited
to numerical calculation. Any other uses (unit conversion, formulas, graphing,
symbolic manipulation, 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.” PHYS 125
Exam #1 . 26 September 2006 Short Answer (40 points) Each of the ten questions below is worth 4 points. 1. A car makes a righthand turn as shown below at a constant speed. Draw
the average acceleration vector. If there is no average acceleration, indicate
it with the Zero vector 0. 2. Two students, Omar and Erika, stand on the edge of Herzstein Hall’s
roof to throw their organic chemistry books off. Omar throws his straight
down to the ground, and Erika throws hers straight up with the same
initial speed. In the absence of air resistance, whose book experiences the greatest mag
nitude of acceleration immediately after it is released? (a) Omar’s
(b) Erika’s
( t is the same. 3. A force acts on an object for some amount of time. If the amount of time
is doubled, what happens to the acceleration? (a) It is one~quarter of its original value. F: W: q (b) It is onehalf of its original value. @t remains unchanged. ((1) It is twice its original value. (e) It is quadruple of its original value. PHYS 125 Exam #1 26 September 2006 4. Two objects, A and B, have position vs. time curves as shown below‘ At
what time(s) do the two objects share the same instantaneous velocity? (a) Os
(b) ls
(d) 35 a
(e) 43 (f) 1sand3s
(g) 13,2s,and3s
(h) Osand4s. time (s) . Three identical balls are simultaneously released down each of the three
tracks below. The tracks are identical eXcept for the portion in the middle.
Which ball will reach the end of the track ﬁrst? 1
(a) 1
Coir?” A (b) 2
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i (c) 3
3 (d) 1 and 3 All at the same time. “X
(L a 2 The speed of a wave on a string could conceivably depend on three quan—
Co’rv‘efiﬂ n 31:" it titiesrwthe' mass of the string m, the length of the string L, and the tension
' ‘ ' the string is’T'under m. ’ ' {"1. Use dimensional analysis to ﬁnd a proportionality relationship for the r V \I': fiw‘ speed of a wave on a string using some combination of m, L, and IT}.
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Exam #1 26 September 2006 7. You jump off a table onto the ﬂoor. As you are landing (from the moment
your feet touch the ﬂoor to the moment you stop), how does the normal
force exerted by the level ground compare to your weight? ‘ @lﬁl > w: "
(b) {73! = le
(c) lﬁl < m S‘w 8. Racing cars often have wings on them. These wings are angled sovthe
wind rushing over them creates a downward force (much greater than the
weight of the car). In one sentence, .how does this help keep the car from sliding off the track? r, » a, , , ,l
T h C. ’m on: mfg A It?” (.4: *1 It 3' f 2 51:: 53%” “‘73! "f 0“ {WAC C0"? (Dr—m ma 6Q wiﬁf a N ka n urnwt i if; f“! gar 5? {Re fyﬂfii..<¢£ f 4 a Jr a: ’ajiﬁr'srxwi emf L: 7’: ,gr,‘
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when stopping, regardless of speed. If you double your initial speed7
what happens to the time it takes to stop?
(a) It decreases to a quarter of the original time. 1)
(b) It decreases to half of the original time. U ’3" «gg’di {l (c) It does not change. i )It increases to twice the original time.
(e) It increases to quadruple the original time. What happens to the distance required to stop.7 (a) It decreases to a. quarter of the original distance. p 0
(b) It decreases to half of the original distance. i I r, i i K ._ , (c) It does not change. (d) It increases to twice the original distance. A / K [x 7; /
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x,"\ PHYS 125 Exam #1 26 September 2006 10. Former Rice Owl, Lance Berkman, is playing in right ﬁeld when the ball
comes to him two plays in a row. He is standing in the exact same spot
for both plays and throws the ball with the exact same speed both times.
In the ﬁrst play he throws the ball to home plate following the solid path.
In the second play he throws the ball along the dashed path to ﬁrst base.
Which ball takes the gleast ltime to travel its path'.7 61/
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f: 30M 5 PHYS 125 Exam #1 26 September 2006 Problems (60 points) 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
on the front of the exam. If you do not, you will be penalized 2 points
on the problem. 1. A lead ball is dropped into a lake from a diving board 10.0 m above the
water. After entering the water, it sinks to the bottom with a constant
velocity equal to the velocity with which it hit the water. The ball reaches
the bottom 5.0 s after it is released. How deep is the lake? '7 A, m t k
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Exam #1 26 September 2006 2. 30 points) This problem is continued on the next page. You are
olunteering for the Doctors Without Borders organization and have been
put in charge of delivering medicine to a remote Village. You will drop a
well—cushioned crate of medicine from an airplane that cruises at a constant
altitude of 1.0 km with a speed of 150 m/s. (a) (10 points) In the absence of air resistance, how much time will it
take for the crate of medicine to reach the ground? u 5 v 2/"; ﬁx \ng A‘ t, “(1,: " é a“? ’11 w
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L ,, TL WO C9 ,1 #57 ('{ac'é jrmkdt A. bf (10 points) In the absence of air resistance, at what horizontal dis
placement from the village should you release the medicine? Be sure
to also indicate the direction, i.e. “. . . '1’ m before/ after you pass over
the village.” 7% tr ' g Craxétg ire : f6 55"“ 21 MM w x. \sewé’ w, a l” ‘ PHYS 125
Exam #1 26 September 2006 (c) (10 points) In real life air resistance is not negligible. In the following
analysis, make sure to include drag. i. (6 points) Draw a free body diagram representing the forces on
the crate of medicine at some point shortly after it has been . N i,
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g f y 4‘6 [0X ; m My we ii. (4 points) If air resistance is taken into account, does the time m to impact get shorter7 longer, or remain constant? This is a \_ conceptual, not a numerical, question. I y y r
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Exam #1 26 September 2006 3. (30 points) This winter Houston is blessed with a nice snow and ice storm.
You decide that you want to tow your friend around on a sled. You use a
3—m long rope and pull on it at an angle of 20° above the horizontal. The
coefﬁcients of static and kinetic friction between the sled and snow are
0.15 and 0.10, respectively. You are wearing cleated shoes, so you need
not worry about slipping. (a) (10 points) Treat your friend and her sled as one 70kg object, and
draw a freebody diagram for the object as you are pulling the sled.
Make sure your axes and forces are clearly labeled. (b) (20 points) What is the minimum force needed to get the sled moving
from rest? Equation Sheet PHYS 125 _ Exam #1
Kinematics:
17“) = dig?) Asia = “amen = flit) = dig) A27(t) = Atf§(t)dt 3an = 2A;
5217— I7 Constant Acceleration: A5 = viysAt + %a3(At)2 'Ufys 2 vi; + asAt 2a3As = 113’s — v35
Forces:
(—1. : Fnet : 21‘ ﬁt
m m
~ .. ~ _. ' a ' 1 .. ..
lfsl S #slnl Ifkl = New lfrl = #rlnl [Di 2 ZpairAv2 w = mg
Good Things toKnow:
ME ' Mass of Earth 2 6 x 1024 kg
Ms Mass of Sun 2 2 x 1030 kg
Me Mass of electron 2 9 X 10‘31 kg
Mn Mass of proton/neutron 2 (5/3) X 10'27 kg
06 Circumference of Earth 2 4 x 107 In
rs/e Sun—Earth separation distance 2 1.5 x 1011 m
ge Grav. acceleration at surface of Earth 2 10 m/s2
2
G Gravitational constant 2 g x 10'10 N 1112/kg2
yr 1 Year 2 7r x 107 3
pair Density of air at 20°C 2 1.2 kg/m3
6 Speed of light in a vacuum 2 3 x 108 m/s
vs Speed of sound in 20° C air 2 340 m/s
6 Fundamental unit of charge 2 1.6 x 10‘19 C
1/47r60 Electrostatic constant . 2 9 X 109 N m2/C2
p0 Permeability constant 2 4W x 10‘7 Tm/ A
BE Magnetic ﬁeld near Earth’s surface 2 5 x 10‘5 T
do Approximate nucleon diameter 2 2.4 fm
Binomial expansion (1 + m)" 2 l + 77.03, for cc << 1
1 in = 2.54 cm Circumference of a circle 27rR
9 lbs 2 40 N Area of a circle «R2
4
9 mph 2 4 m/s Volume of a sphere % R3
1 MeV = 1.6 x 10‘13 J Surface area of a sphere 47r R2
1 rad E 0.010 J/kg 1 Ci E 3.7 x 1010 Bq E 3.7 x1010decays/s ...
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 MUTCHLER
 Acceleration, Force, Speed of light

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