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Exam 2 Physics 125 Fall 2007
Time Allowed: 2 hours
(g=10 m/s2 unless stated otherwise, 1 lb= 4.45N)
Name:
PART I (60 points)
Answer the following 15 questions, worth 4 points each
Numerical answers require supporting calculations
The next two questions refer to the motion of a rock projected with speed 10 ft/s at an angle of 40° with the
horizontal. Use g=32 ft/s2.
1. How long does it take to reach maximum height?
(a)
(b)
(c)
(d)
0.1 s
0.2 s
0.3 s
0.4 s
2. What is its speed at maximum height?
(a)
(b)
(c)
(d)
2.4 ft/s
4.5 ft/s
7.7 ft/s
9.2 ft/s
3. Which of the following quantities is a measure of inertia?
(a) velocity
(b) acceleration
(c) force
(d) mass
4. A 800-kg car is accelerated by tractive force on the tires. Starting from rest, it reaches a velocity of 30 m/s
in a distance of 100 m. Neglecting air resistance, how large is the tractive force, assumed constant?
(a)
(b)
(c)
(d)
3600 N
2400 N
1200 N
800 N
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5. Referring to the last question, calculate the force experienced by a 120 lb driver from the back of the seat,
if the friction from the seat can be ignored.
(a) 360 N
(b) 240 N
(c) 120 N
(d) 80 N
6. The graph below is a plot of the net force on an object against its acceleration along a straight line. What
is the mass of the object?
Ans. Mass = ____________________
F (N)
200
150
100
50
0
2.0
4.0
6.0
a (m/s2)
7. If you blow up a balloon, and then release it, the balloon will fly away. This is an illustration of
(a)
(b)
(c)
(d)
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
Galileo’s Law of Inertia
8. A astronaut travels to Mars, where the acceleration due to gravity is about 40% that of the Earth. The
astronaut weighs 140 lb on Earth. Her weight and mass on Mars are
(a)
(b)
(c)
(d)
Weight = 140 lb, mass = 56 kg
Weight = 140 lb, mass = 62 kg
Weight = 56 lb, mass = 62 kg
Weight = 56 lb, mass = 25 kg
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9. A 40-kg suitcase on a rough floor is dragged by a horizontal force of 50N. It is observed to accelerate at
0.5 m/s2. How large is the friction force?
(a)
(b)
(c)
(d)
10 N
20 N
30 N
40 N
10. Referring to the previous problem, if the drag force remains at 50N, but is at an angle of 30º with the
horizontal, how large is the normal force on the suitcase?
(a)
(b)
(c)
(d)
400 N
375 N
357 N
325 N
11. Refer to the diagram below showing three blocks A, B, and C on a frictionless table connected by two
massless strings. The block C is pulled by an unknown force. The blocks move together with acceleration
4 m/s2. The masses of blocks A, B and C are 3 kg , 2 kg and 1 kg respectively. Determine the tension in
the string connecting blocks B and C.
A
(a)
(b)
(c)
(d)
B
=
B
C
12 N
24 N
20 N
16 N
12. The diagram below shows a block A on a smooth table connected via a massless string to a hanging block
B. The weights of A and B are 50N and 20N respectively. Find the tension in the string.
A
B
(a)
(b)
(c)
(d)
46 N
35 N
29 N
14 N
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13. If the table in the previous question has a rough surface with a coefficients of kinetic friction of 0.3 and a
coefficient of static friction of 0.5, will the hanging block falls when released from a position at rest? Give
reason to support your conclusion.
(a) Yes
(b) No
14. When an object falls in the atmosphere, it eventually reaches a constant velocity called the terminal
velocity because of air resistance. If a brick and a feather fall to the earth at their terminal velocity, which
one experiences the greater force of air resistance?
(a)
(b)
(c)
(d)
The feather
The brick
Neither, both experience the same amount of air resistance
It cannot be determined because there is not enough information given.
15. A truck weighing 2000 lb is being pushed up an inclined plane that makes 10° with the horizontal.
Neglecting friction, what is the minimum force to move the truck?
(a)
(b)
(c)
(d)
2000 lb
1970 lb
582 lb
347 lb
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PART II (40 points)
16. (a) An astronaut on an unknown planet climbs to the top of a 50 m cliff and releases a rock from rest,
finding that it takes 5 s for the rock to land. What is the value of the acceleration due to gravity on the planet? (3
points)
(b) He then fires a rocket straight up with a speed of 20 m/s. How long does it take for the rocket to return to the
launch point? (2points)
(c) If a rocket with twice the mass of the one used in (b) is used, but the initial speed is the same, what is the
return time to the launch point? (2 points)
(d) If the rocket in (b), after passing by the launch point, goes straight down to the bottom of the cliff, how
long will it take? ( 3 points)
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17. (a) (5 points) A coffee cup of mass 0.5 kg sits on a table in a train. It is found that the smallest push force
to move the cup on the table is 1.5 N. What is the coefficient of static friction?
(b) (5 points) What is the largest forward acceleration of the train without causing the cup to slip?
(c) (5 points) Determine whether the cup slips when the train is traveling with constant velocity up a 15°
hill, and find the friction force.
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18. A boy stands on a scale in a moving elevator. His mass is 50.0 kg, and the mass of the elevator is 200
kg. The elevator is suspended from a cable, and descends with a slowing down rate of 3.0 m/s2.
(a) ( 5 points) Draw a free body diagram for the combined mass of the elevator and the boy and indicate the
direction of the acceleration vector on the diagram. Determine the tension in the cable.
(b) ( 5 points) Draw a free body diagram for the boy alone. Determine how many Newtons the scale read
during the descent?
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