PHYS 211 – MT3 Fall 2012 Sample 1

PHYS 211 – MT3
Fall 2012
Sample 1
1. A stationary astronaut sees a small asteroid collide into a large planet which was
previously stationary in the astronaut's frame of reference. The asteroid is embedded
deep within the planet after the collision. Neglect gravity and assume that no debris
leaves the planet after the collision.
Which of the following is true of the asteroid + planet system after the collision
(according to the astronaut's measurements)?
A.
B.
C.
D.
E.
the momentum of the system is definitely reduced.
the momentum of the system may be reduced or may stay the same.
the kinetic energy of the system stays the same.
the center of mass of the system is stationary.
the center of mass of the system moves at the same velocity after the collision as
it did before.
F. None of the above
2. Each of the following true-false questions is worth 2 points (so these 5 questions
together are worth 2 of the multiple choice questions, 10 points)
A Physics 211 student is late for a lab activity and runs up the stairs to the third floor of
Osmond Lab.
A. T or F: The power expended by the student is greater than if he had leisurely
walked up the stairs.
B. T or F: The work done on the student by gravity during the ascent is positive.
C. T or F: The change in kinetic energy of the student from the ground to the third
floor has the same magnitude as the work done by gravity.
D. T or F: The sum of the kinetic plus potential energies of the student is not
necessarily conserved here.
E. T or F: If the student runs up the stairs at constant speed, the power expended
against gravity is the magnitude of the student's weight times the magnitude of
the vertical component of his velocity.
3. A baseball is hit from home plate and is caught by an eager fan sitting high up in the
upper deck, 35 meters above the playing field. Considering both the effects of gravity
and air resistance, over its entire flight the work done by gravity and the work done by
air resistance on the ball, respectively, are:
A.
B.
C.
D.
E.
Negative, Negative
Positive, Negative
Negative, Positive
Positive, Positive
Cannot say from the given information.
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PHYS 211 – MT3
Fall 2012
Sample 1
4. The total work required to accelerate a car from 0 to 30 mph compared to that
required to accelerate from 30 to 60 mph is
A.
B.
C.
D.
E.
larger
smaller
equal
dependent on the time interval and the effect of the acceleration.
dependent on the force used to accelerate the car.
5. Two blocks are initially at rest on a frictionless horizontal surface. The mass m A of
block A is less than the mass mB of block B. You apply the same constant force F and
pull the blocks through the same distance d along a straight line (constant force F is
applied for the entire distance d).
Which one of the following statements correctly compares the kinetic energies of the
blocks after you pull them the same distance d?
A. The kinetic energies of both blocks are identical.
B. The kinetic energy is greater for the smaller mass block because it achieves a
larger speed.
C. The kinetic energy is greater for the larger mass block because of its larger
mass.
D. Not enough information, need to know the actual mass of both blocks to compare
the kinetic energies.
E. Not enough information, need to know the actual magnitude of force F to
compare the kinetic energies.
6. Two boxes, A and B, slide along a frictionless floor with equal magnitude momenta
(pA = pB). If the mass of A is twice the mass of B (mA = 2 mB), how do their kinetic
energies compare?
A.
B.
C.
D.
E.
KA = 4 K B
KA = KB / 4
KA = KB
KA = KB / 2
KA = 2 K B
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PHYS 211 – MT3
Fall 2012
Sample 1
7. A hawk and a quail (mhawk > mquail) are moving directly toward each other. The birds
collide and stick together (via the hawk's talons). Which of the following is true
concerning this collision?
A. The total kinetic energy of the two birds after the collision is the same as before.
B. The center of mass of the two birds moves with the same velocity after the
collision as before.
C. The total linear momentum of the two birds is reduced in magnitude by the
collision.
D. The magnitude of the impulse on the quail is larger than that on the hawk.
E. The magnitude of the impulse on the hawk is larger than that on the quail.
8. A 5 kg particle has potential energy U(x) as shown in the figure below. When the
particle is at x = 10.0 m, its velocity is 2m/s. Between what limits of x does the particle
move?
A.
B.
C.
D.
E.
F.
G.
xmin = -, xmax = +
xmin = 0 m, xmax = 20 m
xmin = 5 m, xmax = 15 m
xmin = 6.25 m, xmax = 13.75 m
xmin = 7.5 m, xmax = 12.5 m
xmin = 8.75 m, xmax = 11.25 m
The particle remains at xmin = xmax = 10 m
9. A force F  (4.0iˆ  3.0 ˆj ) N acts on a particle as it moves 5.0 m in the +x direction.
How much work is done by the force?
A.
B.
C.
D.
E.
5.0 J
15 J
20 J
25 J
None of the above
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PHYS 211 – MT3
Fall 2012
Sample 1
10. Your friends Howard and Raj are trying to solve the following problem: "A block of
mass M is against a compressed spring (of known spring constant k) that is
compressed by an amount x. The spring releases launching M to the right (along a
frictionless horizontal surface), where it collides with a second block of mass m. The two
blocks stick together and move off together. They reach a very long frictionless ramp
that slopes upwards. How far up the ramp do the blocks reach before stopping?".
Howard and Raj have set up the problem in three steps, as shown below and asked for
your comments ("bc" means "before collision" and "ac" means "after collision"). What
would be the correct (in terms of physics) thing to say?
A.
B.
C.
D.
E.
I'd rethink step 1.
I'd rethink step 2.
I'd rethink step 3.
Looks good!
Looks good, but you can skip step 2 and just set the initial elastic potential
energy = final gravitational potential energy.
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PHYS 211 – MT3
Fall 2012
Sample 1
11. The following 5 “matching” questions are all based on the following set-up and
are each worth two points (so these 5 questions together are worth 2 of the other
multiple choice questions, 10 points)
Carts A and B are identical in all respects before a collision.
Figure (i): Cart A starts from rest on a hill at a height h above the ground. It rolls down
and collides “head-on" with cart B which is initially at rest on the ground. The two carts
stick together.
Figure (ii): Carts A and B are at rest on opposite hills at heights h/2 above the ground.
They roll down, collide “head-on" with each other on the ground and stick together.
For each of the below questions, choose either
(1)
Case i
(2)
Case ii
(3)
Case i and case ii are the same
(4)
Neither of the cases
Just before the collision on the ground,
(a) In which system (two cars together) is the kinetic energy greater?
(b) In which system is the momentum greater?
(c) In which system is the momentum zero?
Just after the collision, on the ground,
(d) In which system is the kinetic energy greater?
(e) In which system is the momentum greater?
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PHYS 211 – MT3
Fall 2012
Sample 1
12. A frog sits on the front of a log in still water. Initially, neither the log nor the frog are
moving. He then jumps to the back of the log and sits there. Afterwards the log:
A.
B.
C.
D.
E.
is forward of its original position and moving forward.
is forward of its original position and moving backward.
is forward of its original position and not moving.
is rearward of its original position and moving backward.
is rearward of its original position and not moving.
13. A 1.0 kg ball moving at 2.0 m/s towards a wall bounces off the wall with a velocity of
1.5 m/s directly away from the wall. The impulse on the ball from the wall is:
A.
B.
C.
D.
E.
F.
0.5 kg·m/s away from wall
0.5 kg·m/s toward wall
0.0 kg·m/s
3.5 kg·m/s toward wall
3.5 kg·m/s away from wall
None of the above
14. A force of 10 N holds an ideal spring with a 20 N/m spring constant in compression.
How much work does the spring do if the force is very slowly doubled?
A.
B.
C.
D.
E.
F.
G.
H.
-10 J
-5 J
-2.5 J
0J
2.5 J
5J
10 J
None of the above
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PHYS 211 – MT3
Fall 2012
Sample 1
15. A student is standing on a frictionless sheet of ice when a small penguin slides into
him and bounces off in an elastic collision as shown in the figure (arrows indicate
direction of motion, not magnitude of velocity). The student's mass is 100 times larger
than that of the penguin. After the collision, the magnitude of the penguin's momentum
is:
A.
B.
C.
D.
E.
many times larger than the magnitude of the student's momentum.
many times smaller than the magnitude of the student's momentum.
about equal to the magnitude of the student's momentum.
about twice as large as the magnitude of the student's momentum.
about one half as large as the magnitude of the student's momentum
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PHYS 211 – MT3
Fall 2012
Sample 1
16. Spiderman swings around a construction crane, with his spider thread stretched taut
at a length R. He makes a full vertical rotation around the crane, but at the top barely
loops the loop at speed Vt, i.e., the tension in his thread is momentarily zero there. How
fast is Spiderman going at the bottom of the circular swing around the crane? Ignore
any effects of air resistance.
A. Vb  gR
B. Vb  2 gR
C. Vb  3gR
D. Vb  2 gR
E. Vb  5gR
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PHYS 211 – MT3
Fall 2012
Sample 1
17. Two masses are connected as shown in the figure. The system is released from rest
and mass A falls to the floor through a distance of 1.5 m (mass B remains on the table).
The table and pulley are frictionless. What is the kinetic energy of mass A just as it
reaches the floor? (Use g = 10 m/s2)
1.5 m
A.
B.
C.
D.
E.
5.0 J
6J
25 J
30 J
180 J
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PHYS 211 – MT3
Fall 2012
Sample 1
18. A bullet of mass m and unknown speed passes completely through a pendulum bob
of mass M, emerging on the other side. The pendulum bob is suspended by a string of
length l and negligible mass. Assuming that the pendulum bob just barely makes it
around a complete vertical circle, what average force did the bullet apply to the bob
during the (very short) time t it took to push through the bob?
M
gl
A. F 
t
M
2 gl
B. F 
t
M
3gl
C. F 
t
2M
gl
D. F 
t
M
5gl
E. F 
t
F. None of the above
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