# Practice exam 2A answers (1) ```1 of 14
Exam 2 – Version A
Physics 2010
Name
Student ID #
Circle the Day and Start Time of your recitation/lab
Section
Tues
Wed
8am 10am
Thu
12pm 2pm
Please do not open the exam until you are told to do so. Thanks!
This exam consists of 25 multiple choice questions, each worth the same amount.
Your exam should have 14 pages (7 pieces of paper). The last 3 sides are blank, which you may use
for scratch work. There is also room for work after each question.
Check each box as you complete the instructions below:
Print and bubble in your name and student ID number on the bubble sheet. (Carefully, please!)
VERY IMPORTANT: Bubble in your Exam version, (this one is version “A”), in the LOWER LEFT of
your bubble sheet in the area marked “TEST VERSION”
As you take the exam, mark the correct answer on the bubble sheet. You can circle your
answers on the paper copy of the exam for your future reference. But, the bubble sheet always
takes precedence over anything written on your exam
Relax! A TA will announce when you can start.
“On my honor, as a University of Colorado at Boulder student,
I have neither given nor received unauthorized assistance on
this work”
Signature ____________________________________________
Version A
2 of 14
Average velocity:
v =
x
t
Average acceleration:
a=
v
t
Equations for motion with constant acceleration:
1
x = x0 + v0 Dt + aDt 2
2
v 2 = v0 2 + 2a(x - x0 )
v + v0
v=
2
x = x0 + vDt
Magnitude of acceleration from gravity at the Earth’s surface: g = +9.81 m/s2
(by convention, the symbol g is ALWAYS positive!)
π ~ 3.14
sin() = opposite/hypotenuse
√2 ~ 1.41, √3 ~ 1.73
Newton’s 1st law: An object travels with constant velocity unless acted on by a net force.
Newton’s 2nd law: 𝐅𝑛𝑒𝑡 = ∑ 𝐅𝑖 = 𝑚𝐚
Newton’s 3rd law: 𝐅𝐴 𝑜𝑛 𝐵 = −𝐅𝐵 𝑜𝑛 𝐴
2
Uniform circular motion, with radius 𝑟 and speed 𝑣, has acceleration 𝐚 = 𝑣 ⁄𝑟
pointing towards the center of the circle (centripetal).
Newton’s universal law of gravity between masses 𝑚1 and 𝑚2 , separated by 𝑟12 :
𝐺𝑚1 𝑚2
𝐹𝑔𝑟𝑎𝑣𝑖𝑡𝑦 =
⁄𝑟 2 with 𝐺 = 6.67 x 10-11 N m2/kg2
12
Kinetic friction is given by 𝐹𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 𝜇𝐾 𝑁, where 𝑁 is the normal force magnitude
The maximum static friction is given by 𝐹𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 𝜇𝑆 𝑁
Work 𝑊 by constant force 𝐅 over displacement ∆𝐫 = (𝐹)(∆𝑟) cos 𝜃, where 𝜃 is the angle
between the force and displacement vectors.
Kinetic Energy 𝐾𝐸 of mass 𝑚 with speed 𝑣 equals 12𝑚𝑣 2
Work-kinetic energy theorem: 𝑊𝑛𝑒𝑡 = ∆𝐾𝐸
Version A
3 of 14
1. A heavy truck of mass M collides with a small sports car of mass m on an icy (frictionless) flat
road. What is the ratio of the acceleration of the heavy truck to that of the sports car during the
collision?
A)
B)
C)
D)
E)
M/m
m/M
1
M2/m2
m2/M2
2. A student slides her 50.0-kg desk across the level floor of her dormitory room a distance 5.00 m
at constant speed. If the coefficient of kinetic friction between the desk and the floor is 0.408.
How much work did the student do?
A)
B)
C)
D)
E)
1J
1 kJ
10 J
10 kJ
100 J
3. Two equal mass asteroids, labeled 1 and 2, are traveling through outer space. The kinetic
energy of asteroid 1 is 9 times the kinetic energy of asteroid 2. What is the ratio of the speed of
𝑣
asteroid 1 to asteroid 2, 1⁄𝑣2 ?
A)
B)
C)
D)
E)
0.111
0.333
1.000
3.000
9.000
4. Which of the following statements are always true?
1) A truck initially moving at a constant velocity will slow down unless a small net force is applied.
2) If a net force acts on an object, the object's velocity will change.
3) If a net force acts on an object, the object's speed will change.
4) If two objects have the same acceleration, they are under the influence of equal forces.
A)
B)
C)
D)
E)
1) and 2) are always true
1), 2) and 3) are always true
1), 2) and 4) are always true
only 2) is always true
only 4) is always true
Version A
4 of 14
5. A 35.0 kg block located on a horizontal floor with friction is pulled by a cord that exerts a force
F=24.0 N at an angle q =30◦ above the horizontal, as shown. What is the work done by the cord
on the block as it accelerates over a displacement of 2.00 m to the right?
A)
B)
C)
D)
E)
24.0 J
37.2 J
41.6 J
48.0 J
Not enough information given
Displacement
6. Which one of the following free-body diagrams best represents the free-body diagram, with
approximately correct relative force magnitudes, of a person in an elevator that is traveling
upward but is gradually slowing down at a rate of 3 m/s2? ⃗⃗⃗
𝐹𝑓 is the normal force of the floor on
the person and ⃗⃗⃗
𝐹𝑔 is the force of gravity on the person. If two forces appear equal length in the
diagram, assume they are equal.
7. A ball of mass M = 1.0 kg is pulled upwards by a rope. If the acceleration of the ball is a = 1.0
m/s2 upward, the tension in the rope must be closest to
A)
B)
C)
D)
E)
Version A
1N
2N
5N
10 N
11 N
5 of 14
8. If an object travels at a constant speed in a circular path, the acceleration of the object is
A)
B)
C)
D)
E)
larger in magnitude the smaller the radius of the circle.
in the same direction as the velocity of the object.
smaller in magnitude the smaller the radius of the circle.
in the opposite direction of the velocity of the object.
zero.
The following three questions refer to this
situation: Two blocks are in contact on a table.
A horizontal force F =5 N is applied to M2, as
shown. The masses are M1=1 kg, M2=2 kg.
9. What is the magnitude of the acceleration of the blocks if the friction between the blocks and
table can be neglected?
A)
B)
C)
D)
E)
0.83 m/s2
1.0 m/s2
1.67 m/s2
2.5 m/s2
5.0 m/s2
10. What is the magnitude of the contact force between the two blocks if the friction between the
blocks and table can be neglected?
A)
B)
C)
D)
E)
Version A
0N
1.0 N
1.67 N
3.34 N
5.0 N
6 of 14
11. Which diagram below best represents the free-body diagram of the left block M2 when the
blocks accelerate across the table to the right with friction between the block M2 and the table
surface? 𝐅𝐟𝐫 denotes the force of friction, 𝐅𝟐𝟏 the force block M1 exerts on M2, N is the normal
force and W is the weight due to gravity. The force vectors are approximately to scale.
12. A 4.5 kg block located on a horizontal frictionless floor is pulled by a cord that exerts a tension
force F = 12.60 N at an angle q =30◦ above the horizontal, as shown. What is the speed of the
block 4.0 seconds after it starts moving?
A)
B)
C)
D)
E)
Version A
2.8 m/s
4.35 m/s
5.6 m/s
9.7 m/s
11.2 m/s
7 of 14
13. A frictionless pulley with zero mass is attached to the ceiling. The mass
M2 = 0.250 kg is observed to accelerate downward with a = 5.00 m/s2 .
To two significant digits, what is the mass m1?
A)
B)
C)
D)
E)
0.012 kg
0.041 kg
0.05 kg
0. 081 kg
E) 0.16 kg
14. A baseball of mass m is hit by a bat and travels upwards. Its initial speed is 𝑣0 and it leaves the
bat at an angle of 60 above the horizontal direction. It reaches height h above its starting
position before starting to come back down. Which formula below gives the work done by
gravity on the baseball between the time it leaves the bat and the top of the motion at the
height h?
A)
B)
C)
D)
E)
+𝑚𝑣02 /2
−𝑚𝑣02 /2
−𝑚𝑔ℎ
+𝑚𝑔ℎ
Depends on whether we can ignore air resistance.
15. A person is standing in an elevator that is moving
upward with an increasing speed. How does the
magnitude of the normal force N on the person's
feet compare to the magnitude of the person's
weight mg?
A) N &gt; mg
B) N &lt; mg
D) impossible to tell
Version A
C) N = mg
moving up
|v| increasing
8 of 14
16. A ball of mass M = 0.30 kg on the end of a string
revolves at a uniform rate in a vertical circle of radius
75.4 cm as shown in the figure. If the speed of the
ball is 5.0 m/s, what is the tension in the string when
the ball is at the top of its path?
A)
B)
C)
D)
E)
7N
10 N
13 N
23.3 N
43. N
17. An object moves in a circle of radius R at constant speed; the period of the motion with this
speed is T. If you want to change only the period but not the radius, in order to reduce the
object's acceleration a by 2 (change to a/2), the new period should be
A) T/4.
B) T/2
C) T / 2
D) 2T
E) 4T.
18. An astronaut weighs 800 N on Earth. What is her weight on planet X, a planet which has radius
RX =REarth/2 and mass MX= MEarth/8?
A)
B)
C)
D)
E)
Version A
50 N
100 N
200 N
300 N
400 N
9 of 14
19. A 6.5 kg crate slides down a frictionless ramp with a 60 incline (relative to horizontal). What is
the magnitude of the acceleration of the crate while it is sliding down the ramp?
A)
B)
C)
D)
E)
1.3 m/s2
4.9 m/s2
5.6 m/s2
7.2 m/s2
8.5 m/s2
20. A box of weight 20 N is resting on a board which is initially horizontal; the surface of the board
is rough, so there is friction. The coefficient of kinetic friction k = 0.1 and the coefficient of
static friction s = 0.4. One end of the board is slowly raised upward; let  describe the angle
between the board and the horizontal. Pick the answer below which is closest to the angle at
which the box just begins to slide down the ramp.
A) 5.7
B) 11
C) 22
D) 68
E) 83

21. The moon (mass 7.3 x 1022 kg) orbits the earth (mass 6.0 x 1024 kg) every 27.3 days in a circular
orbit of radius 3.84 x105 km. NASA launches a small satellite that has an orbit around the earth
of the same radius as the moon. The mass of the satellite is 3.2 x 103 kg. How does the
acceleration of the satellite compare to the acceleration of the moon? (You can ignore the
gravitational attraction between the satellite and the moon.)
A) The satellite’s acceleration is less than the moon’s acceleration.
B) The satellite’s acceleration is equal to the moon’s acceleration.
C) The satellite’s acceleration is greater that the moon’s acceleration.
D) Not enough information given.
Version A
10 of 14
22. A heavy mass of 100 kg is attached by an ideal string to a hanging 0.001 kg mass using a
frictionless pulley. The heavy mass is resting on a frictionless horizontal table. Which of the
following will occur?
A) The masses will remain at rest.
B) The heavy mass will move towards the pulley with constant velocity
C) The heavy mass will accelerate towards the pulley.
23. A 0.1 kg box on a rotating flat turntable moves around the center of the turntable with a speed
v = 0.05 m/s at a radius of 10 cm without sliding on the turntable surface. The coefficient of
static friction is 0.5 and the coefficient of kinetic friction is 0.25. What is the magnitude of the
friction force on the box?
A)
B)
C)
D)
E)
Version A
0
1.25 x 10-3 N
2.50 x 10-3 N
0.245 N
0.491 N
11 of 14
24. Two blocks connected by an ideal rope are located on a frictionless surface. They are
accelerating to the right with a pull from force F (by a second ideal rope). How does the
magnitude of the tension T in the rope between the blocks compare to the magnitude of the
pulling force F?
T
A)
B)
C)
D)
F
T=F
T&lt;F
T&gt;F
Which is greater depends on the actual masses of the two blocks.
25. A planet in elliptical orbit around a star moves from the point in its orbit furthest from the star
(A) to the closest point (P) and then back to A. Where on the orbit is the speed of the planet
decreasing?
P
A)
B)
C)
D)
From A to P
From P to A
Nowhere on the orbit (speed is constant).
Everywhere on the orbit
Version A
A
12 of 14
Version A
13 of 14
Scratch Page
Version A
14 of 14
Version A
```