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PHYSICS 160
Spring 2015
NAME________________________
ID_______________
Exam 2: Version A
When you are done, PLACE YOUR SCANTRON SHEET
INSIDE YOUR TEST and put these on the correct color pile at
the front of the room
Useful Equations
! ! ! !
A ⋅ B = A B cosθ AB = Ax B x + A y B y + Az B z
! ! ! !
A × B = A B sin θ AB = ( A y B z − Az B y ) iˆ + ( Az B x − Ax B z ) jˆ + ( Ax B y − A y B x ) kˆ
v = v0 + at
1
x = x0 + v0t + at 2
2
2
2
v = v0 + 2a ( x − x0 )
Newton’s second law
!
!
∑ F = ma
Definition of work
! !
W = ∫ F ⋅ ds
a rad =
v 2 4π 2 r
=
r
T2
Magnitude of kinetic friction
fk = µ k N
Magnitude of static friction
f s ≤ µs N
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_________________
14 questions total = 12 regular + 2 extra credit
All questions are 2 pts each, unless noted otherwise
1. An rock tied to a string is swung in a circle at constant speed. The work done by the
tension force is zero because:
a. the tension is perpendicular to the velocity
b. there is no friction
c. the magnitude of the acceleration is zero
d. the displacement for each revolution is zero
e. the average force for each revolution is zero
2. Three sailboats are blown by the wind
across a calm bay. The boats start from rest
right next to each other and, assuming the
force of the wind is the same on each boat,
rank the boats according to their kinetic energy
at the other side, in order of least to greatest.
(Neglect water resistance).
a) A, B, C
b) B, A, C
c) they are all tied
d) C, then B and A are tied
e) not enough information to tell
3. A 80 kg man is marooned at rest on frictionless ice. In desperation, he hurls his 4 kg
boot to the right with a force of 40 N. As a result of this:
a. the man is stationary but the shoe moves at 10 m/s
b. the shoe has an acceleration twice that of the man
c. the shoe has an acceleration twenty times that of the man
d. the shoe has an acceleration two hundred times that of the man
e. not enough information to tell
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4. Three identical blocks move either on a horizontal surface, up a plane, or down a
plane, as shown below. They all start with the same speed and continue to move until
brought to rest by friction. Rank the three situations according to the heat generated by
friction in the block-floor system, least to greatest.
a. 1,2,3
b. the same for all cases
c. 2,1,3
d. 3,2,1 e. 1, then 2 and 3 tie
5. Three identical blocks, each having mass M, are connected by strings as shown.
Block C is pulled to the right by a force F that causes the entire system to accelerate.
Neglecting friction, the net force acting on block A is:
a. 0
b. F/3
c. F/2
d. -2F/3
e. -F
6. EXTRA CREDIT: For the three blocks above, what is the magnitude of the tension
in the string between block B and block C?
a. 0
b. F/3
c. F/2
d. 2F/3
e. F
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7. A heavy crate is dragged by a force F along a rough surface, as shown in the figure
for two situations. The magnitude of F is the same for the two cases. The frictional
force in (ii), compared with that in (i) is:
a) the same
b) greater
c) less
d) answer depends on the mass
e) answer depends on the magnitude of F
8. A student weighs 700 N. She stands on a bathroom scale in an elevator which is
moving down and decelerating (coming to a stop) at 3.0 m/s2. The scale reading, or the
apparent weight of the student, is:
a. greater than 700 N
b. less than 700 N
c. 700 N
9. A marble moves along the x-axis. The potential-energy function is shown below. At
which of the labeled x-coordinates is the force on the marble zero?
A)
B)
C)
D)
E)
b
b and d
a and c
d
No way to tell.
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10. [5 pts] A book (mass 2.5 kg) is on the outer edge of a merry-go-round that has a
radius of 10 m and revolves once every 20 seconds. What is the minimum value of
coefficient of static friction between the book and the surface for the book to stay on the
merry-go-round?
a. 0.01
b. 0.03
c. 0.04
d. 0.08
e. 0.10
11. [5 pts] A vertical spring (ignore the mass of the spring) has a spring constant of 100
N/m. It is compressed 0.300m. If a 0.500 kg ball is placed on top of the spring and then
released, how high above the original position (the original position is the compressed
position) will the ball fly? (in meters) (Use g = 10 m/s2 and ignore air resistance)
(a) 4.50 m (b) 24.0 m (c) 9.00 m (d) 0.900 m (e) 4.00 m
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12. [5 pts] If air resistance cannot be neglected and the air drag force does -2J of work
on the ball in the previous question, by how much would the maximum height be
reduced? (in meters) (Use g = 10 m/s2)
(a) 0.4 m (b) 0.04 m (c) 4.0 m (d) 0.05 m (e) 5.00 m
13) [5 pts] The figure shows a block of mass m=2 kg held against a spring (spring
constant k=100 N/m) that’s compressed by a distance x=20 cm. The block is released
from rest and slides down a frictionless hill (height h=10 m) to the flat ground below
where, after a short distance, it encounters a rough patch (kinetic friction µk=0.4). After
traveling a distance d it comes to a stop.
The distance d is closest to:
(a) 1 m
(b) 15 m
(c) 25 m
(d) 35 m
(e) 5 m
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14) EXTRA CREDIT: [5 pts] Two blocks are connected by a string and pulley as
shown. Assuming that the string and pulley are massless and that there is no friction,
the magnitude of the acceleration of each block is: (Use g = 10 m/s2)
a. 0.1 m/s2 b. 0.2 m/s2 c. 0.01 m/s2 d. 0.5 m/s2
m/s2
e. 1.0