Exam #3
Physics I
Spring 1999
Name__________________________
Section #
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Section
Value
MW 8-10 (Adams)
(problem)
MW 8-10 (Stoler)
Part A-MC
28
MW 10-12 (Sperber)
MW 12-2 (Stoler)
Part B
17
MW 2-4 (Hayes)
Part C (1)
30
MW 4-6 (Cummings)
MW 4-6 (Kendall)
Part C (2)
25
TTh 8-10 (Adams)
100
TTh 10-12 (Sperber) Total
TTh 12-2 (Roberge)
TTh 2-4 (Cummings)
TTh 4-6 (Kendall)
MW 12-2 (Casabella's laptop section)
Score
If you cheat on this exam, you are in big trouble!
GOOD LUCK!!!
Preliminary Grades will be posted outside of the physics
department office (frist floor of the science center) on
Wednesday after 6 pm.
Part A-Multiple Choice. 4 points each. Choose the best answer and write it on the
line to the left of the question number.
________ 1.
The mass of a hypothetical planet is 1/100 that of Earth and its radius is 1/4 that
of Earth. If a person weighs 150 lb on Earth, what would he weigh on this planet? A) 150 lb
B) 12 lb C) 48 lb D) 6 lb E) 24 lb
________ 2.
When a hard rubber rod is given a negative charge by rubbing it with wool: A)
positive charges are transferred from rod to wool B) negative charges are transferred from rod
to wool C) positive charges are transferred from wool to rod D) negative charges are
transferred from wool to rod E) negative charges are created and stored on the rod
__________3. Points R and T are each a distance d from each of two equal and opposite charges as
shown. If k = 1/4o, the work required to move a negative charge q from R to T is:
A)
B)
C)
D)
E)
zero
kqQ/d2
kqQ/d
kqQ/(2d)
kQq/(2d)
_______4. Positive charge +Q is uniformly distributed on the upper half a semicircular rod and
negative charge -Q is uniformly distributed on the lower half. What is the direction of the
electric field at point P, the center of the semicircle?
A)  B)  C)  D)  E) 
_________5. An electron and a proton each travel with equal speeds around circular orbits in the
same uniform magnetic field, as shown in the diagram. The field is into the page on the
diagram. Because the electron is less massive than the proton and because the electron is
negatively charged and the proton is positively charged:
A) the electron travels clockwise around the smaller circle and the proton travels
counterclockwise around the larger circle. B) the electron travels counterclockwise around the
smaller circle and the proton travels clockwise around the larger circle C) the electron travels
clockwise around the larger circle and the proton travels counterclockwise around the smaller
circle D) the electron travels counterclockwise around the larger circle and the proton travels
clockwise around the smaller circle E) the electron travels counterclockwise around the
smaller circle and the proton travels counterclockwise around the larger circle
_______6.
The earth exerts a gravitational force on the Moon, keeping it in its orbit. The reaction
to this force, in the sense of Newton's third law, is: A) the centripetal force on the Moon B)
the nearly circular orbit of the Moon C) the gravitational force exerted on the Earth by the
Moon D) the tides due to the Moon E) the apple hitting Newton on the head
________7. An electron moves from point i to point f, in the direction of a uniform electric field.
During this placement:
A) the work done by the field is positive and the potential energy of the electron-field system
increases B) the work done by the field is negative and the potential energy of the electronfield system increases C) the work done by the field is positive and the potential energy of the
electron-field system decreases D) the work done by the field is negative and the potential
energy of the electron-field system decreases E) the work done by the field is positive and the
potential energy of the electron-field system does not change.
Part B-Short Answer
1. The torque acting on an object is given by  = 2t2 + 4 What is the change in angular momentum
between time t = 2 seconds and t = 4 seconds? (4 points) Hint: Recall F=dp/dt
2. An electron moves to the left with a velocity v of 106 m/s in an electric field E of
10 N/C that is directed up toward the top of the page.
a) What is the direction of the electric force on this electron? (3 points)
Circle only ONE
Right
Left
Into the Page
Out of the Page
Up (toward the top of the page)
Down (toward the bottom of the page)
b) If an appropriate magnetic field is present, we can cancel out this electric force with a magnet force.
What direction must the magnetic force point? (2 points)
Circle only ONE
Right
Left
Into the Page
Out of the Page
Up (toward the top of the page)
Down (toward the bottom of the page)
c) What direction must a magnetic field point to produce such a force on the electron? (4 points)
Circle only ONE
Right
Left
Into the Page
Out of the Page
Up (toward the top of the page)
Down (toward the bottom of the page)
d) What must the magnitude of the magnetic field be for electron to pass through the fields without being
deflected? (4 points) Show all work to receive credit for your answer.
_________
(unit)
Part C-SHOW ALL YOUR WORK TO RECEIVE CREDIT FOR YOUR ANSWER!!
1. (30 points total) Consider two small balls attached to a horizontal rod that is so lightweight
that we can ignore its mass. Each ball has a mass of 0.3 kg and a radius of 1 cm. The two balls
and the rod are free to rotate as a unit about a vertical axis that is perpendicular to the rod. See
the figure below which is a view looking down on the object. The center of the ball on the right is
1 meter from the vertical axis and the center of the ball on the left is 3 meters from the vertical
axis.
3m
1m
a) What is the rotational inertia I of the object (the two balls and the rod) about the vertical axis? (3
points)
_________
(unit)
A horizontal force of 3 N makes an angle of 60o with the rod and is applied to the center of the
ball on the right. A second horizontal force of 5 N makes an angle of 40o with the rod and is
applied to the center of the ball on the left.
40o
60o
5N
3N
b. What is the magnitude of the torque 1 produced by the 5 N force? (2 points)
_________
(unit)
c. What is the direction of the torque 1 produced by the 5 N force? Circle only ONE
Right
Left
Into the Page
Out of the Page
Up (toward the top of the page)
Down (toward the bottom of the page)
(2 points)
d. What is the magnitude of the torque 2 produced by the 3 N force? (2 points)
_________
(unit)
e. What is the direction of the torque 2 produced by the 3 N force? Circle only ONE
Right
Left
Into the Page
Out of the Page
Up (toward the top of the page)
Down (toward the bottom of the page)
(2 points)
f. What is the magnitude of the angular acceleration  of the object (the two balls and the rod)? (5
points)
_________
(unit)
g. The object is at rest before the two torques are applied. What direction will the object rotate?
Circle only ONE (2 points)
Clockwise
Counterclockwise
h. After some time, the angular velocity  of the object is 5 rad/sec. What is the magnitude of the
angular momentum L of the object? (4 points)
_________
(unit)
i. A 300-gram wad of clay is now thrown at a speed of 5 m/s. The clay strikes the rotating object,
perpendicular to the horizontal rod connecting the balls, 2 meters to the right of the vertical axis of
rotation. The clay sticks to the rod. What is the magnitude of the angular velocity  of the object
now? (8 points)
_________
(unit)
2.
(25 points total )Consider the two charges shown in the figure below.
Hint: C=10-6 C
y
+ 6 C
60o
3.00 m
5.00 m
+2 C
4.00 m
x
a. What is the x component of the net force on the 6 C charge at y = 3m? (6 points)
_________
(unit)
b. What is the x component of the E field at y = 0, x = 0 m? (4 points)
_________
(unit)
c. What is the electric potential energy of the two charge configuration? (6 points)
_________
(unit)
d. What is the electric potential at y =0 , x=0 due to the two charges? (4 points)
_________
(unit)
-6
e. Assume that the 6 C charge has a mass of 10 kg. At t=0, this charge is released from rest and
moves out to infinity. What is the speed of this particle at infinity? (5 points)
_________
(unit)