Exam #2
Physics I
Spring 1999
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need the following information:
Name _______________________________
Section #
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Question
Part A (MC)
MW 8-10 (Adams)
MW 8-10 (Stoler)
Part B - #1
MW 10-12 (Sperber)
Part B - #2
MW 12-2 (Stoler)
MW 2-4 (Hayes)
Part C - #1
MW 4-6 (Cummings)
MW 4-6 (Kendall)
Part C - #2
TTh 8-10 (Adams)
TTh 10-12 (Sperber)
TTh 12-2 (Roberge)
TTh 2-4 (Cummings)
TTh 4-6 (Kendall)
MW 12-2 (Casabella's laptop section)
Value
Total:
No collaborative group work (cheating) allowed on this exam.
GOOD LUCK!!!
Score
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. Two ice hockey pucks collide on a frictionless surface. In considering
conservation of momentum of the two-puck system, we would break the total momentum
into x and y components and apply conservation rules in each direction separately. If we
also use conservation of kinetic energy,
A) We also would need to break KE into x and y components and apply conservation of
kinetic energy in each direction separately.
B) We would not need to break KE into x and y components because we do this for
momentum.
C) We would not need to break KE into x and y components because KE is a vector.
D) We would not need to break KE into x and y components because KE has no
direction associated with it.
E) None of the above
________2. A cart moves up an inclined plane without any friction. Consider the
gravitational potential energy of cart, the kinetic energy of the cart and the total
mechanical energy of the cart. Which of these types of energy are conserved.
A) potential energy only
B) kinetic energy only
C) potential energy as well as kinetic energy
D) potential energy as well as total mechanical energy
E) all three are conserved
F) None of the above
________3. Two objects start from rest and are acted on by equal forces for equal times.
Object A has twice the mass of Object B. Just before the force is removed, which of the
following is correct?
A) The velocity of A is equal to the velocity of B.
B) The acceleration, velocity and momentum of A is greater than the acceleration of B.
C) The acceleration, velocity and momentum of A is less than that of B.
D) The acceleration and velocity of A is less than that of B, but the momentum of A is
equal to that of B.
E) None of the above
________4. The center of mass of a system of particles is moving with a constant
velocity in the x direction and a constant velocity in the y direction. Then an explosion
occurs. During the explosion, the sum of all external forces in the x direction is zero but
the sum of all external forces in the y direction is not zero. Which of the following
statements regarding the cm after the explosion is correct?
A) The velocity of the cm in the x direction is changing and the velocity of the cm in the
y direction is not changing.
B) The velocity of the cm in the x direction is zero and velocity of the cm in y direction
is changing
C) The velocity of the cm in the x direction is constant and velocity of the cm in y
direction is changing
D) The velocity of the cm is changing in both the x and y directions.
E) None of the above
________5. As a particle moves along the x axis it is acted by a conservative force. The
potential energy is shown below as a function of the coordinate x of the particle.
Rank the labeled regions according to the magnitude of the force, least to greatest.
A)
B)
C)
D)
E)
AB, BC, CD
AB, CD, BC
BC, CD, AB
BC, AB, CD
CD, BC, AB
_______6. A thick uniform wire is bent into the shape of the letter "U" as shown. Which
point indicates the location of the center of mass of this wire?
A)
B)
C)
D)
E)
A
B
C
D
E
_________7. The center of mass of a system of particles remains at the same place if:
A)
B)
C)
D)
E)
it is initially at rest and the external forces sum to zero
it is initially at rest and the internal forces sum to zero
the sum of the external forces is less than the maximum force of static friction
no friction acts internally
none of the above
_______8. Two objects undergo a collision. If we know the masses of the two objects
and their velocities before the collision, we can calculate the velocity of one of
the two objects after the collision using the following equation:
v1 f 
m1  m2
2m2
v1i 
v 2i
m1  m2
m1  m2
Only if:
A) momentum is not conserved but kinetic energy is conserved
B) total mass is not conserved but momentum is conserved
C) kinetic energy and momentum are both conserved
D) momentum is conserved but kinetic energy is not conserved
E) the total impulse is equal to the change in kinetic energy
Part B-Short Answer
1. The angular position of a point on a rotating wheel is given by the function:
 (t) = 8t3 + 3t + 7 radians
a) What is the angular velocity of this point as a function of time?
b) What is the angular velocity of the point after 4 seconds?
2. The disk shown below is rotating about an axis through its center such that the point
marked with the X is moving to the right.
a) What is the direction of the angular velocity of the disk? (Circle the best choice)
Up
Down
Clockwise
Into the page
Counterclockwise
Out of the page
Right
Left
Some other Direction
b) If the disk is rotating as shown and slowing down, what is the direction of the angular
acceleration? (Circle the best choice)
Up
Clockwise
Down
Into the page
Counterclockwise
Out of the page
Some other Direction
Right
Left
Part C-SHOW ALL YOUR WORK TO RECEIVE FULL CREDIT!!
1. A 3.5 gram bullet is fired horizontally at two blocks resting on a smooth tabletop (no
friction). See the figure below. The bullet passes through the first block with has a mass
of 1.2 kg and embeds itself in the second, with a mass of 1.8 kg. Speeds of 0.63 m/s and
1.4 m/s respectively are imparted to the blocks. Neglect the mass removed from the first
block by the bullet.
A) What is the momentum of block A after the bullet passes through it?
B) What is the total momentum of block B and the bullet after the bullet is embedded in
the block?
C) What was the speed of the bullet after it had passed through block A but before it hit
block B?
2.
Consider the elevator shown to the left. It has a mass of 200 kg and is
moving downward at 5 m/s when the cable breaks. Fortunately, there
is a safety stopping mechanism at the bottom of the elevator shaft, 40
meters below the bottom of the elevator. The safety stop is basically
just a very stiff spring (k=2500 N/m).
M=
200 kg
V= 5 m/s
A)
How fast is the elevator moving just before it
hits the spring? (There is no friction between the
elevator and the shaft)
40 m
k = 2500 N/m
B)The elevator then hits the spring (k=2500 N/m) and the spring is compressed by 7
meters. How fast is the elevator going at that point? ( There is still no friction between
the elevator and the shaft)
C) If there is a frictional force of 500 N between the elevator and the shaft, the elevator
would be moving at about 25 m/s when it hit the spring. In this case, how fast is the
elevator going once the spring has been compressed by 7 meters? (There is now a 500 N
frictional force between the elevator and the shaft- along the entire shaft, including
the part where the spring is located)