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Physics 221 2005 Su Exam 2
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For all problems, use g=10m/s
2
The correct answers are indicated in
blue italics
.
16.
(1 point)
In which of the following solid objects of uniform density does the center of
mass NOT lie within the mass of the body?
A. A cube
B. A doughnut
C. A sphere
D. A cylinder
E. None of the above
A simple examination of the symmetry should be enough to give the answer to this one.
17.
(1 point)
An 80 kg man standing on a frictionless surface kicks forward a 0.04 kg
stone lying at his feet so that it acquires a velocity of +4.0 m/s. What velocity does the
man acquire as a result?
A. +0.002 m/s
B. +0.09 m/s
C. 0.002 m/s
D. 0.09 m/s
E. None of the above.
Momentum is conserved, and the initial momentum is zero, so
0
f
man man
stone stone
p
m
v
m
v
=
=
+
.
Solving this for
v
man
and plugging in the numbers gives
the answer.
18.
(1 point)
Two masses of 1 kg are placed at
x=4, y=5
and
x=4, y=5
and a 3 kg mass
is placed at the origin, as shown in the figure below.
Where on the Yaxis will the center
of mass lie?
Y
X
3 kg (0,0)
1 kg (4,5)
1 kg (4,5)
A.
y
=1
B. y=2
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View Full DocumentPhysics 221 2005 Su Exam 2
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C.
y
=3
D.
y
=4
E.
y
=5
First take the CM of the two 1 kg masses.
It appears like a mass of 2 kg at the point
(0,+5). Then, combining this with the 3 kg mass, we find that the overall CM is at
y=2
,
x=0
.
19.
(1 point)
Two masses collide as shown in the figure below. Their masses and
velocities before and after the collision are as shown. Which of the following choices
describes the collision?
Before:
After:
0.5 kg
0.5 kg
2.0 kg
2.0 kg
10 m/s
3 m/s
10.8 m/s
2.2 m/s
A. Inelastic
B. Explosive
C. Elastic
D. Not enough information
The quick way to get the answer is to observe that the relative velocities are unchanged,
which is one of the characteristics of an elastic collision.
Alternatively, you can calculate
the kinetic energies before and after the collision and see that they are the same.
20.
(1 point)
What is the velocity of the center of mass of the particles in question 19?
A. 0.4 m/s
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 Fall '08
 HerreraSiklody
 Center Of Mass, Mass

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