Exam D: Momentum and Rotations
(Total points=85)
Name:
Signed:
PHY 161 General Physics I: Mechanics and Thermodynamics
Physics Department — Mercer University — Spring 2010
• Except for multiple choice questions, answers provided without explanation will receive little or no credit,
so you must SHOW ALL YOUR WORK.
• All answers should be decimal numbers. Please do not leave answers in terms of π, fractions, square roots,
trig functions, or inverse trig functions.
• Except as noted, provide numerical answers that are either EXACT, or accurate to THREE SIGNIFICANT
FIGURES, or WITHIN 1% of the exact answer.
• Acceleration due to gravity at the Earth’s surface: g = 9.81m/s2 .
• For all problems in this exam, ignore the effects of air resistance.
A:(30 points total, 5 points @) Multiple Choice.
Circle the letter of the correct answer.
1. Two masses M and 5M rest on a horizontal frictionless table with a
compressed spring of negligible mass between them. When the spring is released,
the energy of the spring is shared between the two masses in such a way that
A)
B)
C)
D)
E)
M gets 1/6 of the energy.
M gets 1/5 of the energy.
M gets 4/5 of the energy.
M gets 5/6 of the energy.
None of these is correct.
2. In the center-of-mass reference frame of a system,
A)
B)
C)
D)
E)
the system's kinetic energy is zero.
the system’s mechanical energy is zero.
the system’s kinetic energy is conserved.
the system’s total momentum is zero.
None of these is correct.
3. A pitcher throws a baseball with a speed of 27 m/s. After being struck by a bat
the ball travels in the opposite direction with a speed of 40 m/s. If the ball has a
mass of 0.11 kg and is in contact with the bat for 3 ms, what was the average
force exerted by the bat on the ball?
A)
B)
C)
D)
E)
Cannot be determined without the mass of the bat.
24.1 N
476 N
2.46 kN
24.1 kN
A (continued). Circle the letter of the correct answer.
4. You have a friend who lives in northern Georgia, and you live in central Georgia. As
Earth rotates, your linear velocity is ______ hers, and your angular velocity is _______
hers.
A)
B)
C)
D)
E)
greater than; greater than
greater than; equal to
equal to; equal to
less than; equal to
less than; less than
5. For a solid sphere of mass M and radius R that is rolling without slipping, which is
greater, its translational or its rotational kinetic energy?
A)
B)
C)
D)
E)
Its translational kinetic energy is greater.
Its rotational kinetic energy is greater.
They are equal.
The answer depends on the radius.
The answer depends on the angular velocity.
6. A thin massless string is wrapped around a 0.25 m radius grindstone supported by
bearings that produce negligible frictional torque. A steady tension of 20 N in the string
causes the grindstone to move from rest to a speed of 30 rad/s in 12s. The moment of
inertia of the grindstone is
A)
B)
C)
D)
E)
1 kg · m2
2 kg · m2
2.5 kg · m2
5 kg · m2
10 kg · m2
C:(20 pts) Air Track
Two masses, m1 = 3kg and m2 = 6kg, are sliding along a frictionless track with velocities v1 = +2m/s and
v2 = −4m/s. What is Ptotal , their total momentum? What is VCM , the velocity of their center of mass?
Suppose that they undergo an inelastic collision. If v10 = −2.5m/s, what is v20 ? What is the impulse I1 that
mass 1 receives in the collision?
•(5pts) Ptotal =
•(5pts) VCM =
•(5pts) v20 =
•(5pts) I1 =