CENTER OF MASS AND LINEAR MOMENTUM - Mustofa Abi Hamid Official Site

## Saturday, June 12, 2010

1. Which one of the following statements is true?
A. the center of mass of an object must lie within the object
B. all the mass of an object is actually concentrated at its center of mass
C. the center of mass of an object cannot move if there is zero net force on the object
D. the center of mass of a cylinder must lie on its axis
E. none of the above
ans: E

2. The center of mass of a uniform disk of radius R is located:
A. on the rim
B. a distance R/2 from the center
C. a distance R/3 from the center
D. a distance 2R/3 from the center
E. at the center
ans: E

3. The center of mass of the system consisting of Earth, the Sun, and the planet Mars is:
A. closer to Earth than to either of the other bodies
B. closer to the Sun than to either of the other bodies
C. closer to Mars than to either of the other bodies
D. at the geometric center of the triangle formed by the three bodies
E. at the center of the line joining Earth and Mars
ans: B

4. The center of mass of Earth’s atmosphere is:
A. a little less than halfway between Earth’s surface and the outer boundary of the atmosphere
B. near the surface of Earth
C. near the outer boundary of the atmosphere
D. near the center of Earth
E. none of the above
ans: D

5. Block A, with a mass of 4 kg, is moving with a speed of 2.0m/s while block B, with a mass of
8 kg, is moving in the opposite direction with a speed of 3m/s. The center of mass of the two
block-system is moving with a velocity of:
A. 1.3m/s in the same direction as A
B. 1.3m/s in the same direction as B
C. 2.7m/s in the same direction as A
D. 1.0m/s in the same direction as B
E. 5.0m/s in the same direction as A
ans: B

6. At the same instant that a 0.50-kg ball is dropped from 25m above Earth, a second ball, with a
mass of 0.25 kg, is thrown straight upward from Earth’s surface with an initial speed of 15m/s.
They move along nearby lines and pass each other without colliding. At the end of 2.0 s the
height above Earth’s surface of the center of mass of the two-ball system is:
A. 2.9m
B. 4.0m
C. 5.0m
D. 7.1m
E. 10.4m
ans: D

7. At the same instant that a 0.50-kg ball is dropped from 25m above Earth, a second ball, with a
mass of 0.25 kg, is thrown straight upward from Earth’s surface with an initial speed of 15m/s.
They move along nearby lines and pass without colliding. At the end of 2.0 s the velocity of
the center of mass of the two-ball system is:
A. 11m/s, down
B. 11m/s, up
C. 15m/s, down
D. 15m/s, up
E. 20m/s, down
ans: C

8. At the same instant that a 0.50-kg ball is dropped from 25m above Earth, a second ball, with a
mass of 0.25 kg, is thrown straight upward from Earth’s surface with an initial speed of 15m/s.
They move along nearby lines and pass without colliding. At the end of 2.0 s the magnitude of
the acceleration of the center of mass of the two-ball system is:
A. 0.25g
B. 0.50g
C. 0.75g
D. g
E. g/0.75
ans: D

9. A light rope passes over a light frictionless pulley attached to the ceiling. An object with a large
mass is tied to one end and an object with a smaller mass is tied to the other end. Starting from
rest the heavier object moves downward and the lighter object moves upward with the same
magnitude acceleration. Which of the following statements is true for the system consisting of
the two masses?
A. The center of mass remains at rest.
B. The net external force is zero.
C. The velocity of the center of mass is a constant.
D. The acceleration of the center of mass is g, downward.
E. None of the above statements are true.
ans: E

10. Two 4.0-kg blocks are tied together with a compressed spring between them. They are thrown
from the ground with an initial velocity of 35m/s, 45�� above the horizontal. At the highest
point of the trajectory they become untied and spring apart. About how far below the highest
point is the center of mass of the two-block system 2.0 s later, before either fragment has hit
the ground?
A. 12m
B. 20m
C. 31m
D. Can’t tell because the velocities of the fragments are not given.
E. Can’t tell because the coordinates of the highest point are not given.
ans: B

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