Circle the right answers.

I. An uniform metal sheet is cut into a "C" shape as shown in the figure. Point C is the center of the two concentric circles. Which point best represents the position of the center of mass?

    A.    Point A
    B.    Point B
    C.    Point C
    D.    Point D
    E.    Point E

Ans. B

II. A cart (A) moving at speed v m/s collides elastically with an identical stationary cart (B) on an airtrack. What are their velocities after colliding?

    A.    Cart A -v m/s, Cart B 0 m/s
    B.    Cart A -v m/s, Cart B v m/s
    C.    Cart A v m/s, Cart B 0 m/s
    D.    Cart A 0 m/s, Cart B v m/s
    E.    Not enough information to solve this problem

Ans. D

III. If the velocity of a body is doubled, its kinetic energy

    A.    remains the same
    B.    decreases to half its original value
    C.    increases to two times its original value 
    D.    decreases to one fourth its original value
    E.    increases to four times its original value

Ans. E

IV. The Moon does not fall to Earth because

    A.    The net force on it is zero. As a result, the Moon is not accelerating and it will not fall to Earth.
    B.    It is beyond the main pull of Earth's gravity. When the Moon is very far away from Earth, the gravitational attraction has to be negligibly small.
    C.    It is being pulled by the Sun and planets as well as by Earth. These forces will add up to zero.
    D.    It is in a circular orbit and the gravitational attraction from Earth supplies the centripetal force necessary to keep the Moon in orbit.
    E.    All of above.

Ans. D