14] When brought close together, two magnets attract and stick together. It requ

Question

14] When brought close together, two magnets attract and stick together. It requires a fairly large force to separate them. Compared to when the magnets were slightly separated, the magnets stuck together
(a) have the same energy, it is conserved. (b) have gained energy.

(c) have more positive potential energy. (d) . have less mechanical energy.

[15] A ball is rolling across the floor of a moving train car. Observer A is on the train and observer B is on the train platform as the car passes. Both observers determine tha kinetic energy of tha ball using the ball's velocity relative to them. The kinetic energy is greater for

(a) observer A. (b) observer B. (c) neither, same for all observers. (d) depends on directions and speeds of motion of train and ball.

[16] A block sliding across a rough patch comes to rest. The friction force (a)converted the KE to heat and sound, (b) converted the KE to

potential energy.
(c) destroyed the kinetic energy. (d) did not affect the energy.

[17] A planet is in circular orbit around a star. The work done by the force of gravity on the planet is
(a) 2rF (b) 0 (c) -2rF (d) -rf

[2] A student is looking at your physics problems and says “I remember this stuff from school. When you toss an object up in the air, the acceleration slows down, is zero at the top and changes direction for the return trip.” A correct analysis of this student’s statement would be:

(a) The student is correct.
(b) The acceleration cannot be zero at the top, but does change directions. (c) The student is describing velocity, not acceleration.
(d) Acceleration is constant during the motion.
(e) Both (c) and (d) are correct.

Explanation / Answer

[14] (c) have more positive potential energy.

[15] (d) It depends on directions and speeds of motion of train and ball. since stationary observer sees it with absolute velocity and observer on the train sees it with relative velocity. we don't know which is big and it depends on the above factors.

[16] (a) the frictional force converted the KE to heat and sound. (The velocity of the block is reduced by the frictional force. The rupture with the surface converts the KE to heat and sound.)

[17] (b) 0 Since the gravitational force is always perpendicular to the motion, no displacement along the direction of force implies no work done.

[2] (e) Both (c) and (d) are correct. The velocity decreases till the top, becomes zero and reverses the direction. The acceleration is constant. (only acceleration is acceleration due to gravity)

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