Am I getting this correct?

Question

A rock is held under water in a pond and is dropped from rest. While it sinks to the bottom of the pond, what happens to its motion? (More than one may be correct, so choose as many as are correct.)

Select one or more:

a. The acceleration decreases.

b. The velocity increases and then decreases.

c. The velocity decreases and then increases.

d. The velocity of the rock decreases until it reaches terminal velocity.

e. The friction on the rock decreases as it approaches terminal velocity.

f. The acceleration of the rock in the water depends on its weight.

g. None of the above.

is the answer A and F

Answer 1

Hi Ana S.

That would be (g), none of the above. Initially, the rock accelerates until it reaches terminal velocity (assuming it's nearly spherical; if it is irregularly-shaped, or oddly textured, it may fall with a non-steady pattern). That terminal velocity will depend on both the density and the size of the rock, but not directly on its mass: the negative bouyancy of the rock is the driving force (under gravity), and the rock profile area downwards (so to speak) is the retarding force, causing frictional drag.

You might assume that the impact at the bottom of the pond is not part of the targeted time.

Choice (f) is deceptive: for two rocks of identical density, the more massive (larger) one will reach a higher terminal velocity. But the instantaneous initial acceleration will ALWAYS be -g (if the rocks sink at all!), because the initial velocity of both objects is zero, hence they experience NO drag force initially. After that, the difference in the accelerations of the rocks depend on their density, size, and time elapsed. Since there is a time (t(0)) when the accelerations do not depend on weight at all, statement (f) is not always true; thus it is false!