18. A 0.2 kg rock is dropped into a lake from a few meters above the surface of
ID: 1571068 • Letter: 1
Question
18. A 0.2 kg rock is dropped into a lake from a few meters above the surface of the water. The rock reaches terminal velocity in the lake after 5 s in the water. During the final 3 s of its descent to the lake bottom, the rock moves at a constant speed of 4 m/s. Which of the following can be determined from the information given? Select two answers and explain why.
a) The change in potential energy of the rock-Earth-water system during the final 3 s of the rock’s descent
b) The distance the rock travels in the first 5 s of its descent in the water
c) The acceleration of the rock 2 s before it reaches the lake bottom
d) The speed of the rock as it enters the lake
Answer choices a and b chosen together are incorrect.
Explanation / Answer
(C) ofcourse you dont know whats the accleration is in the first 5 sec when you dont know how the F water changes as the velocity of the rock is changing.all you know its terminal velocity and then you can the Fwater.so the accleration of the rock 2 sec is zero. the change in P.E = mgH = mgvt but you didnot get g. so c will be the only
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