Question

While out hiking, you and your buddy decide to have some fun with the rocks laying all around. While sitting on a high cliff, overlooking a lake, you each drop a rock into the water.
Describe the change in motion of the rock as it enters the water.

Answer 1

When we drop the stone in air then during its motion in air the net force on the stone is due to gravity

so its net force is given as

$F = mg$

here we know that

$ma = mg$

so we have

$a = g$

now as soon as it will enter into the water then it will have resistive force due to water as well as buoyant force due to water on it in opposite direction

so the net force on the stone will decrease

so as soon as it will enter into the water the acceleration of stone will decrease due to which the rate of increase in speed of the stone will decrease as it will enter into the water