✔ 最佳答案
This question is a tricky one. The correct statement is B.
Throughout the process, the weight of the rock always acts in the down direction, even when it is at the top of its path. So, statement D is wrong.
The tricky part is as follows:
If air resistance is not taken into account, then the net force acting on the rock is its weight, pointing downward throughout the process.
If air resistance is taken into account, then the air resistance will oppose the direction of motion of the rock. That is it acts downward when the rock is moving upward, and vice versa. The net force acting on the rock is always downward since it is always accelerating downward. Note that air resistance is proportional to its speed.
So, when the rock is at its top position of its path, it is instantaneously at rest and hence the air resistance acting on it is zero. And the net force acting on the rock is only its weight. So, statement C, D and E are wrong.
