Rain falls vertically downward with speed v. An inclined plane making an angle α with the horizontal moves horizontally. For the rain to appear parallel to the inclined plane in the plane's frame, the required speed u of the plane is:
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Question

Rain falls vertically downward with speed v. An inclined plane making an angle α with the horizontal moves horizontally. For the rain to appear parallel to the inclined plane in the plane's frame, the required speed u of the plane is:

Options

G1 Physics · Accepted Answer

Relative rain velocity is -uî - vĵ. For this vector to be parallel to the inclined plane, the magnitude ratio of vertical to horizontal components must equal tanα: v/u = tanα. Hence u = v cotα. Option B is correct.