A river flows at 2 m s⁻¹. A boat moves downstream; its speed in still water is 3 m s⁻¹. A person on the boat throws a ball at 10 m s⁻¹ relative to himself in a vertical plane perpendicular to the boat's motion, at 60° above horizontal. When the ball reaches the highest point, which statements are correct?
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Question

A river flows at 2 m s⁻¹. A boat moves downstream; its speed in still water is 3 m s⁻¹. A person on the boat throws a ball at 10 m s⁻¹ relative to himself in a vertical plane perpendicular to the boat's motion, at 60° above horizontal. When the ball reaches the highest point, which statements are correct?

Select all that apply

G1 Physics · Accepted Answer

At the highest point, the vertical component relative to boat is zero and the horizontal component perpendicular to boat motion is 10cos60°=5 m s⁻¹, so A. Boat ground speed downstream is 3+2=5 m s⁻¹; combining perpendicular 5 and downstream 5 gives ground speed 5√2 m s⁻¹, so D.