An aeroplane flies horizontally at height , h in a straight line at a constant speed , u . When it is directly above a cannon on the ground , the cannon fires a shell which is supposed to hit the airplane . Ignore the air resistance , determine in terms of u,g and h the minimum muzzle speed of the shell , v.

Firstly , i resolve the velocity into 2 parts (ie and )

-2gh}

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=\sqrt{u^2-2gh}

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The error here is in that sign , why should it be positive .