First slide
Relative motion in 2 dimension
Question

A body is projected at an angle 30° with the horizontal with velocity v1, from the point A. At the same time, another body is projected vertically upwards from B with velocity v2.The point B lies vertically below the highest point of the trajectory of body projected from point A. For both the bodies to collide, the value of v1v2should be___ .

Moderate
Solution

The two bodies will collide at the highest point  if both cover the same vertical height in the same time. So 
v12sin2 302g=v222g or

 v2v1=sin 30=12 v1v2=2

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