Posted by Ligu on Friday, September 24, 2010 at 3:43am.
Consider a projectile fired horizontally from a cliff of a given height. With what speed must it be fired so that it makes a 45 degree angle with the ground when it hits

physics  MathMate, Friday, September 24, 2010 at 9:52am
It hits ground with 45° angle when the vertical velocity, vy, is the same as the horizontal velocity, vx, which equals u (initial velocity) if the projectile is fired horizontally and ignoring airresistance.
vx=u
Let H=given height of cliff
vy²0²=2gH
vy²=2gH
since vy=u,
u²=2gH
u=√(2gH)
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