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

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**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 air-resistance.

vx=u

Let H=given height of cliff

vy²-0²=2gH

vy²=2gH

since vy=u,

u²=2gH

u=√(2gH)

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