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 Post subject: ProjectilesPosted: Wed, 29 Feb 2012 23:00:57 UTC
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Joined: Wed, 5 Oct 2011 21:02:24 UTC
Posts: 25
A stone is thrown upwards from the top of a vertical cliff 56m high falls into the sea 4 seconds later, 32m from the foot of the cliff. Find the speed and direction of projection. (The stone moves in a vertical plane perpendicular to the cliff.)
ans: 10ms^-1 tan^-1(3/4) to the horizontal

I don't get the picture of the problem. Does it mean the cliff is not parallel to the horizon? And I can't get the answers! Please help! THANKS!

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 Post subject: Re: ProjectilesPosted: Thu, 1 Mar 2012 01:26:59 UTC
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Joined: Fri, 1 Jul 2011 01:17:26 UTC
Posts: 322
Ignoring air resistance, the path of the stone is a parabola. The horizontal speed is constant (8m/sec). The first step in the solution is to find out the initial vertical speed (v) given the initial height of 56m and total time is 4 sec.

The equation you need is h + vt - (gt^2)/2 = 0, where g is the gravity constant, t = 4, and h = 56. Once you have v, get the initial speed from the Pythagorean theorem using the two speed components, and get the angle as the arctan of the ratio of the speed components.

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