## Velocity? "A ball is thrown vertically upward..."

Limits, differentiation, related rates, integration, trig integrals, etc.
AmySaunders
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### Velocity? "A ball is thrown vertically upward..."

A ball is thrown vertically upward from the top o f a 100-ft-high building. Its height above the ground at time t is given by h(t)=100+30t-16t^2

At what time is the ball falling toward the earth at 46 ft/sec?

I've worked this problem several times, yet I always come out with the answer as t=-1/2. The answer is 19/8, according to the book.

I take the derivative of the equation, since I'm dealing with velocity, and come out with v(t)=30-32t.
Then I plug 46 in for the velocity, since the unit can only be used for velocity, and I solve.

I know I'm not doing it right, but where am I going wrong?

nona.m.nona
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Joined: Sun Dec 14, 2008 11:07 pm
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### Re: Velocity? "A ball is thrown vertically upward..."

At what time is the ball falling toward the earth at 46 ft/sec?
I've worked this problem several times...

Then I plug 46 in for the velocity...
In which direction is the ball moving? Then what sign does the velocity have?