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

Limits, differentiation, related rates, integration, trig integrals, etc.

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

Postby AmySaunders on Thu Feb 28, 2013 10:52 pm

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?

Please help! Thank you!
AmySaunders
 
Posts: 34
Joined: Thu Aug 20, 2009 6:27 pm

Sponsor

Sponsor
 

Re: Velocity? "A ball is thrown vertically upward..."

Postby nona.m.nona on Fri Mar 01, 2013 2:04 am

AmySaunders wrote:
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?
nona.m.nona
 
Posts: 255
Joined: Sun Dec 14, 2008 11:07 pm


Return to Calculus