Posted by **Jacke** on Thursday, August 28, 2014 at 9:41pm.

1.if a ball is thrown into the air with a velocity of 35ft/s, it's height in feet after t seconds is given by y=35t-10t^2. Find the averag velocity for the time period beginning when t=4 and lasting 0.05s.

2.if a ball is thrown into the air with a velocity of 30ft/s, it's height in feet after t seconds is given by y=30t-20t^2. Find the instantaneous velocity when t=3

- Calculus -
**Reiny**, Thursday, August 28, 2014 at 10:01pm
Last time I was on this earth, gravity would cause an equation of the type

y = -16t^2 .....

So assuming you have a typo and it should have been

y = 35t - 16t^2

Second problem:

the ball would not be in the air for 4 seconds,

so your whole first question is actually just gibberish.

Once you fix it, find y when t = 4

find y when t = 4.05

evaluate: ( y(4.05) - y(4) )/.05

2. same thing here,

after 3 seconds the ball has reached the ground, and no longer has any velocity

if we ignore reality,

dy/dt = 30 - 40t

when t = 3

dy/dt = 30 - 120 = -90 ft/s

1.

when t=4, y = 35(4) - 16(16) = -20

- Calculus -
**Jacke**, Thursday, August 28, 2014 at 10:47pm
There is no error in the problem.

- Calculus -
**Steve**, Friday, August 29, 2014 at 11:16am
I see the typo as specifying feet. In metric units, g=9.8, or roughly 10, so Jacke, redo the calculations, with g=10 and get the correct answer.

- Calculus -
**Jacke**, Friday, August 29, 2014 at 6:37pm
Is it ok if I get a negative answer???

- Calculus -
**Jacke**, Friday, August 29, 2014 at 7:09pm
OK thanks I got it

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