Free Fall Problem: How to Calculate Time in Air for a Thrown Ball?

[VoNi]

Free fall problem, Help!?

Homework Statement

Hi, I am having trouble figuring out this problem:

A ball is thrown directly upward with an initial velocity of 10 m/s. If the ball starts at an initial height of 2.3 m, how long is the ball in the air? Ignore air drag.

Homework Equations

The equation that I used to solve this was:

delta-y= (vi)t-1/2gt²

The Attempt at a Solution

I plugged in my numbers so i got:

-2.3= 10t-1/2(9.8)t²
-2.3=10t-(4.9)t²
4.9t²-10t-2.3=0

then i did the pythagorean theorem(but I don't really know how to type it on the computer.. but i will try)

-10 ± √((-10²)-4(4.9)(-2.3))/ 2(4.9)

-10 ± √(100-45.08)/2(4.9)

-10 ± √(54.92)/ 9.8

(-10 ± 7.41)/ 9.8

1.77 s or -1.77 s


So I put the answer 1.77 s and it was wrong. i put 1.8s and it was wrong. I used 1.02 and it was wrong. I used 2.04 and it was wrong. I have no idea what i am doing wrong and I no longer have any more submissions for my answer (which means i oficially have a zero for that question). But i really need to know how to solve this problem! please help me!

thanks!

 

[wy125]

You just made a couple mistakes with your sign:

You have
4.9t²-10t-2.3=0 which is correct but when you use the quadratic formula you have (it's not the Pythagorean btw)

-10 ± √((-10²)-4(4.9)(-2.3))/ 2(4.9)

but it should be

(10 +/- sqrt[(-10)^2 - 4(4.9)(-2.3)])/(2(4.9))

which is

((10 +/- sqrt[100 + 4(4.9)(2.3)])/(2(4.9))


that should give you the correct answer

 

[magwas]

a=4.9
b=-10 (not 10!)
c=-2.3