How long until the ball hits the ground?


by hyde2042
Tags: ball, ground, hits
hyde2042
hyde2042 is offline
#1
Aug25-11, 12:56 AM
P: 25
1. The problem statement, all variables and given/known data

A ball is thrown downward from a height of 30m at 8m/s. How long until it hits the ground?
(Air resistance is negligible and gravity is 9.8m/s


2. Relevant equations
I tried deltaX=Vi+(1/2)a(t^2) and came out with 2.12, but the solution says it's 1.79.


3. The attempt at a solution

30=8+(1/2)(9.8)(t^2)
22=4.9(t^2)
sqrt4.49=sqrt(t^2)
t=2.12
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CompuChip
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#2
Aug25-11, 01:47 AM
Sci Advisor
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P: 4,301
Are you not missing a t?
[tex]V = V_i + a t[/tex]
so
[tex]\Delta x = V_i \mathbb{t} + \frac12 a t^2[/tex]
hyde2042
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#3
Aug25-11, 02:16 AM
P: 25
Thank you, but I'm afraid I'm evenb more confused by the additional t.

I ended up at 30-8t=4.9(t^2)

What I keep doing after that seems to make it more complicated than it should be. COuld you possibly point me in the right direction? Thank you for your time.

CompuChip
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#4
Aug25-11, 03:27 AM
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P: 4,301

How long until the ball hits the ground?


It's a standard quadratic equation, of the form ax2 + bx + c = 0.
How do you solve one of those?
hyde2042
hyde2042 is offline
#5
Aug25-11, 05:55 PM
P: 25
Ah... hate when I over look simple things. Thank you very much.


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