# X = 54*5.51-9.8*5.51(squared)

For problem 11(please see the pdf file), I did this:
0 = 54 - 10t
t=5.51

x = 54*5.51-9.8*5.51(squared)
x = 148.8

What did I do wrong?

Note:
V= Vi + GT
X= Xi + Vi*T+.5*G*T(squared)

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Mistake

I meant 0 = 54-9.8T

enigma
Staff Emeritus
Gold Member

Always keep your units in your calculations. It makes it easier to troubleshoot.

Originally posted by NINHARDCOREFAN
For problem 11(please see the pdf file), I did this:
0 = 54 - 10t
t=5.51
Initial velocity is not 54 m/s, but it washes out in this part, because you know both velocities and can subtract them.

x = 54*5.51-XXX*9.8*5.512
x = 148.8

What did I do wrong?
The initial velocity does not wash in this equation, and you missed a 1/2 term in the acceleration portion.

yeah, .5*108=54. I didn't forget the 1/2. I don't understand when you say, you have to subract both velocities, because if you do that you get the same velocity

enigma
Staff Emeritus
Gold Member

Originally posted by NINHARDCOREFAN
V= Vi + GT
X= Xi + Vi*T+.5*G*T2

0 = 54 - 10t
t=5.51

x = 54*5.51-9.8*5.512
Look again, carefully NIN.

54m/s = 108m/s - 9.8m/s2*t

The two like terms can be subtracted, giving you what you got for the first line.

0 = 54m/s - 9.8m/s2*t

That doesn't mean that the initial velocity is now 54m/s.

For the second line, the 1/2 term is not on the velocity portion, it's on the acceleration portion!

x = 0m + (108m/s * 5.51s) + (1/2 * 9.8m/s2 * t2)

<i>The initial velocity does not wash in this equation, and you missed a 1/2 term in the acceleration portion.</i>

so I just use the original velocity? I meant to put 1/2 in the equation, it was a typo.

enigma
Staff Emeritus
Gold Member
Yep, that's it.

If you think to calculus, the x= equation is merely the integral of the first equation. Since the Vi term is a constant, it just gets a t added to it, but it doesn't change itself.

Thanks a lot.

Another question, for problem 13(please see the pdf file) what do i do? I'm getting 0 as an answer
21.6(squared)= 16.2m/s(squared)-2*9.8m/s(x-10.4m)

HallsofIvy