Finding acceleration from two velocities and positions

[sundrops]

kay well here goes:

An object moving with constant acceleration has a velocity v = 55.2m/s when it is at x = 6.9m and a v = 82.4m/s when it is at x = 12.4m. What is its acceleration?

Now this problem qould be simple if time were given but it isn't so I'm going to need a little bit of help please.

What I did with this one was I plugged the numbers into the following equation:
Vf^2 = Vi^2 +2a(xf - xi)

which gave me,

(82.4)^2 = (55.2)^2 + 2a(12.4 - 6.9)
6789.76 - 3047.04 = 2a(5.5)
a = 340.25m/s^2

whish is needless to say, well...impossible. Is there another way to attack this problem or maybe if someone could please point out my mistake?

 

[futb0l]

That should be right.

 

[sundrops]

but it isn't! I submitted my answer (my h/w is done on the net) and it said it was incorrect. too hard!

 

[futb0l]

Hmm, let me think for a bit.

 

[sundrops]

thanks for taking the time! :)

 

[futb0l]

I don't know what's wrong, that answer should be right, maybe I missed something.

 

[learningphysics]

thanks for taking the time! :)

Can you double check the question? Maybe it was 55.2 m/s at 12.4m and 82.4 m/s at 6.9m?

 

[sundrops]

nope its right.

and like its a three part question so I can't attempt the other two parts until i get the first one correct - maybe I'll try submitting it again (we get 3 tries before its marked as wrong)

 

[sundrops]

but can something accelerate that quickly? it seems like its accelerating a little too fast... hmmm

 

[learningphysics]

Don't submit it yet! futbol got a different answer! How did you get 134.52? I get the same thing as sundrops: 340.25 m/s^2

 

[sundrops]

good thing I tried submitting again! I had too many digits in my answer - not the proper amount of sig figs I guess. So the answer is correct! [340.2] (man I hate computers *grumbles*)

thanks to everyone who posted! :D

 

[sundrops]

wow! you guys are awesome - thanks again!

 

[futb0l]

Lol, you confused the hell out of me :P Time to get some rest now :P

 

[learningphysics]

good thing I tried submitting again! I had too many digits in my answer - not the proper amount of sig figs I guess. So the answer is correct! [340.2] (man I hate computers *grumbles*)

thanks to everyone who posted! :D

Cool! Yeesh, that's annoying though.

 

[sundrops]

g'nite futbol thanks again!

 

[sundrops]

it sure is! whatever happened to the good ol' days where h/w was done by hand - and marked by people??! lol

 

[sundrops]

thanks again!