What is the horizontal acceleration of a 7.00-kg block with two given forces?

[KachinaEarth]

Homework Statement

Two forces, F1 and F2, act on the 7.00-kg block shown in the drawing. The magnitudes of the forces are F1=65.9 N and F2=22.7 N. Take the positive direction to be to the right. Find the horizontal acceleration of the block, including sign.

http://img522.imageshack.us/my.php?image=forcett9.jpg

Homework Equations

\SigmaF = ma or a = \SigmaF / m

\SigmaF - Net Force (F1 + F2 = Net F)
m - mass
a - acceleration

The Attempt at a Solution

F1 = 65.9N
F1 * Sin = 61.9N
F1 * Cos = 22.6N

F2 = 22.7 N
F2 Opposite Force = -22.7 N

(-) 22.7 N + (+) 22.54 N = 7 kg*a
(-).16 N = 7 kg*a
(-).16 N / 7 kg = a
(-)0.023 m/s^{2}


I'm not quite sure what I'm doing wrong as when I try to use this answer it is marked as incorrect so any help would be most appreciated.

 

[learningphysics]

-0.023 m/s^2 was marked as incorrect?

 

[KachinaEarth]

Correct; my answer is marked as incorrect but I'm not sure why as it seems to be the only answer I can arrive at using the formula taught to me both in my class and in my recitation lab.

 

[learningphysics]

Correct; my answer is marked as incorrect but I'm not sure why as it seems to be the only answer I can arrive at using the formula taught to me both in my class and in my recitation lab.

Did you post the question exactly as it is? Did you maybe mix up F1 and F2?

 

[KachinaEarth]

Directly as is. I was very careful to type everything word, and number, for word.

Is there perhaps an error with the homework you think?

 

[learningphysics]

Directly as is. I was very careful to type everything word, and number, for word.

Is there perhaps an error with the homework you think?

yeah I think so... I suspect maybe there's a coefficient of friction that's supposed to be in the problem... but they left out...

Unless the program is sensitive about significant figures or something?
Maybe -0.0230m/s^2 will work? Not sure...

 

[KachinaEarth]

I tried the extra zero at the end of -0.0230 m/s^2 and it took the answer, finally, so I think the program is just that sensitive. Thank you so much for your help. I was starting to stress so much I don't think I would have thought to keep adding on significant numbers.

 

[learningphysics]

I tried the extra zero at the end of -0.0230 m/s^2 and it took the answer, finally, so I think the program is just that sensitive. Thank you so much for your help. I was starting to stress so much I don't think I would have thought to keep adding on significant numbers.

No prob. Glad it went through.