# Homework Help: Exercise in Net Force

1. Sep 13, 2007

### KachinaEarth

1. The problem statement, all variables and given/known data
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

2. Relevant equations

$$\Sigma$$F = ma or a = $$\Sigma$$F / m

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

3. 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.

2. Sep 13, 2007

### learningphysics

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

3. Sep 13, 2007

### 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.

4. Sep 13, 2007

### learningphysics

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

5. Sep 13, 2007

### 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?

6. Sep 13, 2007

### learningphysics

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...

7. Sep 13, 2007

### 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.

8. Sep 13, 2007

### learningphysics

No prob. Glad it went through.