# Homework Help: Newton's second law with vectors

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1. Oct 8, 2014

### Shadow236

1. The problem statement, all variables and given/known data
A 9 kg object is subjected to two forces, [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif1 [Broken] = 7 N i - 4 N j and [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif2 [Broken] = 7 N i - 9 N j. The object is at rest at the origin at time t = 0.

(a) What is the object's acceleration?

(b) What is its velocity at time t = 14 s?

(c) Where is the object at time t = 14 s?

(All answers in unit vector notation)

2. Relevant equations
(a) Fnet=ma

(b) V = V0 + at

(c) X = X0 + V0t + 1/2at2

3. The attempt at a solution
(a) Fnet = F1 + F2 = 14i - 13j

Fnet = ma -> a = (14i-13j)/9
a = 1.56i - 1.44j (MARKED INCORRECT)

Why is this incorrect? If a = F/m then all I should have to do is add up the forces to get Fnet.

(b and c) I've used the equations above and got the answers wrong. Overall, I won't be able to get these answers correct unless I get the correct acceleration.

Last edited by a moderator: May 7, 2017
2. Oct 8, 2014

### LCKurtz

Probably because $1.56\ne14/9$ and $-1.44\ne -13/9$. They are just decimal approximations to the fractions, which are the correct answers.

Last edited by a moderator: May 7, 2017
3. Oct 8, 2014

### Shadow236

So you think that I was correct to add F1 and F2? If that is true I should just leave Fnet as 14/9i - 13/9j?

4. Oct 8, 2014

### LCKurtz

Yes. You could always ask your teacher, or if it's an online problem, try typing in the fractions. In that case be sure to use parentheses as in (14/9)i

5. Oct 8, 2014

### Shadow236

Alright, I will try that. Thank you very much. Are my equations for (b) and (c) okay? I just want to check because I've already submitted this question a few times.

EDIT: Hey, I got it right! =D Fractions for life, haha. Thanks a lot!

Last edited: Oct 8, 2014
6. Oct 8, 2014

### vela

Staff Emeritus
Those equations (b and c) are valid only if you have constant acceleration. Do you have constant acceleration? If so, you can use them.

7. Oct 8, 2014

### Shadow236

Yes, I had constant acceleration. Thank you.

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