# Homework Help: Physics I (Connected Masses)

1. Oct 14, 2012

### eddiepi2012

1.I am stuck on this problem can anyone help me?

A 20-kg object is subjected to three forces which produce an acceleration a = -8 m.s-2 i + 6.0 m.s-2 j on the object. Two of the forces are:
F1 = 3.0 N i + 16.0 N j
F2 = -12.0 N i+ 8.0 N j
Find the third force.

2. Relevant equations

F = F1 + F2 + F3

a = Fnet / m

3. The attempt at a solution

F1 = 3.0 N i + 16.0 N j
F2 = -12.0 N i + 8.0 N j
Fnet = -9 N i + 24 N j

a = Fnet / m = -9 N / 20 kg i + 24 / 20 kg j = -0.5 i + 1.2 j

2. Oct 14, 2012

### tiny-tim

welcome to pf!

hi eddiepi2012! welcome to pf!

(try using the X2 button just above the Reply box )
but that's not Fnet

Fnet is the total force

3. Oct 14, 2012

### eddiepi2012

Ok so how do I find the total force I understand that I am searching for F3 but how do I find it based on the information given?? Any help will be greatly appreciated. Thanks tiny-tim.

Last edited: Oct 14, 2012
4. Oct 14, 2012

### eddiepi2012

Thank you tiny-tim for welcoming me to PF. :)

Last edited: Oct 14, 2012
5. Oct 14, 2012

### eddiepi2012

Re: welcome to pf!

Thank you tiny-tim for welcoming me to PF. :)

6. Oct 14, 2012

### eddiepi2012

Re: welcome to pf!

I believe this (-9 N i + 24 N j) is the Resultant of F1 + F2. Having this information how do I find F3?

7. Oct 15, 2012

### tiny-tim

hi eddiepi2012!

(just got up :zzz:)
first, use Fnet = ma to find Fnet

then subtract F1 and F2