# Homework Help: Finding the Force

1. Sep 18, 2009

### Amel

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

Two forces F1 and F2 are acting on a mass M=4kg. The force F1 = 11i + 5j + 7k, in N. The acceleration of the mass is given by a = 3i + 7j + 4k, in m/s2. Calculate the magnitude of F2 .

2. Relevant equations
F = ma

3. The attempt at a solution
Ok so what I tried here is doing F1 + F2 = ma
so F2 = (ma)- F1

I took a = 3i + 7j + 4k and multiplied all of the components by 4kg got 44i+20j+28k and then subtracted F1 = 11i + 5j + 7k and got 41i+13j+24k kg*m/s^2

what do I do?

2. Sep 18, 2009

### rock.freak667

ma=4(3i+7j+4k)=12i+28j+16k

now subtract this from F1

3. Sep 18, 2009

### Amel

so i+23j+3k ? what units do I use for this? its saying im not using the right units. Isn't it N*m/s^2

4. Sep 18, 2009

### rock.freak667

'ma' is in N

F1 is in N

so if F2= ma - F1, then F2 is in N as well.

5. Sep 18, 2009

### Amel

cool I got it right it was 1i+23j+9k square the coeficents add them up and then square root to find the force N.