# Homework Help: Vector Forces

1. Jan 31, 2008

### Go Angels

1. The problem statement, all variables and given/known data
Three forces, given by 1 = ( -2.35 i - 2.10 j ) N, 2 = ( 4.90 i + 3.05 j ) N, and 3 = (49.0 j ) N, act on an object to give it an acceleration of magnitude 3.65 m/s2.

(a) What is the direction of the acceleration?
86.1° (from the positive x axis)

(b) What is the mass of the object?
_____kg

(c) If the object is initially at rest, what is its speed after 10.0 s?
_____m/s

(d) What are the velocity components of the object after 10.0 s?
(____ i + ____ j) m/s

2. Relevant equations
F = ma
Vf = Vi + at

3. The attempt at a solution
F = ma
m = F/a
To find mass (kg), I divided 2.55 by 3.65 and got 0.7 kg as the answer. That should be the correct answer but it isn't. Can someone please tell me what I did wrong? Thanks.

2. Jan 31, 2008

### mjsd

where did you get 2.55 for magnitude of force?

3. Jan 31, 2008

### Go Angels

2.55i + 49.9j were the magnitudes.

I found I was supposed to use 49.9 as the F, and that resulted in a mass of 3.68 kg, which is the correct answer.

Now I just need to solve for the i component for part d.). I got 36.42j for the j component, which is right, and 2.48i as the i component, which is wrong.
Help?

4. Jan 31, 2008

### mjsd

the magnitude of this vectorial quantity (2.55, 49.95) is given by

$$\sqrt{2.55^2 + 49.95^2} \approx 50$$ and NOT 49.9, although in this case it turned out that it is close to 49.9

5. Jan 31, 2008

### Go Angels

Ok that is right.

However I wasn't able to find the i factor of velocity components after 10 second

6. Jan 31, 2008

### Go Angels

For part d

Last edited: Jan 31, 2008
7. Feb 2, 2008

### mjsd

did you mean to say mass is about m= 13.7 Kg?
then 2.55/m x 10 = 1.86
49.95/m x 10 = 36.4