# Forces and vectors

1. Sep 26, 2013

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

Two forces, vector F 1 = (4i+ 6j) N and vector F 2 = (4i+ 8j) N, act on a particle of mass 1.90 kg that is initially at rest at coordinates (+1.95 m, -3.95 m).

a)What are the components of the particle's velocity at t = 10.3 s?

b) In what direction is the particle moving at t = 10.3 s?

(c) What displacement does the particle undergo during the first 10.3 s?

(d) What are the coordinates of the particle at t = 10.3 s?

2. Relevant equations

3. The attempt at a solution
For part a I tried dividing by the mass of the object to get Newtons to cancel to m/s^2
I also tried adding the two forces together, and adding -F2 to +F1 because in my textbook it says that once force is basically the opposite of the other
All of those were wrong
For part b, I took the arctan of 1.95 and -3.95 to get 63.7° but webassign said I was around 10% off on my answer
I havent been able to do c or d yet
Any help or understanding as to what I did wrong would be greatly appreciated

2. Sep 26, 2013

### Enigman

The net force is always the sum of all forces acting on a body, you don't have to worry about the directions vectors will take care of that themselves.
Calculate the net force and use that to get acceleration and then the rest is easy just resolve everything into x and y components and use the kinematic equations.

3. Sep 26, 2013

So your saying I just need to add the two forces together to get the components of the particles velocity?
So Final F= (4i+6j)N + (4i+8j)N and that is what would be the answer for part a?
I did that though. That was my first attempt at an answer
And it came out wrong
Maybe Im not understanding calculating the net force?

4. Sep 26, 2013

### Enigman

Noooo....
Components of force ≠ Components of velocity.
When you add the forces you get the net force which when you divide by mass you get the acceleration. Now you already know initial velocity is zero and you now have the acceleration too. So just use the kinematic equations.

5. Sep 26, 2013

Ohhhh got it
Duh haha
Lets see if I can get it

6. Sep 26, 2013

No :/
Ok so I found the Final F to be (8.00i+16.00j)N
Acceleration is [(8i+16j)N]/1.90kg
Or [(8i+16j)m/s^2]/1.9

Then I used the kinematic equation where Vf=Vi+at
Because the Vi is 0 and I know the acceleration and time
But its still wrong
????

7. Sep 26, 2013

### Enigman

8. Sep 26, 2013

### Staff: Mentor

Please show us the details of your calculations to get the components of the velocity at 10.3 seconds. In your equation for the acceleration, why didn't you divide the 8 and the 16 by 1.9? Anyhow, the 16 should be a 14.

9. Sep 27, 2013

Yeah I messed up with the addition, so its (8.00i+14.00j) right?

10. Sep 27, 2013

In my calculations I did divide by 1.9
And yeah i fixed the 16 to a 14

11. Sep 27, 2013

Oh nevermind I got it
Thanks for helping guys :)

12. Sep 27, 2013

Actually quick question
For c and d

so c is asking for the displacement the particle undergoes during the first 10.3s?
Ive tried using the answer from part a for this part but every variation i use is wrong
The answer for a was (43.3i+75.9j) m/s
What am i doing wrong?

13. Sep 27, 2013

### Staff: Mentor

You can't use the velocity after 10.3 seconds to get the displacement after 10.3 seconds. That is because the velocity is changing during those 10.3 seconds. The velocity at time zero was zero. Show us the equations you are using to get the velocity and displacement. Show us your calculations in detail.

Chet