# Finding Work with velocity

1. Oct 14, 2007

### B-80

The only force acting on a 1.9 kg body as it moves along the positive x axis has an x component Fx = - 6x N, where x is in meters. The velocity of the body at x = 3.0 m is 8.0 m/s.

(a) What is the velocity of the body at x = 4.0 m?

well work is the integral of the f(x) function, so its -3x^2. Knowing that I can get the work done from x=3 to x=4. which is -3*9 and -3*16. after omitting the sign, which I figured I need to do after about an hour of this problem, I get a net change of 21J. so I plug that into 1/2MVf^2-1/2MVi^2 and get 1/2M(Vf^2-Vi^2)=21J

21J/(.5*1.9Kg)=Vf^2-Vi^2
22.105+Vi^2=Vf^2
Vf=sqrt(22.105+8^2)

this yields me an answer of 9.28 m/s
I must have done this problem about 8 times now, and I cant get it right.

(b) At what positive value of x will the body have a velocity of 5.0 m/s?

2. Oct 14, 2007

### cevdet.erturk

Does the force begin to act the object where x=0?

3. Oct 14, 2007

### B-80

I don't really understand what you are trying to say?

4. Nov 6, 2008

### cafercan72

heyy f is in negative direction vfinal must be less than 8m/s