What are the forces and displacement involved in this body's movement?

[nabilsaleh]

Homework Statement

The only force acting on a 2.3 kg body as it moves along the positive x-axis has an x component Fx = -3x N, where x is in meters. The velocity of the body at x = 2.8 m is 11 m/s. (a) What is the velocity of the body at x = 3.6 m? (b) At what positive value of x will the body have a velocity of 3.0 m/s?

Homework Equations

The Attempt at a Solution

i got the answer for part A (10.73m/s) but I'm stuck in B! HELP!

i tried vf^2=vi^2 +2Ax for part a. But i just don't know wat to substitute in B.

 

[ehild]

This is not a constant force, so the equation vf^2=vi^2 +2Ax is not valid. What motion is it when the force is proportional to the displacement and opposite to it?

ehild

 

[nabilsaleh]

What motion is it when the force is proportional to the displacement and opposite to it?

ehild

negative acceleration, right?

 

[ehild]

And what is the sign of the acceleration when x is negative?

ehild

 

[nabilsaleh]

And what is the sign of the acceleration when x is negative?

ehild

positive...so?

 

[ehild]

The acceleration is not constant. Is the force conservative?

ehild

 

[nabilsaleh]

The acceleration is not constant. Is the force conservative?

ehild

no the force is not conservative. so do we use f=ma? but in that case wat is a ?!

 

[ehild]

a=F/m. And how do you know that this force is not conservative?

ehild

 

[nabilsaleh]

a=F/m. And how do you know that this force is not conservative?

ehild

obviously the force changes according to displacement. but how could a=f/m benefit us? i didn't get where you're trying to get with this...

 

[ehild]

Have you learned calculus? Or have you learned about simple harmonic motion?

ehild

 

[nabilsaleh]

Have you learned calculus? Or have you learned about simple harmonic motion?

ehild

i did. but can you elaborate please. I am having a hard time following you here :S

 

[nabilsaleh]

Have you learned calculus? Or have you learned about simple harmonic motion?

ehild

i tried integrating (w= (integral) fd )
and i got x=9.7

PLEASE CHECK MY ANSWER! this is important!

 

[ehild]

i tried integrating (w= (integral) fd )
and i got x=9.7

PLEASE CHECK MY ANSWER! this is important!

The result is all right. But you get proper help earlier if you show your work in detail.

ehild