# Homework Help: Spring and block of mass problem

1. Oct 7, 2009

### tigerlili

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

a block of mass m lies on a horizontal frictionless surface and is attached to one end of a horizontal spring (spring constant k) whose other end is fixed. The block is initially at rest at the position where the spring is unstretched (x = 0) when a constant horizontal force in the positive direction of the x axis is applied to it. A plot of the resulting kinetic energy of the block versus its position x is shown in Fig. 7-38.

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c07/pict_7_38.gif
2. Relevant equations

f= -kx
k= 1/2 mv^2
ui + ki = uf + kf

3. The attempt at a solution

i'm really confused about how to start this one :/

2. Oct 7, 2009

### Delphi51

What are you supposed to find? I see no question there.

3. Oct 7, 2009

### tigerlili

oh, woops! i forgot to paste the rest :X

The scale of the figure's vertical axis is set by Ks= 8.0 J. (a) What is the magnitude of ? (b) What is the value of k?

4. Oct 7, 2009

### Delphi51

Your part (a) question still isn't complete.
For (b), k is the maximum kinetic energy which the graph says is at distance 1 m.

I don't think you can use your k = 1/2*m*v^2 directly. You'll have to think about the work being done by that force and the spring energy (have to look up the formula for the energy of a stretched spring). Give us a start on that and someone will help you if you need help.

5. Oct 8, 2009

### tigerlili

the vector notation didn't actually paste properly

and i really don't remember even learning the equation for a stretched spring, i'm just so lost
it no longer really matters, because the homework was already due
i just want to understand what's going on in the problem

6. Oct 8, 2009

### Delphi51

Take a look here: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
E = .5*k*x^2 where x is the distance the spring is stretched.
The idea is that the work done goes into a combination of kinetic and spring energy:
Fd = .5*k*x^2 + .5*m*v^2
Might be worth doing even though too late to hand in. Problems like this will come back to haunt you on exams.

7. Oct 9, 2009

### tigerlili

i know you're right, i've just been so overwhelmed by this class lately
i think i understand it now, thanks for your help :)