# Question on S.H.M

1. Mar 4, 2008

### mysqlpress

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

provided in picture

2. Relevant equations
provide

3. The attempt at a solution
I know how to show ,but I don't understand why we consider one mass only

[PLAIN][PLAIN]http://b.imagehost.org/0195/questionb.jpg [Broken]

Last edited by a moderator: May 3, 2017
2. Mar 4, 2008

### Staff: Mentor

That looks like two masses connected by a spring. You describe the motion of each mass (with respect to the center) separately, but they are definitely connected.

3. Mar 4, 2008

### mysqlpress

they strecthed at the same time.
so

ma =-kx
a=-(k/m) x
ma=-kx
a= -(k/m)x

but I don't know how to combine as

a=-(2k/m)x

I think it should be a=-(2k/2m)x = -(k/m)x

4. Mar 4, 2008

### Feldoh

I believe that instead of stretching both, you could hold one down and stretch the other twice as far.

5. Mar 4, 2008

### mysqlpress

That means

ma = -k(2x)?
a=(-2k/m)x ?

I am doubt why ma is used instead of 2ma ?
the whole mass of system is 2m ?

6. Mar 5, 2008

### Staff: Mentor

If x is the displacement of each mass, the amount of stretch in the spring is 2x.

7. Mar 5, 2008

### mysqlpress

but why should we use

ma=-2kx
but not
2ma=-2kx ?
since there are two bob with mass m..

8. Mar 5, 2008

### Staff: Mentor

Because we are analyzing the motion of each mass separately:

What's the mass? m
What's the stretch of the spring? 2x
What's the force on each mass? -2kx

9. Mar 5, 2008

### mysqlpress

Thanks Doc Al.
you do help me a lot