# Homework Help: Oscillations - mass suspended from string

1. Jun 21, 2014

### desmond iking

1. The problem statement, all variables and given/known data
the answer is a for this question. but i dont understand how to get the solution.

2. Relevant equations

3. The attempt at a solution
mg-k(l-a+b)=ma

since mg=k(l-a)

-kb=ma

a=-(k/m)b

k/m=a/b

so my ans is sqrt root( a/b)

#### Attached Files:

• ###### IMG_20140621_074427[1].jpg
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2. Jun 21, 2014

### Staff: Mentor

Hi, desmond. Welcome to the forum.

a, b, and l are constants, so the equation you came up with shows a fixed, constant acceleration. As you realize, this isn't correct; a fixed acceleration is not a characteristic of oscillatory motion. Somewhere in that equation you'd need a variable x which is the extension at any moment.

It might be easier if you were to start with the options, and maybe eliminate them one by one? Look for a reason why any particular option could not be the formula for ω2.

Last edited: Jun 21, 2014
3. Jun 22, 2014

### desmond iking

so if the option of (a/b) is given, can i choose it?

4. Jun 22, 2014

### Orodruin

Staff Emeritus
Try getting to the differential equation given in the problem by considering what forces are acting on the mass for an arbitrary displacement.

5. Jun 22, 2014

### desmond iking

can you explain further?

6. Jun 22, 2014

### Staff: Mentor

Do you understand the mathematics behind x with a pair of dots above it, as it appears in the textbook question in your attachment? (Sometimes it is written x’’ )

Last edited: Jun 22, 2014
7. Jun 22, 2014

### Staff: Mentor

Choose it if you wish. It's wrong though.

Oh, I just noticed: aren't you using the letter "a" for two different things in your equations in this thread??

Last edited: Jun 22, 2014
8. Jun 22, 2014

### rude man

Hint: determine the spring constant k, then use the formula for radian frequency w of a mass bobbing up & down:
w = w(k,m).

If you don't know the formula you'll have to proceed per post #4.