# Pendulums frequency problem

1. Aug 11, 2009

### dance_sg

1. The problem statement, all variables and given/known data
A pendulum is pulled to the side and released. What is its frequency if it passes through its rest position 0.150 s after it is released?

2. Relevant equations
f=1/T

3. The attempt at a solution
im not too sure, but in this equation, is all i do is divide one by 0.150 s?

2. Aug 11, 2009

### Feldoh

Re: pendulums

What is it's rest position? Where the pendulum started, or where it would be after friction dissipates all of the energy?

3. Aug 11, 2009

### kuruman

Re: pendulums

Can you figure out T from the given information? Hint: How is T defined?

4. Aug 11, 2009

### dance_sg

Re: pendulums

0.150 is T is it not?

5. Aug 11, 2009

### kuruman

Re: pendulums

It could be. I guess it depends on the meaning of "rest position". If it means "instantaneous rest", then there are two such positions, one on each side of the swing. So T could be either 0.150 s for a full swing to "rest" position or 0.300 s for half a swing to the "rest" position. A third possibility, as Feldoh suggested, is that "rest position" means the equilibrium position. In this case 0.150 s is a quarter of T and T is 0.600 s. It is confusing, I know, but "rest" position is an unconventional word in this context.

6. Aug 11, 2009

### tms

Re: pendulums

It is not (assuming the rest position is when the pendulum is hanging vertically).

7. Aug 11, 2009

### dance_sg

Re: pendulums

ok i understand what a rest position is, but i still dont understand how that helps me figure out the question.

8. Aug 11, 2009

### tms

Re: pendulums

How is the frequency (which is what you are asked to find out) defined?

9. Aug 11, 2009

### dance_sg

Re: pendulums

1/T = F and the units for period is S

10. Aug 11, 2009

### tms

Re: pendulums

What is the period, then? Describe it in words.

11. Aug 11, 2009

### dance_sg

Re: pendulums

the time it takes an object to make one complete cycle

12. Aug 11, 2009

### tms

Re: pendulums

And what is one cycle? Describe, in words, the pendulum's motion for one cycle.

13. Aug 11, 2009

### bm0p700f

Re: pendulums

I was just about to post that question.

Given the wording of the question, the pendulum reaches the rest position after 0.150 s, I think this must mean it reaches the other side of the swing after 0.150s .

Define the time period T and will see what the time period is.

14. Aug 11, 2009

### dance_sg

Re: pendulums

arg im lost

15. Aug 11, 2009

### tms

Re: pendulums

Just imagine the trajectory of the pendulum. You pull it to one side and let go. It swings down, passes the vertical, and goes up on the other side, reaches a maximum, then comes down again past the vertical and back up to the starting point (where you let it go). Then it repeats that motion indefinitely (assuming no friction).

In those terms what is the period?

Are there any symmetries you can use, given the information you already have?

16. Aug 11, 2009

### bm0p700f

Re: pendulums

Well what is the definition of the the Time period of an oscillator (in this case a pendulum). Google it if you don't know or one of your text books. Without that clear in your head it is difficult for us to help you.