# Free body diagram of pendulum

1. Nov 30, 2008

### -EquinoX-

1. The problem statement, all variables and given/known data
I am asked to draw a free body diagram of a pendulum and a bob with it's maximum
amplitude of 30 degrees. Below is my attempt, I just forgot to say that theta is equal to
30 degrees, other than that is it right?

2. Relevant equations
none

3. The attempt at a solution
attached as a picture

2. Nov 30, 2008

### Staff: Mentor

Looks good to me.

3. Nov 30, 2008

### -EquinoX-

reason I am not so sure is because of the amplitude of 30 degrees, what does that mean?

4. Dec 1, 2008

### Staff: Mentor

The amplitude is just a measure of how far the pendulum bob swings away from equilibrium (the vertical). An amplitude of 30 degrees means that the pendulum swings until it's 30 degrees from the vertical before swinging back.

You've reflected the angle in your diagram, so you're OK.

5. Dec 1, 2008

### -EquinoX-

ok, and the period of this pendulum is basically the time it takes the position I move the pendulum 30 degrees away from the equilibrium then I release it and it goes back again to the same position right?

6. Dec 1, 2008

### kevtimc

I think your theta should be flipped to the bottom. Currently, your amplitude is 150$$^{o}$$.

Last edited: Dec 1, 2008
7. Dec 1, 2008

### Staff: Mentor

Right. The period is the time it takes for one complete cycle. If it starts out to the right at 30 degrees at t=0, then in one period it will have gone to equilibrium, moved to the left at 30 degrees, come back to equilibrium and then back to the starting point at 30 degrees to the right.

8. Dec 1, 2008

### Staff: Mentor

No, the angle shown is the correct amplitude.

9. Dec 1, 2008

### -EquinoX-

ok, thanks that helped a lot... so in one period it passes the equilibrium point twice then

10. Dec 2, 2008

### kevtimc

EDIT: Nevermind, I see the image now, and I missed the T part. It's correct.