# Simple Pendulum

1. Apr 12, 2006

### lydster

A simple pendulum is 8.00 meters long.

(a) What is the period of simple harmonic motion for the pendulum if it is place in a truck that is accelerating horizontally at 2.00 m/s^2

There were 2 other questions with accelerating upward and downward in an elevator, and those numbers I either added or subtracted to the gravational acceleration. And that gave me my right answers, but I don't know what to do with the horizontal acceleration.

Last edited: Apr 12, 2006
2. Apr 12, 2006

### Staff: Mentor

Well don't just add or subtract. How do the directions of the accelerations affect the pendulum mass? What determines the motion of a pendulum mass?

3. Apr 12, 2006

### lydster

I'm using the equation T= 2 pi square root of (L/G) and I don't know what I'd do with the horizontal acceleration. Because for vertical accleration I just added to G like 9.8 + 2.00 m/s^2 or 9.8 - 2.00 m/s^2 and that was correct for my other two parts of the problem. The mass is not supposed to have anything to do with the problem I guess.

4. Apr 12, 2006

### Staff: Mentor

Did your instructors (or the textbook) give you the derivation of the "canned" pendulum oscillation equations? This question may be kind of hard to address without the basic equations...

5. Apr 12, 2006

### lydster

No, just whatever I posted above is what he gave us. What do you mean by "canned"?

6. Apr 12, 2006

### Staff: Mentor

Often in physics classes, you will be given a formula to apply. But when you are given a different setup or initial conditions or situation, the canned formula will not apply. That is when you need to go back to the original derivation to figure out the answer for the new problem statement.

Like, in SHM instruction, you will end up with formulas for omega-0, damping factor, Q, etc. But what happens when there is lateral acceleration or viscous damping added in? In that case you need to go back to the original equations that you/others used to derive the expected behavior, and be sure that the new effects are part of the derivation.

This comes up a lot in my EE work. When you push the edges of datcomm stuff, with unique and varied comm channels, you need to be sure that you're still working correctly in your simulations. Like when you change your carrier frequency, several different things can happen to many of your variables in your simulations (and the resulting real world effects).

7. Apr 13, 2006

### arildno

lydster:
Try and formulate the equations of motion in the rest frame of the truck.
1. Is this an inertial frame?
2. If the pendulum is at rest relative to the truck, is the pendulum hanging straight down?

3. We generally derive the oscillatory motion of a pendulum by assuming small oscillations ABOUT THE EQUILIBRIUM POSITION OF THE PENDULUM!
In light of your answer to 2., how should you tweak your equations of motion to account for such small oscillations?

8. Apr 13, 2006

### lydster

I don't know if it's an internal frame. I think we're assuming it might be? Yes the pendulum is hanging straight down, because we're assuming again that it starts from rest.

I'm just not sure where to go from there.

9. Apr 13, 2006

### arildno

I asked you if the REST frame of the truck was an INERTIAL frame.
Do you know what those terms mean?
Wherever is that assumption stated?

Besides, I asked you about the pendulum's position when it was at rest with respect to the truck.

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