# Pendulum periodic motion; period parameter

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1. Oct 11, 2015

### **Mariam**

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

A pendulum with a mass of 0.1 kg was released. The string made an angle of 7 ° with the vertical. The bob of the pendulum returns to its lowest point every 0.1 seconds.
What is the period, frequency?

2. Relevant equations
T= 1/f
T=sec/cycles
F= cycles/sec
3. The attempt at a solution

I am really confused here s the period 0.2 seconds of 0.4 seconds? Because it is really ambiguous what this 0.1 second refers to.

Some questions I do in class turns out to be four times the given time period and others is only twice. How do we know the difference?

And the real answer is 0.2 sec

2. Oct 11, 2015

### ehild

"The bob of the pendulum returns to its lowest point every 0.1 seconds."
Think: Assuming the bob starts from the left highest point. What time we call period? How does the bob move during one period? How many times does it go through the lowest point in one period?

3. Oct 11, 2015

### **Mariam**

I

If the bob starts from right max then it takes 0.1 seconds to reach equilibrium then another 0.1 section move to left max then back to equilibrium in 0.1 sec then back to right max (which completes a period)

4. Oct 11, 2015

### ehild

No, 0.1 s is the time from equilibrium to back to equilibrium. "The bob of the pendulum returns to its lowest point every 0.1 seconds."

5. Oct 11, 2015

### **Mariam**

Oh ok I think I get it.
But then how do you know that and be able to apply it to future problems? What can help one indicate whether it is like what I said or like the answer you said.

6. Oct 11, 2015