Pendulum periodic motion; period parameter

[**Mariam**]

Homework Statement

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?

Homework Equations

T= 1/f
T=sec/cycles
F= cycles/sec

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

 

[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?

 

[**Mariam**]

I

"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?

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)

 

[ehild]

If the bob starts from right max then it takes 0.1 seconds to reach equilibrium

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."

 

[**Mariam**]

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."

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.

 

[ehild]

Read the problem text carefully.
Remember a pendulum or an oscillator goes through the equilibrium point twice during a whole period.