Frequency of a Pendulum

[G1SG2]

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Okay, this might be a REALLY dumb question.

If you're told that a pendulum passes through its initial rest point every 0.1 second, what would be the frequency of its motion?

Let's imagine the start point to be the point all the way to the left, where the bob is at it's maximum displacement. Here's an animation:
http://en.wikipedia.org/wiki/File:Pendulum_animation.gif

Now, if it takes 0.1 second to pass through the equilibrium/middle/rest point, won't it take another 0.1 second to reach the other end, resulting in a total of 0.4 seconds (0.2 seconds for half the cycle)?

For example, bob at left---(0.1s)rest/middle point---(0.1s) bob at right. That's HALF a cycle-taking 0.2 seconds.

Then we'd have bob at right---(0.1s)rest/middle point---(0.1s) bob at left. This is one the second half of the cycle taking 0.2 seconds for a total of 0.4 seconds.

Wouldn't that be a total of 0.4 seconds to complete 1 cycle, and then we can get the frequency by using 1/T? Why is the answer 1/0.2s and not 1/0.4s?

The question is about a pendulum block that is swinging back and forth in SHM after being shot by a bullet (completely inelastic collision). The block is a rectangle and is suspended by two vertical cables.

 

[Dr Gerrard]

So it starts at the very right.

It arrives at the center in 0.05 sec.
It arrives at the very left in 0.05 sec.
It leaves the very left and arrives back at the center in 0.05 sec.
It arrives at the very right from the center in 0.05 sec.

Now if you look at the above, you can see that this passes through the center every 0.1 sec.

 

[G1SG2]

So it starts at the very right.

It arrives at the center in 0.05 sec.
It arrives at the very left in 0.05 sec.
It leaves the very left and arrives back at the center in 0.05 sec.
It arrives at the very right from the center in 0.05 sec.

Now if you look at the above, you can see that this passes through the center every 0.1 sec.

But the problem says it passes through the center every 0.1 second, not 0.05 second...

 

[hqt331]

Think about it. If it goes from the left to the center in 0.05 s, it will pass through the center every .1 s.

 

[thebillsfan]

that means the period is .2 seconds...the frequency is 5Hz

 

[amine2086]

The pendulum passes twice through the center in a period. Thus, period = 2 X time it takes to pass through the center. Period = 0.1 sec X 2 = 0.2 sec. Frequency = 1/period = 1/0.2 sec = 5 Hz.

 

[capostat]

If you're told that a pendulum passes through its initial rest point every 0.1 second, what would be the frequency of its motion?

and then you said we started on the left?

so i think you just described the duration of 1 cycle.

if it starts on the left at maximal displacement, and then comes back to its initial rest point (the left) then the period is 0.1 seconds per cycle. right?

was that the question exactly as stated? because a 0.2 second cycle would make sense if it took 0.1 seconds for the pendulum to reach the right side, and thus 0.05 seconds to pass through the equilibrium position.

 

[loveoforganic]

This thread makes me dizzy.

 

[G1SG2]

This thread makes me dizzy.

lol, haha. I don't like that question 🙁