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Topic: Determining accleration due to gravity (Read 11 times) previous topic - next topic

AWOL

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a) Why?  A swinging chain keeps time.

That would be a compound pendulum.
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liudr


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with a known string length L

A pendulum should be rigid.


It just needs to be light compared with the bob and not springy. And the angle of oscillation be quite a bit less than 50 degrees with vertical.

wwbrown

Why is the photogate method I described a far easier method of measuring gravity than an accelerometer?

Well there was quite a bit of discussion about using cheap off the shelf accelerometers and I never did see anyone having a solution that actually did measure the acceleration due to gravity, without first assuming what it was to begin with.  By using the photogates you can easily derive the solution for the acceleration due to gravity based on first principles, and not by assuming what the result is you are looking for.  The photogate method is used in first-year physics labs everyday, I used to have my first year students derive the solution for homework, tests, or labs.

liudr


Why is the photogate method I described a far easier method of measuring gravity than an accelerometer?

Well there was quite a bit of discussion about using cheap off the shelf accelerometers and I never did see anyone having a solution that actually did measure the acceleration due to gravity, without first assuming what it was to begin with.  By using the photogates you can easily derive the solution for the acceleration due to gravity based on first principles, and not by assuming what the result is you are looking for.  The photogate method is used in first-year physics labs everyday, I used to have my first year students derive the solution for homework, tests, or labs.


First principle is not always easy. I consider pendulum easy. You need a cheap string, some weight and a cheap watch. There is no need to set things up and there is no concern for electronics not working. photogates are fine but not cheap or easy if you compare with yarn and some deadweight.

JimboZA

This reminds me of how we measured it back in high school...

It involved a battery operated bell and a huge long strip (as long as the drop from the window to the ground) of carbon paper with a weight on the end. Then dropped the weight while the bell was ringing, with the strip between the bell and the ringer lever thingy. The impact of the ringer made marks on the carbon paper, and of course the marks got further apart as the weight accelerated.

Measured the time it took to drop from the science lab window to the ground. Counted the total number of marks. That gave the number of marks per second or seconds per mark.

Measured the (varying) distance between marks. That, with the now known time between marks, gave the (varying) velocity at any instant.

And hence the acceleration....

But using a pendulum is by far the easiest way to do it... (As long as the bob is very heavy compared to the string, the whole mass may be deemed to be at the bob's CoG.) Beauty of the pendulum method is that since the period T is constant regardless of how wide the swing is, it can as suggested above be measured over 100s of swings and that reduces the impact of the reaction time when the stop watch is started and stopped.

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