Methods of measuring 'g'

I need to find information on the possible methods available to measure acceleration due to gravity (symbol g). Some ways are: strobe flash photography, a ticker timer, and a data logger. However, i cannot find any sites with much information on these.

Does anyone know any sites, or has some information?

Many Thanks

:confused:

Reply 1

teachercol

Most accurate method in a school lab uses the period of a simple pendulum

Reply 2

iceman_jondoe

^^ yeap pendulum is best way

Reply 3

Eau

Pendulum Method

T = 2 pi sqrt(L/g), where T = period, L = length of pendulum

So, squaring both sides gives

T^2 = (4 pi^2)/g * L

Measure the period for different lengths of pendulums. Then plot T^2 against L.

Find the gradient, and then you can use this gradient to work out g.

Feel free to ask if you need more help.

Reply 4

phys981

teachercol

Most accurate method in a school lab uses the period of a simple pendulum

Ooh, I don't know - last year our 2 AS groups got 9.9 and 9.7 using the freefall method with a little orange unilab timer.

Yes, I was amazed aswell.

Reply 5

teachercol

Yeah - but you can get 9.81 to 3sf accurcacy with a pendulum based experiment.

Reply 6

phys981

Ah well, the pendulum doesn't come into our course until A2 anyway, so we´ll stick to practising the equations of motion.

Reply 7

AlphaNumeric

Since dropping a ball is very quick and short of using light gates hard to decent timing, you could roll a marble down a ramp. Using trig you can work out the value of g because you know the distance a marble rolls, the angle of the ramp and the time it takes.

I think Galileo might have done somethign similar (I'm stealing what I think is his idea :wink:

)

Reply 8

phys981

The light gates are a nightmare with the the freefall method. You do it instead with a digi timer that starts when the ball is released and stops when it hits a landing plate.

if you want to use light gates you can measure the acceeration of a trolley down a ramp and then assume that this is the along-ramp component of g (ignoring friction, of course).

Reply 9

sebbie

teachercol

Yeah - but you can get 9.81 to 3sf accurcacy with a pendulum based experiment.

nice, your experimental technique must be much better than mine!

Reply 10

Theorist

sebbie

nice, your experimental technique must be much better than mine!

That's the beauty of the simple pendulum - you don't have to be a great experimental physicist to get accurate results. Just make sure:

- The amplitude of the pendulum isn't too large.
- The pendulum travels in a straight line, not an elliptical orbit.
- You measure the length of string from the centre of the bob (i.e. the centre of mass of the pendulum, as you assume the string has no mass)
- You use your stopwatch correctly

That's it. And you get good results. I remember a boy in my GCSE set who got a perfect correlation for his coursework and he didn't cheat. We even tested his results using a Spearman Rank, and he got 1 (which means perfect correlation).

Reply 11

teachercol

You need to use a LOT of swings. 100 is good. This means that you need a long piece of very thing thread and a heavy bob.

Reply 12

phys981

teachercol

You need to use a LOT of swings. 100 is good. This means that you need a long piece of very thing thread and a heavy bob.

Good grief.

Just make sure you have more than one person counting. Just in case.

Reply 13

ultrapower

Don't forget the theoritical formula---g=2h/t^2
To derive it, use any of your practical methods that use a ball which is dropped from rest and then use---
s=ut+1/2at^2
a=g
s=h
u is 0 (because dropped from rest) and therefore, this is the forumula
h=(gt^2)/2 (u is 0 so u * t is zero, so no point writing that)
2h=gt^2 (cross multiplied)
2h/t^2=g (cross multiplied)
g=2h/t^2 (reversed, for clearer understanding)

regards,
ultrapower.

Reply 14

Theorist

teachercol

You need to use a LOT of swings. 100 is good. This means that you need a long piece of very thing thread and a heavy bob.

Ideally, you could put the pendulum in an evacuated chamber and use light gates to measure the time period of oscillation.

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