The Weekend Experiments – Gravity

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Do you remember those days at school when your physics teacher would tell you that the strength of gravitational acceleration is 9.8 m/s2? You just took it on blind faith that that was the strength of gravity right? Well, in the first of my Weekend Experiments series we are going to look at a very simple experiment anyone can do to find the strength of gravity in their local area (or planet). All we need is some string, a weight and your mobile phone to do some timings and a few calculations.


Myself and students of St Saviours High School, Leribe, Lesotho performing the Pendulum Investigation – 2010

What you need

A long piece of string – the longer the better

Somewhere to hang it from

A stop watch – just use the one on your phone

A weight – even a small stone will do

And a measuring tape or metre stick

Calculator – again use the one on your phone if desperate

What you do

Tie the weight to the end of the string and hang the piece of string up from the other end (remember the longer the string the better).

Measure the length of the string in metres to the nearest mm if you can.

Let the weight rest down and give it a little push – just a few cm will do.

The weight should now start moving back and forth in a pendulum motion.

Using your stopwatch we need to time how long the pendulum takes to go back and forth (We call this an oscillation). Human beings don’t have great reaction times so I suggest you time 10 swings or more in one go and find an average.

The Calculation

So here comes the maths – don’t freak out.

g = L4π2/T2

L is the length of your string in metres

π is that circle thing equal to 3.142 (approximately)

T is the time taken for one swing

An example

L = 3.56m

T = 3.8 seconds

g = 3.56 x 4 x 3.142 x 3.142 / (3.8 x 3.8)

and I get..

g = 9.73m/s2

not a bad answer with just some basic equipment!



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