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!



All your education are belong to us..

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1393060065639What should you do when parents have unrealistic expectations of their children? Recently at school the students received their IGCSE and AS level results. There was very large crowd outside of the principal’s office, crammed full even, with lots of expectant children and parents. Many of the students were being congratulated as they received their certificates and one of the girls fainted when she got her results. One parent asked me if I thought their child would get an A or B for their AS level but as that student only got a C grade at IGCSE, the level of expectation was simply way too high. It is always difficult to know what to tell a parent when they simply have too much expectation of what their son or daughter is capable of but the simple fact is students generally do not do better at AS level than IGCSE because the subject level is so much harder.

Recently I went to my own daughters parents evening. She has just started grade R. Some of the parents in the meeting wanted to know how quickly their child could be moved into grade 1. The principal quite rightly responded that with differentiation in the classroom that student would still be pushed and it is not necessarily wise to jump up a grade. One of the parents would not give up and was convinced that their little darling was a genius. The honest truth is that most children are just average at most things and with time develop specialist skills with positive feedback from parents and teachers playing an important guiding role. What’s the rush to get a child through high school? School is not just about grades and academics, perhaps more importantly it’s about social interaction, making friends, understanding how to interact with other people. Children should always be encouraged and supported throughout their education but over-expectation is dangerous, puts stress and pressure on the child and must be avoided at all costs.

I HATE exams!

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I HATE exams! I probably despise them even more than my students. They are probably the worst kind of assessment going and yet we rely on them so much. There is no situation in the employment world where you have to work individually and secretively, you’d be fired pretty quickly if you did! In the world of work you have to collaborate and be innovative, using key skills and knowledge to solve unique problems – how could we re-invent what is going on in the classroom to reflect this?

When it comes to revision time I have a flock of students around my desk asking for past papers. They know there is a chance that what2013-11-18 08.26.09 has been covered in the past might well come up again, which of course it does. We run Cambridge international examinations here at my school and of all the exam boards I have worked with they do seem to push the key skills element of examinations and test if the student can apply the knowledge they have learnt to a different scenario but is this still enough? People have bad days, (I should know) and is it fair to test someone for one hour on about 200 hours’ worth of material? I don’t know if it but I can’t think of an alternative. I wish there was some kind of practical examination where the students get dropped in a room with some basic supplies and they have to use their physics knowledge to build a Rube Goldberg machine to get out! I guess the closest I’d ever seen to this is the physics game – which I strongly recommend. Can you imagine getting an IGCSE for your ability to play physics computer games, that’s the kind of syllabus I want!

I’m a student as well as a teacher

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I’ve always loved taking things apart. I remember being about 6 years old and re-wiring the bike shed with wires swinging from overhead – luckily there was no electricity applied! Later in my teens I took apart my dad’s PC and unfortunately couldn’t remember the order in which I had dismantled it. I always remember that my dad didn’t seem to mind too much and in fact commented that he had done exactly the same with my grandmothers TV when he was younger. I guess tinkering has been in my blood since birth. Maybe be that’s where my love of Physics came from. With Physics you can take apart the universe and have a look at how it works, and so far without breaking it!

I’m very lucky that I havemy profile pic a job that I enjoy rather than endure. I’m not saying that teaching doesn’t come with its grinds, it can be frustrating at times but when you have all your students around you in the laboratory and you are demonstrating something exciting, the “oohs” and “arrrs” come out. You can see the students “getting it” and the light bulbs in their minds are switching on. It’s a very privileged position to be the first person to show someone how something works. I’m lucky that here in Africa I am teaching in a school that understands the importance of practical laboratory time. I have seen so many schools here that do not have a laboratory or even any real practical equipment. No simulation or reading from a textbook can beat actual tangible experience of experiments. An educational sales rep came to the school the other day to try to sell a 3D projector to us. She commented that “with this there would be no further use for the laboratory and that we would save so much money from not having to buy chemicals”. I love ICT in the classroom, it saves me a whole lot of time but replacing experiments it will never do!