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Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Thursday, 26 May 2011

SI Units: The Secret Weapon

When faced with a list of equations “vital” for the upcoming test, I used to panic quite a lot. You see my memory for these things has always been terrible, as if my brain knows what I should be remembering and seems to purposefully mix it up. Flash cards, reciting by rote and repetitive writing out of the equations in question have all taken up vast amounts of my time with very little success, so I though I’d share my secret trick with you.

I’ll admit right off the bat that it’s not a magic pill that will get you out of doing any work, it’ll hopefully just help when you can’t remember the vital equation mid-exam. Because as we all know, sometimes you can get a mark just for writing down the correct one.

So what are SI units? Well these are the base measurements that we talked about in the “What is a kilogram” blog a few weeks back, and in particular they are the units of these measurements. So seconds (s) metres (m) kelvins (K) kilograms (kg) and amperes (A) are probably the only ones you’ll need here. All other units are derived from these and with a few equations you can break down the stranger units like Newtons (N) and Joules (J).

Then you’ve got to really read the question, because the clues will all be there, what you’re trying to find, and the data that you’ve been given to work it out, units and all. If I think I remember the equation then I’ll write it down and work out the answer and see if it seems sensible. But that’s a big IF in there and more than likely, I’ll need to use my fall back plan. That is, to look at the units and see if I can work out the equation. Alternatively, you can just use this method as a fail-safe check on your results so you can leave the test feeling smug!

Let’s take an example here, using potential energy, which is the energy you give to something when you raise it off the ground. That object then has the potential to fall back down to earth, hence the name.
In the example, the ball, which weighs 200g (changed to standard units this is 0.2 kg) is at the top of a 6m high wall. What is the potential energy? Take gravity to be 10 ms-2.

Uh-oh, I have no idea what the units are for potential energy, but luckily one of my fall-back equations (see, I said there were no short-cuts, just every now and then a couple of equations should stick, and this is how to use them to your advantage) is to do with work because this seems to make sense to me.

So now we know the units we’re aiming for we can start to sort out our equation first by matching up the units and moving them around until you get what you're after, then it’s nothing but child’s play to put the numbers in and get the answer. Don’t forget to include the units in your result, you can get marks for them even if you happen to get the wrong answer from your calculator by, er, mashing the wrong buttons. 

This might seem quite a long-winded way of getting to know your equations but it’s really useful. So identify a few key equations that you can remember and get working on figuring out the rest with units! Good luck!

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