Monday, 24 September 2012

The Area Under A Distance-Time Graph

Teaching Mechanics

I love teaching A level maths, but I must admit to feeling a bit disappointed when I found out I would be teaching the Mechanics module (M1 from MEI, fact fans) because... Should I really be saying this? OK, I'll just say it: I find it boring. No, that's not the whole truth, I'm making excuses now. I have to admit: I don't always get it.

I did Mechanics 1 at A-level (also using the MEI exam board) and that's the full extent of my knowledge. No Mechanics 2, none at Uni, I didn't do Physics A level or even separate Physics GCSE. I've never really been a "scientist", preferring to see myself as a creative type. I was always better at English and French than at science.

Anyway, this year (2012) I've been team-teaching M1, sort of. I've basically been watching someone else teach it. And suddenly I'm learning things I never really understood the first time round. I'm starting to get it!

The Area Under a Distance Time Graph

But then I went and spoiled it all by doing something stupid like Googling "What's the area under a distance-time graph?"

I would hope that those reading this are aware of the concepts illustrated below:
differentiate displacement and you get velocity. Differentiate velocity and you get acceleration. Integrating goes backwards.

When you draw a velocity-time graph, for example, the area underneath the curve gives you the displacement, and the gradient of the curve gives you the acceleration.

The other teacher of the class posed the question to the class "What's the area underneath a displacement-time graph?". My mind was immediately blown. With a bit of jotting down (the diagram above), I could quite easily work out that the units had to be ms (metre seconds, a bit like kilowatt hours), so I knew it was something to do with distance multiplied by time.

I knew that what I was looking for would be the blank in this sentence: "displacement is the rate at which BLANK changes". What could fit? Nothing seemed intuitive.


That was where that much-relied on search engine came in. After a bit of research (Googling), I found that the word I was looking for was "absement" (oh of course, you all exclaim) and that I was by no means the only geek in the world wondering what it was.

Absement is a port-manteau of the words absent and displacement (knowing which makes it no easier to understand) and there are not many results on Google for it (the eighth result is an English to Urdu translation page. Erm, cheers for that).

I will attempt to explain absement using an example. For another example, visit Wearcam.

You live 2km from school. You walk to school in 30 minutes, stay there for six hours, then return home, also taking 30 minutes.

A displacement-time graph would look like this:

Now let's consider the area under this graph. It would be given by integrating the curve above. You can sketch this curve easily: in the first section, the gradient is constant and positive, so the corresponding bit of our new graph would be increasingly increasing (curving upwards). The middle bit has zero gradient so our new graph will have a constant positive gradient. The last bit is the opposite of the first bit, so our new graph's last section will have a decreasingly increasing bit.

My sketch:

The next thing to do is calculate the numbers on the vertical axis.

Your absement at any point is given by the (average) distance you are from home multiplied by the time that you're there for.

So when you've just arrived at school, your absement for that point is 1000m (your average distance from home) multiplied by 1800. So that's 1 800 000 ms. Up to that point, your absement has been increasingly increasing. When you're halfway to school, for example, your average displacement was 500m, your time is 900, so your absement is 450 000ms (note that this is not half of 1 800 00ms). At the end of your second part (when you've just finished school), your displacement has been constant at 2000m, your time has been 6 hours which is 21600 seconds, so your absement at that point is 43 200 000 ms. You can calculate your absement at many different points so that you can plot a nice, smooth curve. It looks a bit like a cumulative frequency curve. I'll leave this to you as an exercise.

What's the point of absement?

Well if you were on a spaceship and you had some kind of mobile communication device, you might imagine that the further from Earth you are, the more power the device uses. Therefore you might measure its battery usage in metre seconds.

Emma x x x


  1. Nice handwriting.

  2. really great! Thnx I was find the same stuff>>>>>>

  3. Looks great. One slight improvement: if you reverse the right and left sides of the 2nd plot and keep the middle as-is, you'll have a more smooth absement plot.

    1. Ah true, but then wouldn't time be running backwards? That would just freak me out.

  4. Since the inertial frame of reference, or "perspective", is implicit in the above discussion I thought it'd be appropriate (and amusing) to share this quote:

    "Geography is just physics slowed down, with a couple of tree stuck in it."

    Terry Pratchett


    I'm studying for the MCAT and came across a passage in my physics book that reads, "The area beneath the curve has no meaning for a displacement versus time graph." Googling lead me here.

  5. Love the quote!
    I can't believe your physics book says the area has no meaning! OK, absement isn't exactly useful but it's still something.

  6. Emma,
    First of all --thanks for your help--I googled it too! LOL Second of all if your hair is falling out at an alarming rate--have you considered that it might be hormonal? Do you have other hormonal symptoms. I have a natural answer for you that has helped me and a lot of others. If you are interested let me know and we can have a personal conversation :)

  7. Thanks a lot for this info :) You explained the issue to me perfectly.
    If I were to explain this to someone, I would call it a unit for homesickness! Im sure you can see why :)

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  10. How about 'wear' as a name for absement? If you keep walking round in circles you would be worn out?
    If I remember, there is a thing called 'entropy' which always increases and progresses, never reducing. A measure of disorder in a system. Not the same units but is it a similar concept?

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