Area Under a Graph (excel)

Nitro_Junkie · 22 Mar 2007 at 14:46

in short how do I do this?

need to work out the area (+ and -) under the graph to get the co-efficient of pressure drag

OzyOly · 22 Mar 2007 at 14:52

Nitro_Junkie said:
in short how do I do this?

need to work out the area (+ and -) under the graph to get the co-efficient of pressure drag

Don't suppose excel can work out the equation of that curve? I've nearly finished my A-level maths so could work it out. But I doubt the equaiotn of that curve can be worked out.

Nitro_Junkie · 22 Mar 2007 at 14:59

lecturer said something about using mid-ordinate system? so wouldn't be perfectly accurate but give a good idea, just don't know how to do it

sassora · 22 Mar 2007 at 15:02

How accurate do you need it?

You could use the trapezium rule or trapezoid method seperately in each +tive and -tive section.

See http://www.duncanwil.co.uk/areacurv.html

If you need it to be more accurate, you could find the points in between by linear interpolation (since your joining lines are straight). Too be picky, is it the linear connection accurate enough. If yes, then linear interpolation could be a possible option.

sassora · 22 Mar 2007 at 15:08

I reckon you +tive section has got enough points on there to give a reasonable estimation but probably not for the negative bits. It's up to your judgement really.

Good luck

Nitro_Junkie · 22 Mar 2007 at 15:14

trapezoid rule will be fine but I cant quite get my head round that tutorial

sassora · 22 Mar 2007 at 15:36

Area of a trapezium is 0.5(a+b)h

h here is the change between two consecutive time points
a and b are two consecutive temperature points

Time Temp Area of trap
0 100 0.5(100+71)1 = 85.5
1 71 0.5(71+50)1 = 60.5
2 50 ...
3 35 ...
4 25 0.5(25+12)2 = 37
6 12 ...
8 6.2 ...
10 3.1 ...

I think he has made an error for time = 4 onwards because the step sizes aren't the same.

In excel you might have something like

**** 0.5(B2+B3)(A3-A2) in the area cell C2

You can the drag it all down and the values will fill in.

(The numbers in the table start in row 2 in the example and time is in the A column and temp is in the B column)

sassora · 22 Mar 2007 at 15:40

Nitro_Junkie · 22 Mar 2007 at 17:23

thanks for your help mate, its appreciated

got it working and know roughly what I'm doing now just trying to get it sorted for the specific example