Ben Craven - Magnitudes

This week, we had Ben Craven providing a very interesting lecture on gauging the feasibility of ideas. Often it is hard to get a scale of something - for example, how much does it cost? Precise numbers may be difficult to work out, but sometimes you can work out a good estimate - is it £100 or £100,000?

The point here is to start to put numbers in to an idea to quickly find out if the idea is feasible or not. Ben pointed out how he has seen final year students nearing the end of a project who still don't have a vague idea if their idea is feasible or not!

Exact numbers are not needed - usually educated guesses will give and answer at least in the right order of magnitude.

Example - can we get power from closing filing cabinet drawers?

Let's assume that the drawer weighs 20kg (this is being deliberately optimistic.)

Let's say the drawer moves at 1m/s.

Using Ke = 0.5*m*v^2 gives 10J per drawer close.

Let's assume (optimistically again) that the drawer is closed every 5 minutes for 8 hours every day. This is about 100 times per day, so the total energy is 10J*100 = 1kJ.

Dividing this by the time in a day gives a power output of 0.01W.

So even with optimistic assumptions, and assuming that the process was 100% efficient, we still get a useless power output, and the idea is not viable.

It works the other way too - if you use pessimistic assumptions and still get good results, the idea is likely to be viable.

Ben also raised the point of the difference between accuracy and precision:
Accuracy is how close you are to the actual answer;
Precision is the "smallness" of the units used.

Using the same techniques, the class worked out approximate values for a wide variety of things in the lecture. These included the number of breaths taken in a year, the mass of 1km of road and the steps taken to walk 100 miles. More worryingly, the area of land needed to grow enough biomass to power the UK worked out to be roughly the same as the area of the UK. Similarly, we would need to cover all of the land of the UK with wind turbines if we were to power the country with them alone, and this still probably wouldn't be enough.

Overall this was a fascinating and useful lecture in how to see if an idea will work - an extremely useful tool for a product design engineer.