Think Like a Physicist

Nov 19 2013

This week DARTofPhysics asked ‘How many leaves fall in Dublin every Autumn?

Every October I put a question like this to large first-year classes. The response is always similar: a look of complete confusion as young minds rifle through the years of rote-learned science hoping to recall the required formula to answer this question perfectly. When I tell them there is no defined way to answer these questions it seems to make matters worse. So, armed with a calculator and a piece of chalk we start to build an answer.

To answer questions like this, you need to make guesses and assumptions. While a great deal of the public view science as definitive or absolute, scientists often rely on assumption and estimation. It is their ability to quantify that uncertainty that matters. If you wish to increase the accuracy of your answer, then you can be more exact in your estimation – the accuracy of the final figure depends on how you intend to use it.

For instance, if you wanted to gauge the size of the crowd at Croke Park for the All Ireland final, you might be sufficiently happy to hear that there were 86,000 people present. But 86,000 spectators would be an estimate that neglects fine detail like people leaving early, or perhaps leaves out the players and park staff.  But you do not need to know the exact number of attendees to gauge the size of the crowd.

Scientists use this sort of approach to many aspects of their work. Simplifications make the otherwise improbable, possible. Whether it’s estimating how much heat people give off sitting on a DART carriage or how leaves on the track affect the ability of the DART to grip the track, scientists use estimation to work out ball-park figures.

Have a think about some of these:

Q. If all the people in Dublin were to stand as close as possible to one another, what area would they occupy?

Q. How many cups of water are in a full bath?

Q. How fast does your hair grow?

Q. What is the weight of all the rubbish you throw out in one year? What about all of Dublin?

So, back to my first year class. I told you that each year the class look at me aghast when I ask such random questions. What I haven’t told you is how they react when they realise that they too can easily work out solutions to the seemingly impossible. Tutorials and classes are noisy engines of creative thought as students use mathematics and physics concepts as tools to figure something out. It’s a central part of learning to think and work in a scientific way – give it a go! 


School of Physics & CRANN, Trinity College Dublin

Dart of Physics Blog


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