• Question: There is a car with all the windows and doors shut. A pigeon is flying around inside. Does the car way way more with the pigeon?

    Asked by 738fasp29 to Stewart, Miriam, Marton, Laura, Kathryn, David on 13 Nov 2018.
    • Photo: David Ho

      David Ho answered on 13 Nov 2018:


      This is a brilliant question. I think the answer is actually very complicated. Let’s imagine the car is on a set of scales and think about what we’d see.

      If the pigeon is still, the weight would remain constant because everything’s in “equilibrium” (this comes from Newton’s first law).

      If the pigeon jumps, without flying, the weight would rise at first because the pigeon has to push off the car/ground (Newton’s third law).

      But if the pigeon flies, it needs to interact with the air in the car, and this would make lots of very complicated air current motions we wouldn’t be able to predict. In fact, if anybody can solve the equations governing air currents, they can get a million pounds! http://www.claymath.org/millennium-problems/navier%E2%80%93stokes-equation

      I think what you would see is that the number on the scale would fluctuate up and down as the pigeon and the air in the car moves. But unfortunately I can’t tell you exactly what would happen, as then I’d be a millionaire!

    • Photo: Stewart Martin-Haugh

      Stewart Martin-Haugh answered on 13 Nov 2018:


      That’s a fun question!

      When the pigeon flaps its wings, it pushes the air down. The air will then move around the car a lot, as David says, so probably you would see some fluctuations.

      I think the question is easier with a helicopter – helicopters displace the air beneath them in a more regular way:

      so you would get a clearer reading on the scales.

      So overall, the car weighs more, but it would be harder to measure.

      If you like these kinds of questions, I can recommend https://what-if.xkcd.com/archive/ – it answers a lot of these questions.

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