Why didn't my drink fall over?

19 April 2016

Question

I dropped my coffee and while the coffee shot everywhere, the cup itself didn't fall over and stayed completely still. Why?

Answer

Kat Arney put this question to Chris Smith...

Kat - Okay. So we are going to reenact your scenario. Here we have a plastic cup in the studio with some water and Chris is standing very close to me. Alright, here we go.

Chris - Drop the cup straight down onto the floor?

Kat - Yes. Wow. Okay that did not exactly work. So the cup kind of bounced on the floor, all the water has come out but, presumably Chris, if you had made a flat hit on the floor, would it have stayed upright - what are the chances of that happening?

Chris - Okay. Our experiment is slightly cheaty because this is a little plasticky cup. It's not a decent, high density, heavy cardboard cup. But, the fact is, it's all down to basic physics. If you have got a cup and you have got a liquid in it, you're going to hold that cup upright, and you can see whether it's upright by whether the liquid looks straight in the cup because you don't want to spill your coffee. If you then drop the cup; if you drop it having held it upright, then the only force acting on it is going to be gravity, which is going to pull it down in a straight line. It's therefore likely to hit the ground in a straight line and, as Isaac Newton taught us, everything is going to keep at a constant velocity unless some kind of force acts on it. Now if there's no force pushing the cup to one side or the other, then actually all the force is the acceleration due to gravity straight down. If the cup lands straight on the ground - now it's got a lot of kinetic energy and, therefore momentum. It's going to hit the ground, there's going to be compression of both the liquid by itself, hitting itself, and the cardboard in the cup. There will probably then be some elastic recoil as the cup goes back straight again, this will apply a force back on the liquid. Because the cup's circular, all things being equal, it will have stretched the cup outward when it landed, so the cup will rebound inwards and it will just squirt the liquid straight back up symmetrically in a column up in the air. So it should all coming flying out all over the lady next door and possibly all over you Kat. We just probably didn't drop this cup very well.

Kat - Also we've dropped it onto a carpet and a carpet isn't necessarily a flat surface.

Chris - Ah, but it is only water - not coffee. So it will be alright.

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