Dr Chris Smith and Georgia Mills, The Naked Scientists
At the Naked Scientists no question is too big or too small. We had lots of fun at easter with our eggcellent easter special and possibly everyone’s favourite moment was when Georgia persuaded Chris to, quite literally, stand on eggshells…
Georgia - We’re going to test the strength of eggs today and see if they together can support your weight. So, I've got a standard 6 eggs carton on the floor with 6 eggs in it. I haven't done anything to them. I've checked that there aren't any cracks in them and that's all. You're going to stand on them now.
Chris - This is this sort of cardboard carton that you'd get from the supermarket and there are 6 rather nice looking eggs in there and you want me to just stand with one foot to stand on that.
Georgia - Yes. It will be a shame if this goes wrong, but I have faith in the structural integrity of these eggs.
Chris - I have taken my shoe off. I'm putting my socked foot on top of the eggs in the egg box.
Georgia - And now, I want you to make sure your weight is evenly distributed across all of the 6 eggs.
Chris - Okay, I'm going to put my arm on you, just so I can just stay stable.
Georgia - Okay.
Chris - Okay, you wouldn’t believe it and I'm very nervous doing this, but I'm standing on one leg, on one foot, on a box full of eggs. I'm genuinely doing that and they haven't broken.
Georgia - I'm just as surprised as you I think.
Chris - I'm very relieved. Honestly, I thought that my feet were going to go straight to them and I was going to have a very eggy sock.
Georgia - That’s the amazing property of an egg. If we did this on their side, this wouldn’t work at all. Do you want to try that Chris?
Chris - I'm not doing that, but let’s just grab an egg. So, these really are genuine eggs. They're not hardboiled or anything. I can shake it. I can feel the sort of yolk and stuff slushing around inside. So, what's special about this that I can stand on it in that way?
Georgia - Eggs have a really good shape for this kind of compression. If you look at them, they're ovals. They're like an arch shape and we use this kind of shape in architecture all the time. you think about bridges, you think about domed ceilings. If you push on the top of an egg because it’s such a narrow arch, the force is distributed all throughout the body of the egg. Meaning, it won't break. However, if you put a smaller force on the long edge of the egg, this actually squeezes in the inside of the egg as well and the eggs are really bad withstanding this kind of pressure.
Chris - What you're saying is, this egg is – because of its shape, when I apply force to the curved top of the egg, basically, I'm compressing all of the material in the egg in all directions. It’s sort of transferring the force down all of the sides evenly whereas if I turn the egg on its side and I were to stand on that and I'm not going to do it – much to your disappointment, I won't be doing that. But if I were to stand on the side of the egg, then I would be bending the shell a bit and there wouldn’t that transfer of load all over the egg. It would just basically bend the shell and it’s not very strong when you make it stretch. It’s going to bust open.
Georgia - Exactly, which is why when you crack an egg, you do it on the side. You'd never do it on the top.
Chris - Did eggs evolve to have this shape for that reason? Is that why the egg has got this property to stop the chicken literally cracking its own eggs?
Georgia - That's right. So, when a mother hen sits on her eggs, the last thing she wants to do is to squish all her babies. So, with these eggs pointing upright, it distributes her weight all around. But then when these tiny weak chickens need to get out, a small force on the side of the egg enables them to crack out of the egg.