Why Does Only One Egg Break
When You Smack ’em Together?
Why Don’t They Both Break?
by Dallas Doctor
If you’re like me, you love a good detective story… and this one’s a doozie!
It all started back in a year called 2002, in a place called Akershus County (that’s in Norway). A young student (let’s call her Karoline, because that’s her real name) was learning how to bake a cake.
“Well, I wonder why that happened?”
Karoline noticed that when she smacked two eggs together, only one of them broke. Karoline wondered “Well, why didn’t both eggs break?” So she tried it again with two more eggs. And then with two more. And then two more. The same thing happened every time. Only one egg broke.
So the next morning at school, Karoline asked some of her friends. They had never thought about it before, but they decided it was a fun and interesting question, so they asked their teacher. She, in a moment of inspiration, recommended that they figure it out…
[I gotta stop right here and tell you that this is one of my favorite stories. It turns out that the explanation is not as easy as you might expect.]
So what’d they find out?
I’ll jump to the end: Karoline’s class won Norway’s Nysgjerrigper Prize for 2002. And I, for one, am thrilled that they did, because they did a lot of good work, and learned a lot … BUT …
(Don’t tell anyone…)
They did NOT QUITE discover the answer!
They got close though, and they went about it in a scientific way.
They came up with hypotheses. They designed a number of experiments. They broke a bunch of eggs. They collected data. And had a lot of fun. And learned a lot.
But they missed it.
They missed it, because science is about MORE than lab coats and measurements and data collection and experiments. Science is a way to know things — a way to look at a the world — it’s not enough to just do experiments, you need to design the experiments that address your questions. And that’s where the kids got off track. I, for one, am glad they did, because it gives me the perfect chance to illustrate one incredibly important scientific idea. Here it is:
“Even when we think we get it right, we very often get it wrong, BUT THAT’s OK, because as long as we remain willing to test our beliefs, we still at least have a chance to find the answer. AND THAT’S HOW SCIENCE WORKS!”
By the way, there is no shame at all in the fact that Karoline’s class didn’t find the answer. I scoured the internet and found a boatload of scientists and engineers who fared no better.
So what did the kids do?
First, they wondered why it happened! They talked it over and came up with a list of possible explanations. Here’s their actual list:
Let’s call Them Hypotheses:
- If one of the eggs is moving faster than the other, then that is the one that will break.
- They cannot both break because the egg that gets broken is no longer strong enough to break the other one: the broken egg has become weaker.
- Perhaps it has something to do with the weight and size of the egg?
- One of the eggs may have a thicker shell than the other.
- All eggs are different, since they come from different hens.
- It depends on which part of the eggshells are knocked against each other (e.g. narrow end against narrow end).
Did you spot it?
This is where the kids went wrong. ONLY explanation #2 approached the question, (why does only one egg break, and not both?“); ALL of the other explanations are dealing with a very different question: “which egg breaks?”
There’s nothing wrong with that second question. (SPOILER ALERT: The answer is … “There is always a small differences in the structure of the shells and the weaker one will break.”) But that question (and answer) is nowhere near as interesting as the original question “Why Don’t They Both Break?”
If you threw both eggs against the wall, they would BOTH break. … Go ahead and test that theory if you want to… I’ll wait here…
Why does the wall break both eggs? Because of Newton’s Third Law! When each egg smacks the wall, the wall smacks back with an equal and opposite reaction (not good news for the egg)!
The video below is an excellent demonstration of Newton’s Third Law. (You’ve probably seen a toy called “Newton’s Cradle” before.) Notice that the momentum of the first ball (or two, or three, or four) is conserved after each impact. And that’s what we’re dealing with here – the Conservation of Momentum.
Have you figured it out yet?
Well, according to Newton’s Third Law, if you smack an egg with another egg, the first egg should apply a force on the second egg and also the second egg should apply an equal and opposite force on the first egg.
Therefore, if the first egg smacks the second egg with enough force to break an egg, then the second egg will always “smack-back” with an equal amount of force (also enough to break an egg).
So we would expect BOTH eggs to break. But that’s not what happens.
Do Eggs Defy the Laws of Physics?
Probably not … so what’s going on? …
It’s not always easy to design experiments to answer your questions. The path forward is not always obvious. The world can sometimes be messy and confusing. It takes imagination and skill and careful thought to come up with a way to address our questions.
So What did the kids do?
I’ll let them tell you:
[We] “… bought 20 eggs from one shop and 20 from another, so as to be quite sure that they came from different farms. Then [we] numbered the eggs at random and weighed them on a very accurate laboratory balance. To measure the length and width of the eggs [we] used something called a caliper gauge. And to measure the thickness of the shell [we] got hold of a micrometer (an instrument for measuring very small distances).”
Then the kids put all of the data into a table and began their experiments.
Back to the actual report:
“To test the first hypothesis (If one of the eggs is moving faster than the other, that is the one that will break) [we] chose eggs at random and smashed them together in various ways; narrow end against narrow end, blunt end against blunt end, side against side, narrow end against blunt end, blunt end against side and narrow end against side. So as not to miss anything, [we] videotaped the experiment. Every single time, the egg that was not in motion was the one that broke. So much for that theory!”
Notice that the experiments that the kids designed were ALL designed to answer the wrong question. And when I googled the original question, almost every response I found was also answering the wrong question. There was a lot of talk about “strength” and “hardness” and “thickness” and “stiffness” and “stress” and “tensil strength” and “brittle” and “ductility” and so on. But they all end up with basically the idea that “…the egg with the weaker structure will break.” But we already know that.
So lets go back and focus in on hypothesis #2, because, as far as it goes, it seems reasonable. (Remember, it was: “They cannot both break because the egg that gets broken is no longer strong enough to break the other one: the broken egg has become weaker.”) A broken egg certainly wouldn’t be much of a barrier to an unbroken egg.
But there’s a subtle problem with this hypothesis: It ALSO doesn’t address the question. It describes what happens AFTER one egg is broken and the other one isn’t. It assumes that only one egg will break, and then explains the condition AFTER that event. So in point of fact, explanation #2, in spite of being kind’a-sort’a true, is no explanation at all, because it never addresses the question: “Why don’t BOTH eggs break?”
In order to focus in on THAT question, we only need a little tiny bit more help from our old friend, Sir Issac Newton.
Newton’s Second Law states that F=ma (Force is proportional to mass times acceleration).
And remember, we’re concerned here with a mass (egg) that’s in motion, so we’re talking about momentum.
In terms of Momentum, Newton’s Second Law is written:
p (momentum) = m (mass) times v (velocity)
And since we have two eggs, we are talking about a system:
And whenever we crack two eggs together, we change the momentum of each individual egg, but NOT the momentum of the system, because momentum is Conserved).
[NOTE: While momentum and energy are always conserved, kinetic energy is NOT! Upon impact, kinetic energy can be converted into other forms of energy, like sound (crack) or heat (yes a little) or work (work transfers energy from one place to another or from one form to another, like from shell to broken shell).
Whenever we have a change in momentum, we call that change: Impulse (J).
Impulse is the change (J) in momentum (p) and is equal to the product of the average force (F) and duration (t=time).
(For you non-math-lovers, don’t be scared, just skip straight to the video below.)
But if we decide to do the math, then:
And the impulse is the integral of the resultant force (F) with respect to time:
- Wait! No! Don’t run away!
So as we can clearly see now, the key factor (for the egg) is TIME!
Okay, here’s a much better hint:
Here’s a FUN video of some 9th-graders playing with eggs:
- So you’ve probably figured it out by now.
Yes! We’re talking about “Impulse.” Remember that Momentum is simply mass time velocity (p = m • v). And Impulse is the change in an object’s momentum over time. So for an egg, the key factor is time. Specifically: the longer (time) it takes to change the egg’s momentum (impulse), the less impact (force per unit of time) on the egg.
Here’s a super-slow-motion video of a golf ball smacking into a steel beam. Notice how even a very rigid object like a golf ball deforms at extremely tiny segments of time.
(This same thing happens with eggs also. But while golf balls are extremely resilient, eggs are not. Eggs are much more brittle than golf balls. The first egg to break deforms and provides a blanket (longer time period — hence less force per unit of time) and cushions the blow for the second egg.)
Bingo! That’s It! The broken egg (with it’s deformation and moving shell fragments) acts like a blanket for the other egg, increasing the TIME of the impact for the unbroken egg.
So the moment of impact (time of the collision) is short for the egg that breaks, but long for the egg that doesn’t. This is because the direction (momentum is a vector) of the momentary deformation (before breaking) and then subsequently the fragmentation of broken egg (after breaking) both move with the unbroken egg in the same direction as the unbroken egg — thereby increasing the TIME of the impulse (like a blanket) for the unbroken egg. That’s why ONLY ONE egg breaks!
ONLY ONE EGG BREAKS and NOT BOTH: Because the first egg to break CUSHIONS THE BLOW (acts like a blanket) for the other egg.
It’s all because:
Oh, I’m so glad we figured that out!
What’s the point of all this?
The point is that there’s often a LOT more going on than we realize. There’s so much we don’t see, it’s hard to know which questions to even ask. Nobody ever said science was easy. Nobody said it’s not messy. Science is NOT (as some people imagine) a cold-clear-cut-linear-progression-of-ever-increasing-information-gathering. It takes imagination and induction, and yes, maybe even insight and inspiration. But in spite of all the hard work and confusion, it’s still the best — the only — way to figure stuff out. Anything else is just guessing … and then not caring whether you guessed right — or not.
Science kids — it’s where it’s at!
BONUS IF YOU”VE READ THIS FAR:
There’s more than one way to break an egg …
P.S. I’d love to hear what you think. Please leave a reply. As long as it’s not spam or trolling, I’ll happily post it here for all to enjoy! Thank you!
P.P.S. ALSO Here’s MY GoScienceGo Promise: If you EVER think I get something wrong — and if you can provide actual evidence — (not anecdote, but verifiable EVIDENCE) — of such a thing, PLEASE let me know. If your argument is valid, I’ll be thrilled to change my mind and acknowledge my error (because that’s the only honest way to be); all you have to do is give me a good reason…
For Further Reading: