What is the scientifically proven best way to skip a rock?
This is kind of hard to figure out
because every rock you throw is a little different,
not to mention your throwing motion might accidentally vary in between throws.
So to solve that I built a perfect rock skipping robot with my nieces and nephews.
So today I'm going to show you how we used what we learned from the perfect robot
to improve their throws on average from 3 to 13 skips.
Let's get started.
This is Lake Cavanaugh, it's located here in Washington State.
I came here every summer for a few weeks as a kid growing up.
and my memory of it is just this magical place to be outdoors and to be with family,
but to also discover my independence.
And of course to find a way to compete in anything,
[laughing]
including rock skipping.
And this is my first time back in 15 years,
so to commemorate that I wanted to build something really cool.
So we took a clay pigeon thrower and tweaked the spring,
and machined an adapter plate,
and then made some custom throwing arms from wood.
Then we created a base for stability.
And now all that was left
was to challenge my nieces and nephews
to a rock skipping contest.
Only I might have neglected
to tell them about my creation.
Actually forget this
[Surprised Yells]
You guys ready?
3... 2... 1...
[Spring Releases]
[Mark Cheering]
[Others Cheering]
So it actually wasn't that impressive right out of the gate.
But that was intentional,
because for this video I wanted to pull back the curtain
on some secrets about the engineering design process.
There are four steps I follow for any big project I've ever built
on this channel or in real life.
The first is research.
I will go online and look at any published papers
watch any videos on the topic
and then talk via Skype or email with any experts I could find.
That usually gives me a rough idea of where to start
and so the next step is to build a prototype.
A good prototype is cheap
and easy to reconfigure,
so you can try a lot of different things.
And then step three is to do
a sensitivity analysis with the prototype.
That means we try lots of different combinations
of settings to see what actually matters
and what doesn't.
And only at that point
are we ready to take what we've learned and move to step 4
which is building the final version.
[ting noise]
[screaming]
So often people want to jump straight to step 4
which usually won't work,
but it's almost worse if it does,
because you have no confidence
that you're close to the optimal design.
For the rock skipping robot
I'd already done steps 1 and 2
so we all sat down and
brainstormed about step 3
and all the things we could tweak
to find the right conditions for the perfect skip.
Eventually we came up with 4 different things to test,
but before we did that I was told
the robot needed to look a lot cooler.
[Music]
With the mission clearly accomplished
here are the four variables we decided to test.
The first was wrist angle of the robot.
This will change the angle of the rock relative to the water,
as you can see here.
So should it be perfectly flat?
Or really steep?
Or something in between?
Next was finding the right arm angle.
This changes the angle of the path of the rock
relative to the water.
So is this better?
Or is this better?
Or is something in between.
The last two variables had to do with the rocks,
but since every rock is going to be unique,
we needed a way to control this,
so we decided to make our own rocks
out of unfired clay.
That allowed us to make lots of samples
and to see how many more skips you would get
if you varied either the diameter or the thickness.
If you ever seen a primitive technology video
you know clay is just dirt and water.
This is great because it means it has a similar density to a rock,
and if you dry it in the Sun it's hard like a rock,
but then it only takes about 30 minutes
to totally dissolve away once you put it in the water.
Then we set up a testing plan
and for each throw we would change only one thing
and then count the number of skips.
That way if we suddenly saw a huge improvement
we'd know it'd be from the thing that we just changed.
That's where having a robot
is way better than using a human,
because it allows you to independently change one thing at a time.
And so as you can imagine we had a lot of skips that look like this...
or this...
or this [Scream]
But then we started to narrow in on some key parameters
and then they started to look...
more like this.
[Cheering]
[More Cheering]
[Music]
[Spring Clicks]
[More Music]
And I'm not exactly sure you count all those little skips the end,
but depending on that I would think we got some throws well into the 60's.
[Music]
And so in the end
we discovered there are four steps
to achieving the perfect rock skip.
The first is that you want the angle of your rock
relative to the water
to be about twenty degrees.
This is the optimal angle to allow the rock
to sort of crash into the water and then to make a ramp
and then use its forward momentum to ride up that ramp
and shoot back up out of the water.
And that's the exact physics that explains why rocks skip.
Next you want the path of your rock relative to the water surface
to be about twenty degrees.
This means you sort of need to start to throw high
which gives you some potential energy
to put into the system.
In the same way a ball bounce from this height will bounce more times
than a ball bounce from here.
Now before making this video
I assumed you wanted to throw
as low and parallel to the water as possible
like this,
but actually the ideal throw
looks something more like this.
Here's the guy who holds the world record in rock skipping
throwing from a bridge,
to demonstrate you don't need a super low throw to the water
to have it skip.
The third key is to flick the rock your wrist as much as possible
to get it spinning.
As you know
spinning things are more stable.
This is because they have angular momentum
and resist having it changed
according to Newton's first law.
So spinning and therefore stability is important
because it will maintain that magic
20 degree ramp angle with the water
as you can see here.
If it's not spinning enough it will rotate over
and then sink.
And finally rock choice is important
but not in the way I originally thought.
Primarily, it just needs to be flat on the bottom
to create that ramp.
but shape and diameter don't matter nearly as much.
The heavier the better because you put more energy into the system,
but not so big
that you can't reach the terminal velocity of your arm
by the time you release the rock.
This is especially important for kids.
They'll see much more success with small lightweight rocks.
And I should also mention that we looped back and checked
a couple of papers we discovered about rock skipping
and for the most part it matched
really closely to what we discovered ourselves.
And so armed with our new knowledge,
I put all the nieces and nephews through rock skip boot camp
to get the perfect technique into their muscle memories.
And while it started a bit rough,
they all eventually graduated.
[Music]
And the clay discs tended to be a little more forgiving
so we trained with those first,
and then graduated to actual rocks
and the principles translated perfectly.
And so after a hard workout,
like all true performance athletes,
we carb loaded.
[Music]
So there you have it.
In addition to learning to skip rocks like a pro,
hopefully you picked up some tips on the engineering design process.
People ask me all the time
"I have an egg drop competition coming up. Here are the materials I can use. What's the best solution?"
I don't know.
No one does,
without running some tests.
Because it depends on the actual rules of the competition
or maybe the brand of materials you chose
or how you actually put it together.
So you should do some googling for inspiration
and then make a bunch of prototypes
and do small changes on each to see what matters and what doesn't.
And only then go back and build your final version with some confidence
that you're gonna dominate the competition.
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[Music]
[Robot Noises]
[Laughing]
[Slow Motion Laughing]