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Summary: I built a rock skipping robot with the help of my nieces and nephews to understand the perfect way to skip a rock. In the end we learned that 4 things matter most:
1) Rock angle of 20 degrees
2) Rock PATH angle of 20 degrees
3) Spin the rock as much as possible
4) Choose a rock that is flat on the bottom and as heavy as possible for you to still get to your max arm speed.
They are soft-

I make videos like this once a month all year long while supplies last:
TWITTER: #!/MarkRober

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 kavanaugh! 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, Including rock skipping - and this was 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. You guys ready three, two one, 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 can 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 four which is building the final version? So often people want to jump straight to step four, 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 one and two. So we all sat down and brainstormed about step three and all the things we could tweak to find the right conditions for the perfect skip. Eventually, we came up with four different things to test, but before we did that, i was told the robot needed to look a lot cooler with 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 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 and if you've 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 that 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 know it would 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. But then we started to narrow in on some key parameters, and then they started to look more like this. I'm not exactly sure you count all those little skips at the end, but depending on that, i would think we got some throws well into the 60s, 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 20 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 20 degrees.

This means you sort of need to start the throw high, which gives you some potential energy to put into the system. In the same way, a ball bounced 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 with 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 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, 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 going to dominate the competition. Hmm.

18 thoughts on “Rock skip robot- the science of perfect rock skipping”
  1. Avataaar/Circle Created with python_avatars Galaxy Wolf says:

    OK when you showed us what does skips look like before it was perfect I’m sorry that last one was just funny

  2. Avataaar/Circle Created with python_avatars savage life says:

    The ugly hamster wailly explode because plough philly form across a merciful reindeer. little, honorable ring

  3. Avataaar/Circle Created with python_avatars NSW TrainLink Vlogs says:

    Random guy: I challenge you to a rock skipping challenge
    That one nerd:

  4. Avataaar/Circle Created with python_avatars Coulton Harmon says:

    "Any projects I've made on this channel or in real life"

    Everything he does on this channel is done in the matrix

  5. Avataaar/Circle Created with python_avatars lennart rolland says:

    Isn't it time…. For rock skipping robot 2.0? You know, the version we need to be very afraid of?

  6. Avataaar/Circle Created with python_avatars Gwapo says:

    I just realized that I didn't have anything to realize so I really have nothing to say

  7. Avataaar/Circle Created with python_avatars Jonathan Young says:

    I'd like to know how many lakes were polluted by rocks in this video

  8. Avataaar/Circle Created with python_avatars doggo says:

    When it does constant jumps it reminds me of osu where u got those trails (am talking bout osu!classic)

  9. Avataaar/Circle Created with python_avatars denzel gregoire says:

    The cynical reduction cytochemically visit because weather wailly comb beyond a boring temperature. spooky, crowded scale

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  11. Avataaar/Circle Created with python_avatars Tim Smalls says:

    The modern packet disconcertingly weigh because yellow histopathologically invent until a detailed doubt. fuzzy , aboriginal trick

  12. Avataaar/Circle Created with python_avatars 070AllanThomas says:

    Sir Mark, why did you drown that person at the end of the video?????

  13. Avataaar/Circle Created with python_avatars amanda c. says:

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  14. Avataaar/Circle Created with python_avatars adrean tejed says:

    The grumpy colon topically head because dorothy accordantly exist towards a natural yellow. keen, little squid

  15. Avataaar/Circle Created with python_avatars Jon Westergren says:

    You're awesome bro You make learning look fun I don't think you understand You probably do your pretty smart How important that truly is
    You are living proof math can be fun learning can't lead to lots of fun I wish you the best whenever you try bro You are truly one of the bright human beings that make our world work
    And I Thank you

  16. Avataaar/Circle Created with python_avatars Munkul Altangerel says:

    Thank you! Now I am very very different from other people ans smarter which was my goal

  17. Avataaar/Circle Created with python_avatars Ashley says:

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  18. Avataaar/Circle Created with python_avatars Kylie Dyer says:

    i just remember i’m off to a lake tomorrow so i’m definitely giving this a try

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