The Physics of Touching your Toes!

Posted by Ethan on November 13, 2009
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Can you touch your toes? Seems like an easy thing to do for those of us who have the flexibility.

Now, here’s the challenge. Stand with your back and your heels pressed up against a wall, and now try to touch your toes.

You can’t do it! Not without putting your hands down on the floor, you can’t. There’s a super-simple reason for this: center-of-mass.

For human beings, your center-of-mass is somewhere in your abdomen. It’s lower down for women than men, but in the abdomen region for everyone. When you typically bend down to touch your toes, you’ll notice that the upper part of your body moves forwards, while the lower part moves backwards. You do this by default, but the major reason you need to do this is so that your center-of-mass stays over a stable point: your feet!

But as soon as you prop yourself up against the wall, the lower part of your body can’t move back! So as the upper part moves forward, your center of mass moves forward. It goes from being over your heels to being over your arches to your toes, and the instant it extends out beyond your toes, you know what happens?

You begin to rotate, and once that happens, you’re going to fall, like so:

Image credit: Gabrielle Varieschi.

And that’s it! This is a great trick to try on children to teach them about stability. It’s also great for physics teachers to introduce torque, rotation, and center-of-mass. (And on unsuspecting coworkers to teach them about falling on their faces!)

It’s a remarkable phenomenon that such a simple thing as touching your toes would be off-limits with your heels against a wall, but there it is! Don’t believe me? Try it yourself and see what happens, but I warned you…

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Comments

  1. #1 Sili
    November 13, 2009

    Show-off.

  2. #2 claire
    November 13, 2009

    cool

  3. #3 peter
    November 13, 2009

    though do not challenge a member of a circus to do this.

    very carefully, by curling down, I have witnessed this done by a contortionist…

  4. #4 Jerry
    November 13, 2009

    I suppose of you’re quick enough, you can cheat and push against your toes, burying your ass in the wall?

    I can’t even touch my feet normally…

  5. #5 Matt
    November 13, 2009

    hey, hair is coming back

  6. #6 Sophos
    November 13, 2009

    Haha. Pretty funny. But if you stretch ur arms until about your knee level and maintain your balance, then quickly touch your toes…that’s possible. haha

  7. #7 Thomas Neil Neubert
    November 14, 2009

    Onboard my starship in deep space, wearing of course my usual sticky or magnetic shoes, I have no problem touching my toes staring with my heels and back to the wall. I have no idea why an astrobiologist would imply the unreasonable assumption that I am on planet Earth.

  8. #8 Katkinkate
    November 14, 2009

    Those of us with big butts would have trouble just getting into the starting position of that little experiment.

  9. #9 Samantha Vimes
    November 14, 2009

    It put a big strain on my calf muscles, my weight was all on the balls of my feet, and I felt strains in several muscles that don’t put much effort in to normal toe-touching, but I did it. My arms ended up having to carry some of my weight, too, creating a sort of triangle– arms, grabbing the feet, and legs, being two edges of the triangle, with my torso the other side.
    It’s better exercise, I think, than simply stretching, but it can be done, at least by me. I wonder if my short torso/long limb ratio helps.
    And as Peter mentioned about the contortionist, I moved slowly and carefully. Strength and control are key elements.

  10. #10 Tammy
    November 14, 2009

    And me, with my longer torso and shorter limbs… nope. I do yoga- and can get my ELBOWS to the floor away from the wall, but couldn’t get hands much past mid-shin with my heels at the wall… isn’t physics awesome!

  11. #11 neil
    September 15, 2012

    i can touch my toes with my back and heels against a wall its easy.