Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Why is it hard to walk without bending your knees?

  1. Feb 3, 2004 #1
    In the terms of rotational inertia why is it hard to walk without bending your knees?

    The greater the bulk of the object mass to its axis the greater its rotational inertia.
    how can I relate that to walking without bending knees?

    I understand that with rotational inertia an object resist
    any change in its state of rotation. I just down know how to
    phrase this question
  2. jcsd
  3. Feb 3, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The harder something is to get spinning, the more rotational inertia it has. If you want to get something spinning, does moving it closer to the axis of rotation make it easier or harder?

    Let's take an example. Say you are standing with a 10 pound dumbell in each hand. Which way makes it easier for you to get yourself spinning (around a vertical axis): (1) holding your arms straight out and the weights as far from your body as possible?, or (2) holding your arms bent, the weights as close to your body as possible? The configuration that makes it easier to turn is the one with the smaller rotational inertia. Got it?

    So... what are you doing when you walk? You are rotating your leg about your hips, right? What does bending your knee do?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook