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

Foucault pendulum and angular momentum

  1. Mar 19, 2015 #1
    In a discussion with someone claiming to be a physicist (whether PhD or something less he did not say) we got into a hypothetical discussion related to Hitchhiker's Guide to the Galaxy. An alien civilization wants to move the Earth out of the way because it's impeding a galactic highway. Possessing very LARGE equipment, a huge grappling claw grabs the Earth at the poles, stopping its rotation in the process, and proceeds to move it to its new location.

    I pointed out the obvious (at least to me) that ignoring all the other catastrophic results, that everyone would be subjected to being violently moved in an eastward direction, 1000 miles per hour near the equator, relative to the surface of the Earth because the globe stopped spinning suddenly. So there would be huge tsunamis, and all sorts of other effects due to conservation of angular momentum. He maintained there would be no such effects. Here are his quotes:

    1. On Earth, gravity exerts more force on a human being (and everything else) than the centripetal force. That is Physics 101.

    2. Let's do a simple significant figures analysis of gravity versus centripetal force.
    Let's look at a Foucault Pendulum, http://en.m.wikipedia.org/wiki/Foucault_pendulum .
    Take the bob of the pendulum by itself and hold with your hand. It is 28 kilograms, almost 60 pounds. That is the force of gravity on the bob.
    Now, after the bob is in motion, try to stop the pendulum. I'll give you a hint, it doesn't take 60 pounds of force to do so.
    That simple experiment shows how much greater is gravity than the Earth's centripetal force.
    Lesson concluded.

    3. The Foucault Pendulum actually captures the dynamic force caused by the Earth's tangential velocity.
    The force needed to stop the bob is what your body will feel if the Earth suddenly stopped spinning.

    Ok, I only have 2 semesters of university level physics and that was several years ago but his explanation is total Greek to me, especially considering conservation of angular momentum. So ... am I stupid or is he crazy? To me, I have to say that the bob isn't moving at 1000 mph relative to the surface of the Earth so the force it takes to stop it now has nothing to do with what would happen if the Earth was latched onto by a Douglas Adams Earth grabbing claw! I'm the first to admit I don't know everything but I can't see how gravity is going to nullify your angular momentum. So here I am on the physics forum to see if anyone can shed some light on where he's coming from and if he has any merit.

    Thanks to all who can comment.
     
  2. jcsd
  3. Mar 20, 2015 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Welcome to PF!

    This person you were speaking to seems to be confusing rotation and revolution. A person who is standing on earth's surface is both rotating (spinning around an axis through his waist) and revolving about the earth's axis (moving in a 4000 mile radius circle). If earth's rotation were stopped suddenly by a giant claw, you'd continue to do both...at least for the few miliseconds before you slammed into the wall of your room at 1000 mph.
     
  4. Mar 20, 2015 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    As Russ said, this is the crucial error. The pendulum deviation is just due to the rotation of the setup in space (angular velocity relative to an inertial frame). It has nothing to do with the linear velocity of at the equator due to the Earth’s rotation.
     
  5. Mar 20, 2015 #4

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Assuming he's in a stable bunker that stops with the surface. Otherwise he would just fly with the building for a while. Obviously the whole idea of stopping the rotation of a mostly fluid body, with claws at the poles (the worst possible placement even for a rigid body) is meant as a joke.
     
  6. Mar 20, 2015 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Just to clear up a potential source of confusion for the OP: there is no hair to split for my use of the term "revolving" and AT's "linear velocity". Obviously we are moving in a circular path as the earth rotates, but for short distances the motion may as well be linear.
     
  7. Mar 20, 2015 #6

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Yes, the term "linear velocity" just means the physical quantity usually just called "velocity", and does not imply "motion along a line".
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Foucault pendulum and angular momentum
  1. Foucault's Pendulum (Replies: 3)

Loading...