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Angular momentum - length of day change if drive on other side of road

  1. Apr 13, 2013 #1
    This is a question that I remember hearing a few years ago:

    If the law were changed so that traffic in Great Britain travelled on the righthand
    side of the road instead of on the left, would the length of the day increase,
    decrease or be unaltered?

    I remember the answer was increase but not by much.

    The assumed that there is as much traffic west-east as east-west. Changing from left to right side of the road means that the west-east traffic moves a bit further away from the earth axis, the
    east-west traffic moves a bit closer (because generally the north is closer to the axis
    than the south).

    The person that told me this then claimed that this meant that the angular momentum of all traffic increases. (and therefore the day becomes longer due to conservation of angular momentum)

    My question is: why does the two directions not cancel out? and therefore the length of day would remain the same?
  2. jcsd
  3. Apr 13, 2013 #2


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    You have the answer right there.
  4. Apr 13, 2013 #3
    but would one increase the angular momentum and the other decrease the angular momentum? I think im missing something that is staring me in the face...
  5. Apr 14, 2013 #4


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    By different amounts, due to different distance to the axis.
  6. Apr 14, 2013 #5
    but angular momentum is conserved.
  7. Apr 14, 2013 #6


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    Try sitting in an office chair and waving your feet around in a circle. You can get the chair to rotate as long as you keep rotating your legs. When you stop the chair stops.

    I believe the cars would have the same effect. They would cause the earth to rotate as long as they are moving.

    I think the falling cat problem is similar.
  8. Apr 14, 2013 #7


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    • We must be clear on what angular momentum we are talking about. It is the total angular momentum of the cars+Earth system that is conserved.
    • The angular momentum due to the cars changes if Great Britain shifts to driving on the other side of the road.
    • Therefore, the angular momentum of Earth must have an equal-but-opposite change. And since the distribution of Earth's mass does not change, the angular momentum change must come from a change in rotation rate.

    p.s. The angular momentum of the cars can change because there can be net torque exerted on the cars by Earth.
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