1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Newtonian momentum

  1. Dec 21, 2016 #1
    I would like to give you a real world problem concerning Perpetual Motion.

    I have built a Dawn Mission type yo-yo de-spin device. I question if NASA is correct in their assumption that the weights conserve the same energy as the spinning space craft. They claim that when the 3 kilogram weights have the 1400 kg spinning rocket stopped; that the weights have the same kinetic energy as the original spin of the satellite body.

    Just for a point of reference I change the satellite into a one meter diameter hollow cylinder; rotating at one meter per second around the arc of the circle. I think this 1200 kilograms cylinder is a ballpark figure; so we have twelve hundred kilograms moving one meter per second.

    For the satellite energy we have ½ * 1200 kg * 1 m/sec * 1 m/sec = 600 joules

    For energy conservation of the 3 kilogram masses on the end of the tethers we have ½ * 3 kg * 20 m/sec * 20 m/sec = 600 joules. This 20 m/sec is half the speed of the baseball in the majors.

    For the satellite momentum we have 1200 kg * 1 m/sec = 1200 units.

    For momentum conservation of the 3 kilogram masses on the end of the tethers we have 3 kg * 400 m/sec = 1200 units. This 400 m/sec is 20% above the speed of sound.

    Quite a difference 20 or 400; which one is correct?

    I built a yo-yo de-spin device to find out. Because only momentum can be transferred from small objects to large objects; and conservation of energy only gives use 20 m/sec * 3 kg = 60 units of momentum to return 1200 units. If the 3 kilogram masses only have 60 units of momentum they can not restart the spin of the satellite.

    So now for the experimental answer: After a complete stop of the cylinder the tethered spheres completely restore the spinning motion of the cylinder. The original and final spin rates of the cylinder are the same.

    If the Dawn Mission tethers were not released the 3 kilogram masses would have rewound and restarted the spin of the satellite. For a complete restoration of motion.
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted