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Masterphysics again, ARGH!

  1. Mar 12, 2005 #1
    An unmanned spacecraft is in a circular orbit around the moon, observing the lunar surface from an altitude of 40.0km (see Appendix F). To the dismay of scientists on earth, an electrical fault causes an on-board thruster to fire, decreasing the speed of the spacecraft by 30.0m/s.

    If nothing is done to correct its orbit, with what speed (in km/h) will the spacecraft crash into the lunar surface?
    Take the gravitational constant to be 6.673*10^-11 N*m^2/kg^2, the mass of the Moon to be 7.75*10^22, and the radius of the Moon to be 1.74*10^6.

    Ok, um, how do I approach this problem? I can't reason it out. Please don't do the problem for me. I really want to learn how to do this, but I just can't figure it out. Thanks in advance.
  2. jcsd
  3. Mar 13, 2005 #2
    Great approach.

    Now you know that the spacecraft crashes into the moon so you do not need to worry about how it gets to the moons surface.

    Now there is a certain amount of energy gained from falling from the spacecrafts orbit to the surface.<-use energy conservation.

    The spacecraft also starts out with a certain amount of kinetic energy determined by it velocity and mass. The problem does not state the velocity that the spacecraft has before it is decreased by 30.0m/s. You know though that it was in a stable orbit however.

    Have you found the velocity of a spacecraft in a stable orbit before? You have to balance the centripetal force and the centrifugal forces.

    After finding the initial kinetic energy and the energy due to falling you should be able to find the speed at which it hits the planet.

    Was this too much info?
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