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Homework Help: Speed due to gravity

  1. Jun 1, 2010 #1
    This is a revision question for a test ive got. Just want to check my answer.

    An asteroid with mass of 1.8*1015kg and radius of 6.7km attracts a spacecraft that has come into its gravitational influence. What speed will the spcaecraft crash into the surface. intial speed of spacecraft is negilable.

    its a multiply choice and i choose it is impossible to calculate without knowing the mass of the spacecraft.

    the equation i would have used is -Gm1m2/r

    this right? or is there a way of working out the speed without knowing the mass?

  2. jcsd
  3. Jun 1, 2010 #2


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    Yes, it is a way to find the speed without knowing the mass of the spacecraft.

    You said you would have used the equation -Gm1m2/r, but its is not an equation without the other side. What is it equal to?

  4. Jun 1, 2010 #3
    well i worked out the gravitaional pull of the asteroid by using Gm1/r^2 and found it to be 0.0026757m/s which makes sense as its a small object. But im not sure where to go from there as i dont know a, the distance the spacecraft is and b, the mass of the spacecraft so cant use the normal a=f/m
  5. Jun 1, 2010 #4


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    Hint: Energy conservation: E(infinity)=E(distance at which it impacts the asteroid)
  6. Jun 1, 2010 #5
    lol im really lost now
  7. Jun 1, 2010 #6
    The total energy (gravitational and kinetic) of the system remains conserved throughout the interaction ie. total initial energy = total final energy.
    Basically, use the energy approach in tackling this problem.
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