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Escape Velocity Given Radius of 2 Objects

  1. Apr 10, 2005 #1
    Got a dosie of a problem I need checked. I've been playing around with this problem, and after I submit the right answer (the ans. I got) , it came out wrong. Someone tell me what I'm doing wrong PLEASE thanks:

    The prob. is to calculate escape velocity from a white dwarf and a neutron star. They are each 1 solar mass. The white dwarf has a radius of 104 km + the neutron star has a radius of 10 km.

    Step I took:
    sqr. root of 2GM/R = V = Sqroot of (2)(6.67e(-11)(1) all over 10^4th power
    v = 1.15e(-7)

    neutron star v= (2)(6.67e(-11)(1) all over 10
    v = 3.65e(-6)
    i got that, but only got half credit for this prob., what did I do wrong or have to add??
  2. jcsd
  3. Apr 10, 2005 #2


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    Looks like you forgot to convert the mass of the objects to kg. You put in "1" for both cases, but a solar mass is equal to 2e30 kg.

    Also note that the escape velocity from a neutron star is near the speed of light, so the classical calculation you did won't be very accurate.
    Last edited: Apr 10, 2005
  4. Apr 10, 2005 #3
    Converting to Kg

    Oh I see so I did the prob. right, but just messed up on the conversions, so close. Just out of curiousity what the correct answer would be, is it going to be V = 1.15 e-10 and for the neutron star 3.65e-9

    just trying to learn this, so i don't mess up next time

    thanks a lot
  5. Apr 10, 2005 #4


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    Whenever you get an answer, you should always go back and think if it makes sense. Would you expect it to take more or less speed to escape from a neutron star than, say, the earth? Does that check with the equation you used? Next, look at your answer. How fast is 10-10 m/s? Is it faster or slower than you can run? If you start running around, do you escape the earth's gravity?
  6. Apr 11, 2005 #5
    you should also check your units! this will always helps.
    Last edited: Apr 11, 2005
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