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Homework Help: Force of gravity problem

  1. Nov 28, 2004 #1
    Calculate the force of gravity on a spacecraft 6400 km (1 earth radii) above the Earth's surface if its mass is 1400 kg.

    i started by using the Fg = GMm/d

    and i plugged in the numbers so it looked like this


    i don't know what m should equal and i can't get past that much. thanks so much for helping.
  2. jcsd
  3. Nov 28, 2004 #2
    You have the correct formula. One of the M's is the mass of the second object involved in the gravitational attraction, in this case the earth. Find the mass of the earth in your data sheets, plug that in, and your done.
    Last edited: Nov 28, 2004
  4. Nov 28, 2004 #3
    thanks so much!

    i was also stuck on this problem if you can help me.

    A car at the Indianapolis-500 accelerates uniformly from the pit area, going from rest to 300 km/h in a semicircular arc with a radius of 200 m.
    Determine the tangential acceleration of the car when it is halfway through the turn, assuming constant tangential acceleration. Determine the radial acceleration of the car at this time. If the curve were flat, what would the coefficient of static friction have to be between the tires and the roadbed to provide this acceleration with no slipping or skidding?

    i know you use the equation Ac = v^2/r but i don't know for which part to use it for. so i used 300^2/100

    but i know thats not right and i have no idea where to go from there. thanks so much
  5. Nov 28, 2004 #4

    Andrew Mason

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    This is not the correct formula. The correct formula (Law of Universal Gravitation) is:

    [tex]F = \frac{GM_{Earth}m_{craft}}{r^2}[/tex]

    where r= Radius of the earth + d (distance above surface).

    Note the [itex]r^2[/itex] in the denominator. So all you have to really know to solve this problem is that the force would be 1/4 at twice the distance or 350 kg.

    Last edited: Nov 28, 2004
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