Sessumaru Posted November 2, 2008 Share Posted November 2, 2008 Hello! Gotta question here about Circular Motion... we all know that a = (v ^ 2) / r f = 1 / T and F = ma... Then how do I know which formula to use? For example, Calculate the force of gravity on a spacecraft 12,800km (2 earth radii) above the Earth's Surface if the mass is 1400kg? Link to comment Share on other sites More sharing options...
big314mp Posted November 2, 2008 Share Posted November 2, 2008 Usually you are given a velocity also with these problems. Otherwise, you could try g=GMm/r^2, although I don't think that will give you a good answer. Link to comment Share on other sites More sharing options...
insane_alien Posted November 2, 2008 Share Posted November 2, 2008 big314mp, that is the right equation to use. why would you think it would give you a wrong answer? you have M,m and r with G being a constant so you can find F(which you have mysteriously labelled g in this case) Link to comment Share on other sites More sharing options...
swansont Posted November 2, 2008 Share Posted November 2, 2008 F = ma always applies in these problems a = v^2/r always applies to circular motion However, the "centripetal force" is not it's own kind of force, such as gravity, or tension or normal force, that we identify in a free-body diagram. It's an additional condition that holds true when something moves in a circle. GMm/r^2 (as big314mp has given) is the gravitational force (rather than the acceleration) If you were given the period of orbit, you could find the speed, and use the centripetal acceleration equation. Link to comment Share on other sites More sharing options...
big314mp Posted November 2, 2008 Share Posted November 2, 2008 big314mp, that is the right equation to use. why would you think it would give you a wrong answer? you have M,m and r with G being a constant so you can find F(which you have mysteriously labelled g in this case) Oops. It is supposed to be F. Anyways, I just did this problem the other day for my physics homework (it was the Space shuttle, and it was orbiting lower) and we were expected to use the orbital velocity given to calculate the force of gravity. I figure this is the same question, just with a bit of info missing. Link to comment Share on other sites More sharing options...
Sessumaru Posted November 2, 2008 Author Share Posted November 2, 2008 So I have to use F=ma, as a base, right? Link to comment Share on other sites More sharing options...
Flashman Posted November 4, 2008 Share Posted November 4, 2008 Classically, the force is gonna be the same however fast it's moving, however the resultant acceleration actually affecting it would change according to it's velocity component at right angles to the gravitational force. Link to comment Share on other sites More sharing options...
swansont Posted November 4, 2008 Share Posted November 4, 2008 Classically, the force is gonna be the same however fast it's moving, however the resultant acceleration actually affecting it would change according to it's velocity component at right angles to the gravitational force. I can't reconcile those two statements. If the acceleration is different, the force cannot be the same. Link to comment Share on other sites More sharing options...
Flashman Posted November 4, 2008 Share Posted November 4, 2008 Whoops, you're right, not thinking rigorously, had a glitch in my mental free body diagram. Maybe should have said, the resultant displacement towards the center of mass is dependent on the velocity. At classical velocities the force remains the same, independent of the velocity. Link to comment Share on other sites More sharing options...
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