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lucphysics
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Homework Statement
If a satellite is in a sufficiently low orbit, it will encounter air drag from the Earth's atmosphere. Since air drag does negative work (the force of air drag is directed opposite the motion), the mechanical energy will decrease. If E decreases (becomes more negative), the radius of the orbit will decrease. If air drag is relatively small, the satellite can be considered to be in a circular orbit of continually decreasing radius.
a) According to v = (GM/r)^1/2,if the radius of a satellite’s circular orbit decreases, the satellite’s orbital speed increases. How can you reconcile this with the statement that the mechanical energy decreases?
b) Due to air drag, the radius of a satellite’s circular orbit decreases from r to r - △r, where the positive quantity △r is much less than r. The mass of the satellite is m. Show that the increase in orbital speed is △v = +(△r/2)[(GM/r^3)^1/2]; that the change in kinetic energy is △K = + (GMm/2r^2); that the change in gravitational potential energy is △U = -2△K= - (GMm/r^2)△r; and that the amount of work done by the force of air drag is W = - (GMm/2r^2)△r.
Homework Equations
U = -GMm/r
K = 1/2mv^2
E = K+U
[PLAIN]http://media.wiley.com/Lux/82/331282.image4.png[/B]
The Attempt at a Solution
I think (a) can be solved saying that the mechanic energy will decrease due to the negative work done by the air drag, because doing E = K+U the final mechanic energy is negative. (I'm not sure anyway)
But I'm stuck doing (b) , I don't understand where does △r/2 in the velocity equation come from...I don't know how to solve it...any help would be helpful, thanks to all of you who could help me.PS: this isn't homework, it's just a problem I found to prepare final exams
(Sorry if there are any grammar mistakes, I'm not a native English speaker)
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