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Homework Help: Energy and energy changes

  1. Jan 8, 2008 #1
    Hi. I'd like some help with the following please:

    1. When a body accelerates or decelerates, is there definitely a change in K.E.?
    2. When an object moves up an incline, is K.E. = P.E.?
    3. Do satellites orbitting in space lose energy due to air resistance?

    I know they're very simple but I haven't found a textbook that answers these particular questions. I can attempt them but I could be very wrong somewhere.
    I guess my answers would be:

    1. Yes, because the velocity is changing.
    2. Yes, if the object is projected up the plane. I am not sure what happens if a constant force is applied.
    3. My teacher says yes. I am unwilling to believe there is air resistance in space but I wonder if satellites do lose energy, maybe due to something else.

    Any help is welcome.

  2. jcsd
  3. Jan 8, 2008 #2

    Shooting Star

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    1. Correct. (EDIT: as Doc Al had pointed out below, only in st line motion.)
    2. In frictionless motion, KE+PE is constant.
    3. They do. Look up what happened to SKYLAB.
    Last edited: Jan 8, 2008
  4. Jan 8, 2008 #3


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    It depends on the orbital altitude of the satellite. If it is orbiting in earth's atmosphere, then air resistance will exist.

    Like Shooting star mentioned, SKYLAB had this problem (due to solar activity increasing the drag on it). The orbital altitude was 286 miles per one article I read. And since the outer layer of earth's atmosphere (exosphere) is like 400-800 miles, it encountered air resistance.

  5. Jan 8, 2008 #4

    Doc Al

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    Careful here: Velocity is a vector and can change in direction as well as in magnitude.

    Imagine something traveling a circular path at constant speed. Is it accelerating? Is its KE changing?
  6. Jan 8, 2008 #5
    oo and what if its velocity doubles but it loses a quarter of its mass in the process?(like something a rocket might do)
  7. Jan 8, 2008 #6

    Shooting Star

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    Doc Al, you did it again. I considered only rectilinear motion, just because of the simplicity of the questions. Hi Amy T, take note of this.

    Why don't you find out how much the "atmosphere" extends, and how far up some of the satellites are placed in orbit?

    I think the OP is talking about a single object here.
  8. Jan 9, 2008 #7
    And probably just linear motion but we just wanted to make you look bad!

    Incidentally I was asked in a job interview how often I think the ISS's orbit needs to be corrected because of the very little atmospheric drag. I was wrong then, and I forget the answer, but not often!
  9. Jan 9, 2008 #8

    Shooting Star

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    You have to come up with something LOTS better than that.
    Last edited: Jan 9, 2008
  10. Jan 9, 2008 #9


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    2. But the problem asked if KE= PE.
  11. Jan 9, 2008 #10

    Shooting Star

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    It's simple to deduce from what I've written that the answer in no. I have given more information.

    (What's troubling you, HallsofIvy?)
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