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Communication problems! No forces at some place in universe?

  1. Jun 4, 2008 #1
    1. The problem statement, all variables and given/known data

    1a. is the statement "there is no place in the universe where a body can exist with no force acting on it" if aggree or dissagree explain why?...
    b. Is it possible for a centripetal force to do work on an object? explain your answer?
    c. why do roller coasters rides always start by going up hill?
    d. could a slow moving truck have more kinetic energy than a a fast moving car?
    e. you push a book acrooss a horizontal desk at a constant velocity, you are supplying some energy to the book into what form (s) does the energy go?
    2. Relevant equations

    there are no really equations that i am aware to this. I am a noob!>. help!

    3. The attempt at a solution
    well my answer for the first one i think its true.. i mean anywhere in the universe every object has its own gravitational force acting upon it.. satelites for example keep themselves floating in orbit because of the mooons and earth gravity. hmm any expansion on my comment would be appreciated b c and d i have no clue. I think e. the energy goes into mechanical energy?... THANKS IN ADVANCE
    Last edited: Jun 4, 2008
  2. jcsd
  3. Jun 4, 2008 #2


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    Homework Helper

    In what direction does the centripetal force act? And at any point in time,how do you find the direction of motion of a particle moving in a circle? And finally,the definition of work

    For this, I would guess it has something to do with keeping the roller coaster on the tracks while it goes down hill and then back up. and also maybe to gain gravitational p.e. so it can be converted into another form which gives roller coasters something desirable.

    How does one find the kinetic energy of a body?[/QUOTE]

    Firstly,what is the book actually doing? Is it staying still or moving? Secondly, is there any other forces that should be considered other than force you exert on the book?
  4. Jun 4, 2008 #3

    Just to elaborate on rock's statements:

    A. Hhhmmmmm... a force is defined as accelerating mass, F of G produces a constant acceleration for any mass existing in space time, so I think that practically your assumption is correct. But inside the event horizon of a black hole... who knows...

    B. As rock.freak667 said, just look at your definitions of Fcen and Work. Oh and try breaking the equations down to there integral parts i.e.. for Fc:

    Fc = m * r [( change in arc length / r )/ time interval]^2


    Fc = {m [r ( angular displacement / time )]^2} / r

    and for W...

    W = (m * a) * displacement (cos theta)

    C. Look at how roller coasters meet the Conservation of Energy

    D. Simply examine the equation for KE and play around with some values:

    KE = .5 * m * v^2

    You know that Energy is conserved and you know that there are no perfect closed systems, so draw some force diagrams and try and list all the possible forces acting on the book that may use your KE besides motion...
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