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Newtonian Mechanics (true or false)

  1. Mar 9, 2010 #1
    1. The problem statement, all variables and given/known data

    1.The work required to lift a mass one meter depends on the speed at which it is raised..

    2.The acceleration of an object is same in all inertial reference frames.

    3.A particle's kinetic energy can change without having any net work done it.

    4.The work done by a constant force acting on an object is equal to the magnitude of the force times the distance the object moves.

    5.Less power is required while lifting a box up quickly than while slowly lifting it.

    6.A simple machine may reduce the force required to do a task


    2. Relevant equations



    3. The attempt at a solution

    FTTTFT
    FFFTFT
    FTTTFF
    FTFTFT
     
  2. jcsd
  3. Mar 9, 2010 #2

    ideasrule

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    Let's examine each one separately. You're pretty sure that #1 is false, and you're right--it doesn't depend on speed. #4 is not worded clearly, but technically it might be false. Consider the scenario where an object attached to a string is swung in a circle. The tension in the string is always perpendicular to the object's motion, so it does no work. However, the object does move a non-zero distance under the influence of a constant force.

    You knew #5 is false, which is good: it is. Can you explain what you think about the other questions? Why are you not sure?
     
  4. Mar 9, 2010 #3
    #6 A simple machine may reduce the force required to do a task, i believe it reduces the force, thinking of a pulley it reduces the force but not the work because you are still doing the work right? It might take longer but you are reducing the force that you have to apply in order to do a task.

    #2 i am really confused i don't know what inertial reference frames mean but i thought it was false because it referred to ALL

    #3 A particle's kinetic energy can change without having any net work done it, it is talking specifically about net work, not just work so it confuses me and that's why i went from T to F to T and to F again
     
  5. Mar 9, 2010 #4

    collinsmark

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    I agree with your logic. But the statement isn't addressing anything about work. It's only dealing with force.

    I think I can help you with this one. "Inertial frame" means a frame of reference that is not accelerating, itself. A frame of reference called an "inertial frame" can be traveling at an arbitrary constant velocity, but the frame is not accelerating.

    So in question #2, you are not comparing the acceleration to an object relative to something else (namely a frame of reference) which is also accelerating. The fact that the term "inertial frame" is used, means that you are only comparing the acceleration of the object to things (frames) which are not accelerating (even though they may have different constant velocities).

    I'm not sure I follow you on this one. Can you think of any example where a particle's kinetic energy changes with no net work done on it? (Well, using Newtonian mechanics, that is)
     
  6. Mar 9, 2010 #5
    on 6 i believe it is true because it does reduces the force it takes to do a task

    on 2 is true

    on 3 the net work done on an object is equal to the change in its kinetic energy so it will be false for 3

    is this right?
     
  7. Mar 9, 2010 #6

    collinsmark

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    I believe your logic is sound. :approve:
     
  8. Mar 9, 2010 #7
    Got it! it is right

    THANKS YOU SO MUCH collinsmark AND ideasrule I REALLY APPRECIATED
     
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