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Work energy theorem vs Newton's 2nd law and kinematics

  1. Nov 8, 2009 #1
    So i have this general question.
    When the situation arises in which there are velocity and/or position depended forces acting on an object, would it be generally easier to use the work energy theorem or Newton's 2nd law & kinematics in predicting the motion? My guts tell me it will be work energy theorem but I cannot figure out good reasons. Any reasons why work energy theorem will be easier in this situation?
     
  2. jcsd
  3. Nov 8, 2009 #2
    Hi
    My rough thought is as follows.

    If your question is NOT explicitly on time, i.e.
    -What's the maximum height of the thrown up ball?
    -What's the velocity of ball in height h?
    Energy conservation law is useful.

    If your question is explicitly on time, i.e.
    -What's the velocity of ball 1 second after?
    -What's the time for ball to reach maximum height?
    Equation of motion is necessary.

    I hope there's something helpful for you in it.
    Regards.
     
  4. Nov 8, 2009 #3

    Motion = change in position over time.
    Velocity = change in position over time.

    Acceleration = change in velocity over time.

    Jolt, surge = change in acceleration over time.


    Kinematics = branch of physics that describes motion.



    Newton's 2nd law & Kinematics equations
    ----------------------------------------

    1.) a= F / m (change in acceleration)

    2.) v= u + a * delta-time (change in velocity)

    3.) position+= v * delta-time (change in position)

    4.) GOTO 1.)

    Done.



    Work-energy theorem
    --------------------

    1.) W= F * d

    2.) F = ?? / ??

    3.) position = ?? / F

    acceleration? velocity?
     
  5. Nov 9, 2009 #4

    stewartcs

    User Avatar
    Science Advisor

    The Energy approach is typically always easier. The only problem is that it only tells you what is happening at two points (1 and 2) and nothing about what happened in between those two points.

    So as long as you only care about the initial and final states the Energy approach will be the easiest normally.

    CS
     
  6. Nov 9, 2009 #5
    Ok, so I'll say Work Energy Theorem(W.E.Th) might be easier if -

    *We do NOT have to deal with time.
    *We know info(velocity and energy) about the starting and ending point of the object's motion.
    *I also thought W.E.Th will be easier since it only involves scalar multiplication. Scalars are easier to deal with than vectors.
     
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