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Force And Acceleration

  1. Jul 26, 2012 #1
    Why is it necessary that there has to be a force if there is an acceleration?
     
  2. jcsd
  3. Jul 26, 2012 #2
    Remember Newton's laws - an object in motion will stay in motion until acted on by an outside force. So, an object will normally be in a state if uniform motion - moving with constant velocity. If you want to change this uniform motion (accelerate it), you need to apply an outside force.
     
  4. Jul 26, 2012 #3

    sophiecentaur

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    Acceleration implies a change in Kinetic Energy, that involves work being done on or by an object - which implies a force moving through a distance.

    That isn't a true answer to "why" but there never is a complete answer to 'why' questions. All you can hope for is that an explanation helps you to put the unexplained thing in amongst enough familiar things that one is satisfied and it begins to make sense.
     
  5. Jul 26, 2012 #4
    You must be very careful about the terms you use.
    Newtons first law states that an object will remain at rest or move with constant velocity (constant speed in a straight line) unless it experiences a RESULTANT (unbalanced)force. If there is a RESULTANT force then there will be an acceleration.
    It is misleading to associate Kinetic energy with acceleration. Uniform circular motion involves acceleration but no change in Kinetic energy.
    Change in momentum but not KE
     
    Last edited: Jul 26, 2012
  6. Jul 26, 2012 #5

    sophiecentaur

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    Good point.
    So change the argument by saying Momentum Change is equal to Force times Time. The K.E. argument only works for motion in a straight line. The work done during circular motion is zero because the force and motion are at right angles.
    You really have to be on your toes on this forum!!
    But I still stand by my statement about 'why' questions.
     
  7. Jul 26, 2012 #6
    I am not going to be the one to 'change the argument'.
    The original query was straight forward and the explanation is easy straightforward text book stuff.
    Changing the argument will result in yet another 50+ posts about nothing in particular.
     
  8. Jul 26, 2012 #7
    Because of inertia.
     
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