1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newton's III Law :- Help me Understand!

  1. Jan 5, 2005 #1

    Mo

    User Avatar

    Hello all, need a little bit of help understanding Newton's third Law.

    It states that:

    " While body A exerts a force on body B, body B exerts an equal and opposite force on body A"

    Okay, i can understand that.I also understand that the force are acting upon different bodies.

    so (and heres the question..) if i push my desklamp across the .. desk.. then i am exerting a force in one direction on the lamp, and it is exerting an equal and opposite force on my hand.

    If so, why can i still get the desklamp to move. In my textbook it tells me that the forces do not cancel each other out, because they act on different bodies, but .. still ... i obviously don't understand it all that well.

    Okay the next question from the book was

    Explain why forces occur only in pairs"

    Is it simply enough to say: Whenever one body exerts a force on a second body, the second body always eerts a force on the first body, hence forces occur only in pairs.

    Help + Advice please!

    Regards,
    Mo
     
  2. jcsd
  3. Jan 5, 2005 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    The 3 laws of Newton do not act independent of each other.U'll have to use them at the same time.The answer to your 'dilemma' is that,according to the second law,to each force corresponds an acceleration.In your case,the hand that pushes the lamp gives it an acceleration equal to the ratio between the force and the lamps mass.The total force acting on the lamp is just the force u're using to push it along the table.Of course,nthere's the friction force,too,but that,if the coefficent is small,doesn't alter the results significantly.

    Daniel.
     
  4. Jan 5, 2005 #3

    Mo

    User Avatar

    Thanks for your reply, but im still not 100% , am i right in saying that the reason that my hand never acclerated off in the opposite direction (after all the lamp is exerting a force on my hand in the opposite direction) is because of the ratio between force and mass (making it a very small acceleration?)
     
  5. Jan 5, 2005 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    It's not only your hand,yer whole body should be accelerating in the opposite direction with a tiny acceleration (due to the big ratio of masses:yours & the lamps).You don't move,becuase of the friction between your feet and and the floor,or between your butt and the chair your butt uses to relax...

    Daniel.
     
  6. Jan 5, 2005 #5

    Mo

    User Avatar

    Thanks for your help :)

    Regards.
    Mo
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?