Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Newton 3

  1. Dec 9, 2004 #1
    Good day all,

    If I apply a force on an empty cardboard box, it will apply the same force on me. But why does it move? There must be a resultant force, but how? If I apply a larger force it follows that the box will also apply the same larger force to me. Can someone please explain to me why the box can move?
  2. jcsd
  3. Dec 9, 2004 #2
    This is one of the biggest misconceptions in physics.

    The two forces are separate. The force the box exerts on you is entirely different from the force you place on the box
  4. Dec 9, 2004 #3
    But they are of the same magnitude. According to Newton 1, there has to be a resultant force before something can accelerate.
  5. Dec 9, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

    Because u cannot apply third newtonian princple without applying the second and viceversa.These 2 are intimately related (don't take it in a perverted way! :tongue2: ).The cardbox moves,because a nonvanishing force is applied to it.According to the second principle,u move as well,but your mass in (maybe) more than 50kg and the box's is of the order of grams...

    In the enclosed system formed by you and the box,the resulting force is nil.But applying the second principle to each component (either u,or the box),u'd be certain of moving,since the force is not zero.

  6. Dec 9, 2004 #5


    User Avatar

    Another thing is the friction of whatever the box is resting on and whatever you are standying on. On a nonfrictional surface you would both move perhaps noticably but remember the difference in mass. F=m*a
  7. Dec 9, 2004 #6


    User Avatar
    Homework Helper

    phreak is right, both in his explanation and in stating that it's a major misconception.

    Try this: If I push on you with a force of 10 N due north, and my brother pushes on your sister with a force of 10 N due south, would you say that neither of you moves because the forces cancel? Obviously not - they aren't acting on the same object. This may seem contrived, but it's exactly the same situation with action/reaction force pairs. The action force never acts on the same object as the reaction force. Consequently, they cannot be added together to cancel.

    A few possibly useful facts: Action/reaction force pairs are always exactly the same magnitude. They are always exactly opposite in direction. They are always exactly the same type of force (i.e. if one is a frictional force, so is the other. If one is a gravitational force, so is the other. And so on.)

    Keep these rules in mind and you should usually be able to identify the reaction force. Even when you can't, you can almost always tell if you've identified the wrong one.
  8. Dec 9, 2004 #7
    Great, it's starting to make more sense now. I thank all of you. :biggrin:
  9. Dec 9, 2004 #8
    Oh yeah.

    So if there is a stone on the ground, there are actually 4 different forces:
    -the gravitational weight of the stone, by the earth
    -the gravitational attraction of the stone on the earth (it's not much but it's there!)
    -the reaction force from the earth's surface
    -the corresponding opposing electrostatic replusion of the stone's molecules etc.

    I think this is correct..
  10. Dec 9, 2004 #9


    User Avatar
    Science Advisor
    Homework Helper

    If in your "etc."you included the missing force (from the first 3;there should be 4),then u're right.If not,not.

  11. Dec 10, 2004 #10
    I agree with Ba.

    It is actually friction of the floor you are standing on that prevents you from moving.

    Check this out!

    astronauts doing space walks have mini jets on the spacesuits.Why?
    Because, even if they apply a little force on the spacecraft during spacewalk, they would start moving in the opposite direction indefinitely. To control such situations and to maneuovre in space, they have mini jets on their spacesuits.

    Try this!

    try pushin a very heavy object. You could ty pushing the wall. the wall exerts the same force on you and you start moving!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook