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: Questions about Gravity

  1. Oct 24, 2005 #1
    For the sake of the questions, I am going to ask them in regards to the gravity of the earth.

    When gravity pulls down on a certain body, and that body interacts with the ground, a third law force pair is made and the body pushes down on the earth with the same force that gravity pulled it down.

    Now, the normal force is another third law force pair, and it pushes back up on the object with the same force that the object pushed down on the ground. Thus causing equilibrium.

    Is this all correct?

    If it is, then I have some questions:

    If gravity pulls down on an object, and an object pushes down on the ground, then is this an exception to Newton's Third Law? Why aren't the forces opposite? Is this true of any "pulling force?"

    If there are only three forces coming into play in gravity, then is the reaction (a reaction because it reacted to gravity) of the body pressing down on the ground, also an initial action at the same time by causing the reaction of the normal force?

    Is the one force of the body pressing into the ground both a reaction and an action at the same time in both separate third law force pairs? So actions and reactions are not mutually exclusive?

    Thanks in advance to anyone.
  2. jcsd
  3. Oct 24, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Be careful how you identify 3rd law force pairs. When gravity pulls down on an object, the 3rd law reaction force is the object pulling up on the earth. It's true that if the object is supported by a surface and is not accelerating, then the surface will provide a force (the normal force) that will equal the object's weight. That normal force is part of a different 3rd law force pair: The normal force is the ground pushing up on the object; the reaction force is the object pushing down on the ground.
    These forces are not 3rd law pairs! See my comments above.
    The terminology "action" and "reaction" is somewhat misleading (and oldfashioned). The meaning of Newton's 3rd law is that forces are interactions between objects that always come in pairs. It's not that one force "causes" the other; they exist together.
    A given force (an interaction between two objects) can only be part of a single action-reaction pair. But whether it's the action or the reaction is a totally arbitrary distinction. In this example, I see two 3rd law pairs. I'll summarize them here:
    (1) The gravitational pull of the earth on an object; the gravitational pull of the object on the earth.
    (2) The force that the ground exerts on an object (normal force); the force that the object exerts on the ground.​
    Note that 3rd law pairs by themselves never produce equilibrium--they act on different objects. Note also that these 3rd law pairs must exist--it is physically impossible for A to push on B without B also pushing on A.

    Note that if the normal force equals the weight of the object, then the object will be in equilibrium. But that's got nothing to do with the 3rd law.
  4. Oct 24, 2005 #3

    Talk about seeing the light.

    Thank you so much for that. I totally forgot that an object can exert gravity on the earth as well. This was creating an entire host of problems for me, mismatched forces and a bunch of other crazy ideas.

    Thanks for the truth Doc.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook