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

When does the normal force act on something?

  1. Dec 21, 2012 #1
    A bit confused about the operation of normal forces.

    When does a normal force act due to some surface on an object? When the object touches the surface or when the object actually applies a force on the surface?

    The reason I ask this is because I am confused about normal forces experienced by objects travelling in circular loops, such as roller coasters etc.

    When the roller coaster is at the top of the circular loop, it is touching the surface of the circular track, but it isn't applying any force on it. So would there still be a normal force?


  2. jcsd
  3. Dec 21, 2012 #2


    User Avatar

    Staff: Mentor

    "Touching" something IS an application of force. The two cannot be separated.

    Why isn't the roller coaster applying a force against the track? It should apply a force in the direction away from the center of the loop. ("Down" from the perspective of the coaster, which would be "up" from the perspective of someone standing on the ground)
  4. Dec 21, 2012 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Supposing you are atop a building near the edge of the roof, holding 2 blocks of equal mass, one atop the other. There is a normal force between the 2 blocks equal to mg. Now you drop them toward the ground below. They are each now in free fall, assuming no air resistance. Questions:
    1. What now is the normal force between the 2 blocks?
    2. Are they touching??
  5. Dec 21, 2012 #4
    1) There is no normal fource?? :confused:

    2) Yes

  6. Dec 22, 2012 #5
    1) there will be no normal force
    2)yes, they will be in contact.
  7. Dec 22, 2012 #6
    Is normal force a contact force or can it act when objects are not in contact?

    If yes, give an example.
    Last edited: Dec 22, 2012
  8. Dec 22, 2012 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If there is no normal force, there is no contact. Actually, even when there is a normal force, the objects, at the quantum level, never touch..it is the electromagnetic repulsion of electrons that are responsible for the force. So all the more so is there no contact when the normal force is 0.
  9. Dec 22, 2012 #8
    As Drakkith sad, touching something is applying force. However:
    In this case, although it is possible that roller coaster had just enough speed so that it doesn't apply force when on the top of the loop, just at that moment it is not really touching the track. But in that special case the problem will usually state something like "just touching".
    So basically it is the limiting case of touching and not touching.
  10. Dec 22, 2012 #9


    The force (and contact) may be momentary as with two billiard balls colliding or it may be sustained as in PhantomJay's excellent example of the bricks. It may even be zero as with the falling bricks.

    However I would not advise casting it as non existant in the case of the falling bricks since only a tiny variation will introduce a non zero value. There is nothing wrong with zero as a value.

    Here is a further question for PJ about the bricks.

    Let us say the coefficient of friction between these bricks is 1.
    Let us further suppose that whilst falling the top brick is slid sideways over the lower brick.
    What is the force of friction and what work is done in that sliding?
  11. Dec 22, 2012 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Since normal force is 0, friction force is 0, no work done by friction.
  12. Dec 22, 2012 #11
    Do you not find that statement interesting and worth further comment?

    Oh and I'm sorry I should have said work done against friction.
  13. Dec 22, 2012 #12


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No work done by friction. No work done against friction. Work done by the force causing the motion depends on the distance through which it acts. Top brick will accelerate in the direction of the net force (resultant of the gravity and applied force).
    I find it always interesting when talking about 0 ( that's where the secret of the universe may lie , if, indeed, 0 exists).In this falling brick case , since there is no contact between the two bricks and no normal force , it doesn't much matter what the coefficient of friction between the 2 is . Might as well place them side by side with same results. It does seem a bit strange, though.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook