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Homework Help: Question about forces

  1. Dec 15, 2004 #1
    Can someone explain the following forces(equations, different ways to find them,
    Tension Force
    Frictional forces (static and Kinetic)
    Normal Forces
    I have a force labeled as Fp on a Diagram, if you know what that is could you tell me what it is and explain it.

    (I was absent a few days of the week when we learned this, and since then I have been getting test scores in the 30's and 40's because of it)
  2. jcsd
  3. Dec 15, 2004 #2
    Tension force is the same as normal force in that tension is pulled, not pushed.

    For example someone pulls a wagon of 10kg at 5m/s^2. Well, F = ma and since tension is the same as force, T = ma. T = (10kg)(5/m/s^2) = 50N.

    Frictional Force is found by using the formula f=umg, where u = the coefficent of friction.

    Normal force is the opposing force. Lets say a block that weights 10 kg sits on the ground, and gravity is acting on it at 9.8m/s^2. Therefore, the normal force would be F = mg, F = (10kg)(9.8m/s^2) = 98N.

    Fp is probably the force from pushing when you push a block on a floor that has the coefficent of friction of .04 or something.

    If anyone see anything wrong with my post, feel free to correct it. :zzz: And you might want to check your books for better explaination since I am also busy covering 13 chapters worth of notes to prepare for my exam.
    Last edited: Dec 15, 2004
  4. Dec 16, 2004 #3
    I'll try to give you some real-world examples of your forces.

    When you pull a string tight against something, there is a tension force at play. If you tie something to a string and let it hang from the ceiling, for instance, there is a tension force. You can use Newton's second law F=ma to determine this force; since a hanging object undergoes no acceleration, net force must = 0. Net force = T - Mg (tension is directed upward in this case) so we know T must = Mg.

    2. When you drag something across the floor and feel resistance, you are experiencing friction. A static frictional force is one in which an object doesn't move. As you apply a force to a stationary object on a non-frictionless surface, you notice that it dosn't move at first. The frictional force on the object is exactly equal to the force you apply to the object, and this makes since, since net force = F - friction = 0 and there is no acceleration. There is a certain point, however, at which there is no more static friction, and the object begins to move, and the kinetic frictional force comes into play. The coefficient of static friction multiplied by the normal force on an object (usually equivalent to its weight Mg) gives you the maximum force you can apply on the object before it begins to move. The coefficient of kinetic friction multiplied by the normal force gives you the constant frictional force which acts on a moving object once the maximum of static friction has been exceeded.

    I'm not sure what Fp might mean...
  5. Dec 16, 2004 #4


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    The Kinetic Friction force has the expression:
    [tex] \vec{F}_{f,K}=\mu \vec {N} [/tex]

  6. Dec 16, 2004 #5
    One more point is that these forces, like all forces, are vectors, and as such they have direction. Kinetic frictional force is directed in the opposite direction of the motion, and the static frictional force is directed opposite the applied force. Also, note that the coefficient of kinetic friction and static friction are different, with the kinetic friction coefficient being less. Tension forces are always directed away from the object it's connected to. So in the case of an object hanging via a string from a ceiling, both the object and the ceiling feel a force directed toward the center of the string with magnitude [tex]T = mg[/tex] as already correctly pointed out. Although everyday objects may appear smooth at a macroscopic layer, they are actually jagged at a microscopic view. At the atomic level, friction of both forms is the result of atomic bonds that occur at the point of contact between the two surfaces. When you're sliding an object that's experiencing friction, those bonds are breaking and reforming very quickly at different points of contact.

    Mike Fairchild
    "Euclid alone has looked on beauty bare."
    --Edna St. Vincent Mallay
    Last edited: Dec 16, 2004
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