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Homework Help: Friction and normal force

  1. Feb 17, 2010 #1
    More or less just a general question.

    So if the normal force is pretty much the weight of the object (mass times acceleration of gravity) and then the friction is that times the coefficient? So basically weight is what causes MORE magnitude of friction? We take the force vector of gravity, turn it around and multiply it by the coefficient of the frictioning items to get..the static friction...right? I'm starting to think teaching this with vectors ruins the obviousness of this whole thing. Soemone correct me?
     
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  3. Feb 17, 2010 #2

    PhanthomJay

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    No, the normal force is sometimes equal and opposite to the weight of the object, when there is no acceleration in the vertical direction and the object is on a level surface, but generally that is not the case. The normal force is the contact force between the 2 objects acting perpendicular to the plane of the contact surface. Once that normal force is determined from newton's laws and often some trig, then the friction force is as you say...static friction if they are not moving relative to each other, and kinetic friction if there is motion relative to each other.
     
  4. Feb 17, 2010 #3
    Woo that confused me ha. Sorry. Maybe I worded It kinda funny. So the normal force, is the contact for between two objects? That makes sense, so of course the weight of the object on the surface? If I'm correct the weight is the measure of the force of gravity on a certain mass. With gravity however, shouldn't there ALWAYS be acceleration in the vertical direction?

    It would appear to me that the examples in the book shows me, the normal force multiplied by the coefficient of friction being the static friction. So weight DOES contribute to the friction(mathematically at least?). And how is normal force SOMETIMES equal the opposite?

    Yes, it's static until overcome and then kinetic frictions sets into play.

    I'm trying to get this, im sorta behind because I actually care enough to get this through my skull instead of plug and chugging and moving onto friction on an inclined plane and being completely lost.
     
  5. Feb 17, 2010 #4

    PhanthomJay

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    the force of gravity will always accelerate an object in a downward vertical direction IF it is the only force acting in the vertical direction. When an object sits on table, it isn't moving, and it's not accelerating. The weight force acts down on it, trying to accelerate it, but the normal force of the table on the block , per newton 1, is equal to it in the opposite direction, so the NET force is 0. No net force, no acceleration, per newton 1, F_net = 0 for bodies at rest or moving in a straight line with constant speed.
    sometimes static, sometimes kinetic. If the block is moving relative to the table, you must use the kinetic friction coeficient. If it's not moving relative to the table, the friction force is less than or equal to uN. If the block is just sitting there and no one is pushing it or trying to push it sideways, the friction force is 0, right??
    weight may contribute to the normal force, which contributes to the friction...
    a book weighs 10 N and it sits on a level table, motionless. The normal force of the table on the book is 10N up, per Newton 1. If the book is on an incline, the normal force is not 10 N up, it is less. The Normal force acts perpendicular to the incline. It's magnitude is found by using vector components and trig and newton's laws.
    Good move, the incline plane stuff will lose you if you don't get the basics.
     
  6. Feb 17, 2010 #5
    Aha, so it has equilibrium. So its not really much of a vector, but the acceleration of gravity that would be can still be turned around and multipiled by the co to get the static or kinetic friction,right?
     
  7. Feb 17, 2010 #6

    PhanthomJay

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    sure it is, the Normal force has magnitude and direction, so it's a vector; the net force on the block is 0, so i guess the net force isn't much of a vector, but the weight force and normal forces on the block are vectors.
    yes, only if the block is on a level table : if it is , the normal force is mg ,from newton1 : mg -N =0, thus, N =mg. If the block is on an incline, the normal force is not mg, more on this later after you get the basics.
     
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