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Homework Help: Friction Question

  1. Feb 2, 2007 #1
    If there were a trailer with 4 wheels, with a frictional force acting upon the trailer of say 100N. What woudl happen to the frictional force if one set of wheels were taken away so that only two wheels remained?

    I thought that the frictional force would remain the same because the weight of the trailer would then be more concentrated on the single set of wheels, my physics teacher thought that there would be less friction with only two wheels.


  2. jcsd
  3. Feb 2, 2007 #2
    The weight force wouldn't be more concentrated; the pressure would. p = f/a.

    But since half the weight of the total number of wheels would be removed, the friction would be less because the force exerted on the ground by the trailer would be less now, and hence, the vector product of the forces acting upon it would be less.

    It also depends on the surface, velocity and other factors.
  4. Feb 2, 2007 #3
    i forgot about cahnge in mass of the trailer due to actually removing wheels... stupid.

    The trailer was probably a bad example to show what i am trying to find out, how about this...

    If we had a rectangualr prism, and placed it with the larger surface area lying on the floor, would the same amount of friction be present if we were then to stand the prism up on it's end?

    If somene asked me that question, i would say that the friction would be the same, because when the prism is on its end (that with the smaller surface area) the mass would be more concentrated over the smaller area which is in contact with the table or ground or whatever...

    Have i completely lost the plot or does my thinking have some merit?


  5. Feb 2, 2007 #4
    Indeed, the force would be more concentrated, but this would not affect the frictional force.

    Several things need to be ascertained before you can successfully explore your model:

    .Is the surface of the box uniformly smooth/rough on each side? (No jagged edges, different material..)

    . Is the surface (floor) very low friction, or at least, a constant friction constant for the model?

    . Is it traveling at a uniform velocity?

    . Others

    If all this is correct, so they're are no inconsistencies to factor in (which would change the answer) then the answer would be this:

    The friction would not change by swapping the prism to its end. Unless there is a change in:

    .Frictional constant (u (need to find symbol for mu))
    .Air speed (Air resistance is a type of friction as well)

    The friction would remain the same. However, the pressure the prism exerts on the ground, due to it's weight force standing on a smaller area, would increase.

    I hope this answers your question.
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