Mouse Glide Science

  1. I've recently been looking at the friction caused by a computer mouse sliding on a mousepad.

    I was wondering, if I increased the size (area) of the mousefeet (points of contact of the mouse with the mousepad) will I get an increase in static and/or dynamic friction?

    Also, will increasing the weight of the mouse increase static and/or dynamic friction?

    Furthermore, let's take a mousepad with a cloth surface with a foam layer underneath where the mouse slightly sinks in if there is enough perpendicular force acting on it. If I have two mousepads with exactly the same surface but one with a foam layer thicker than the other, how will they compare in terms of static and dynamic friction? (might be wise to consider there will be a hard material under the foam layer)


    Thanks in advance!
     
    Last edited: Aug 14, 2014
  2. jcsd
  3. jedishrfu

    Staff: Mentor

    You might want to start by reading about friction:

    http://en.wikipedia.org/wiki/Friction

    In particular pay attention to the sliding block on an inclined plane. Notice how the normal vector is connected to the friction the block has and also that there are two kinds of frictions static and kinetic each with their own coefficients that are dependent on the surface.
     

  4. I read it all twice and it did not answer my questions. :(
     
  5. Nugatory

    Staff: Mentor

    OK, let's try a few.
    The simplest model says that the frictional is equal to a constant times the normal force: ##F_f=\mu{F}_n## where the "normal force" is the perpendicular force between the two surfaces and ##\mu## is a constant. For a computer mouse, the normal force is the weight of the mouse plus whatever additional force comes from the user pressing the mouse down, resting soem of the weight of his hand on it, or whatever.

    The contact area doesn't show up in this formula, so it doesn't affect the frictional force.

    Yes. That will increase the normal force and hence the frictional force.

    That will depend on the exact details of the materials used, but for most reasonable setups, you would expect that the constant ##\mu## will be higher on the softer surface, so the frictional force would be greater for the same normal force.

    This case can become very complicated because, although the contact area doesn't affect the normal force, it does affect the pressure across the contact area, and that pressure in turn affects how much the softer surface deforms, which in turn affects the value of ##\mu##. So ##\mu## may not be a constant in this situation.

    Generally the straghtforward ##F_f=\mu{F}_n## rule applies only when the two surfaces are rigid enough that they don't deform under the pressure from the normal force.
     
  6. It might be worthy of note that most actual mousepads employ an very dense foam that takes considerable downward force even at the point of a finger to compress. This is the place where surface area has an effect - the deformation of the surface. It is highly unlikely that as large as most mice are that one could substantially compress an actual pad. That said, there are gaming pads that have a cloth surface on one side and a metal surface on the other and the metal side is very fast. The textured cloth surface is a holdover from the days when mice had balls. I refuse to comment on whether this has affected their testosterone levels :biggrin:
     
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