posted
I don't want to sound like I'm asking for help with my homework, so I just want to know if the following problem seems odd in any way. We were doing a lab that involved using a wheelchair at various points on campus for everyday activities--nothing exotic like wheelchair basketball.
quote:Operating a wheel chair requires the coordination and use of the muscles of the upper extremity to produce compensatory locomotion that the lower extremity can no longer provide. Below is a 2-dimensional, planar model of a wheelchair and one arm. Your task is to determine how much force (Fue) the musculature of the upper extremity must produce at the point of contact with the wheel (the hand) to overcome the force of static friction, for a person weighing 200lbs, using a chair with a wheel radius of 2 feet, on a level surface. State your answer in lbs-force.
Keep in mind these equations when calculating your answer:
M = F *d (2-dimensional moment calculation; force times perpendicular distance) Ff = ks * N (frictional force calculation; coefficient of static friction times normal force) ΣM = 0 (The summation of all moments about a single point equals zero for static problems) ks= 1.0 (Frictional coefficient depends on the types of materials in contact and more specifically what their surface condition is)
posted
No, more than that: why are you using sliding (and static) friction in the first place? I'd think rolling friction would be more applicable here. Unless you're talking about the friction between the wheel and the axle, which really shouldn't have a coefficient of 1.0. Or you're planning on dragging the wheel sideways. If anything, the coefficient of static friction between the wheel and the ground will only tell you the maximum force you can apply before you start skidding. But then, maybe that's what the question is asking. Though you don't usually see many wheelchair users revving their engines or peeling out.
Posts: 1810 | Registered: Jan 1999
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posted
A coefficient of static friction of 1.0 is not that odd for rubber on asphalt (I'm at home, or I'd look it up in my CRC handbook, and tell you what the real value/range is).
But mike is right--it's really about rolling friction, not static friction. Think of a car with all four tires flat--the rolling friction is still not even close to 1.0. Think of a skateboard on steel--you're approaching zero (or, at least less than 0.05).
Unless, as Mike says, you're trying to "peel out" in your wheelchair...
posted
Thanks for your help. That's exactly what I was having a problem with. I didn't think it would be a good idea to overcome the force of static friction in a wheelchair, and 1.0, while being reasonable for rubber on asphalt, is far too high for rolling or axial friction. I guess I'll have to take this up with the TAs. I'm glad to know that there's a problem with the problem and not with me!
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