This matter falls outside the domain of this service or is not acceptable for other reasons. For an explanation: http://www.newton.dep.anl.gov/aasinfo.htm http://www.newton.dep.anl.gov/hints.htm Running, Static Friction, Torques
 
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Name: Steve
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: April 28, 2011


Question:
My question is about the forces and torques required to run fast and come to a stop. My thinking is this: When you run fast (to the right say) and set your feet in order to stop, you have your weight back at the moment you set your feet. There are three forces acting on you: gravitation, friction, and normal force from the floor (excluding air). Only the frictional force reduces my horizontal speed. I can sum torques about any point so I choose to sum torque about the center of mass. Gravity produces no torque. Normal force produces torque that rotates my body counter-clockwise and torque from friction rotates me clockwise. The net torque is due to friction, which is why I rotate clockwise. Once I have my center of mass above my feet, I have stopped rotating. So at some point, the Normal force starts producing more torque than friction. But the horizontal distance between my center of mass and my feet is getting shorter as the torque from this force gets stronger relative to the torque from friction. Either the Normal force must get stronger or friction must get weaker as I slow down, or both

Replies:
Steve, Because you choose to use center of mass as the pivot point around which torque is calculated, gravity never produces torque for you. You are correct about friction getting weaker. This is static friction, not kinetic (or sliding) friction. Your shoes do not slide on the ground. As the force needed to keep your feet from sliding on the ground decreases, the friction decreases. Static friction is only as big as it has to be.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


Friction gets weaker. Static friction basically is a force of constraint, it is whatever is necessary to keep contacting surfaces from slipping. The formula with the coefficient of friction gives a maximum value, but the actual friction force can be anything less than that in magnitude. You can feel that as you stop: there is zero sideways force (friction) when you are standing still, unless you have your feet planted very far apart.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


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