Direction of frictional force when running

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Discussion Overview

The discussion revolves around the direction of the frictional force experienced by a person while running. Participants explore the mechanics of friction in relation to running, addressing both theoretical and practical aspects of the forces involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the frictional force is forward when considering the net momentum transfer from the ground to counteract air resistance.
  • Others argue that the direction of the frictional force can vary during different phases of running, with it being backward in the early stance phase and forward in the late stance phase.
  • A participant emphasizes the importance of distinguishing between the two forces of friction: the force exerted by the shoe on the ground and the force exerted by the ground on the shoe, noting that the latter must be forward for propulsion.
  • There is mention of confusion arising from imprecise language regarding the forces of friction involved in running.
  • One participant notes that during acceleration or in the presence of air resistance, the average frictional force will be forward, while on a treadmill, it is approximately zero.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the direction of the frictional force, with multiple competing views presented regarding its behavior during different phases of running and conditions.

Contextual Notes

Some limitations include the dependence on the definitions of forces involved and the varying conditions under which the frictional force is analyzed, such as running on a treadmill versus on solid ground.

Who May Find This Useful

This discussion may be useful for individuals interested in biomechanics, physics of motion, or those seeking to understand the dynamics of running and the forces at play.

sach963
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When a person is running is frictional force forward or backward?
I do not have good knowledge of
 
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Well, they're moving forward, pushing their legs backwards. Newton's 3rd law says the force A exerts on B is equal and opposite of the force B exerts on A. What does this say to you?
 
sach963 said:
When a person is running is frictional force forward or backward?
Both, but for steady speed the net momentum transfer from the ground is forward to counter air resistance (unless on treadmill).

 
So, this means that friction between ground and shoe is forward?
 
sach963 said:
So, this means that friction between ground and shoe is forward?
The horizontal force (friction) on the shoe is backward in early stance phase, and forward in late stance phase, See the smaller component in the plot in the video above: negative = backwards, positve = forward. The bigger, always positive, component is the vertical force on the shoe (normal force).
 
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Much of the confusion in all similar situations arises because of using imprecise language. For example, regarding the question by sach963, "So this means friction force between ground and shoe is forward?", there are TWO forces of friction between ground and shoe. One is the force by shoe on ground, and the other by ground on shoe. It is the force between ground and shoe that needs to be ultimately forward so that the person is propelled forward by it.This is clearly stated by A.T. above, "force on the shoe". When your shoe pushes back on the ground, the ground pushes forward on your shoe.
 
Chandra Prayaga said:
Much of the confusion in all similar situations arises because of using imprecise language. For example, regarding the question by sach963, "So this means friction force between ground and shoe is forward?", there are TWO forces of friction between ground and shoe. One is the force by shoe on ground, and the other by ground on shoe. It is the force between ground and shoe that needs to be ultimately forward so that the person is propelled forward by it.This is clearly stated by A.T. above, "force on the shoe". When your shoe pushes back on the ground, the ground pushes forward on your shoe.
Sorry, the wrong words got cut and pasted in my response, here is the correct one:
Much of the confusion in all similar situations arises because of using imprecise language. For example, regarding the question by sach963, "So this means friction force between ground and shoe is forward?", there are TWO forces of friction between ground and shoe. One is the force by shoe on ground, and the other by ground on shoe. It is the force BY ground ON shoe that needs to be ultimately forward so that the person is propelled forward by it.This is clearly stated by A.T. above, "force on the shoe". When your shoe pushes back on the ground, the ground pushes forward on your shoe.
 
Incidentally, A.T., that was a very nice video. Can I show it in my class?
 
Chandra Prayaga said:
t is the force BY ground ON shoe that needs to be ultimately forward so that the person is propelled forward by it.
With air resistance or during acceleration the average force will be forward. On the treadmill the average horizontal force is approximately zero.

Chandra Prayaga said:
Incidentally, A.T., that was a very nice video. Can I show it in my class?
It's not mine. It comes from here:
http://www.smu.edu/Simmons/Research/LPL
 
  • #10
Thanks
 
  • #11
Thank you all for your replies.:smile::smile::smile::smile::smile::smile:
This question had been breaking my head.:headbang:
 
  • #12
Thank you all for your replies.:bow:
This had been breaking my head.:headbang:
 

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