Force Generated By Leg Muscles in Free Body Diagrams

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Discussion Overview

The discussion revolves around the forces generated by leg muscles during walking, particularly in the context of free body diagrams. Participants explore the nature of internal forces, reaction forces, and the implications of Newton's third law in this scenario, including considerations of potential energy and inertia.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the force of the leg muscle onto the foot is an internal force and not of direct interest, while the reaction force of the ground on the foot is significant and equal and opposite to the force exerted by the foot on the ground.
  • There is a question regarding what force causes the leg muscle to exert force onto the foot, with some suggesting that this is also an internal force.
  • One participant emphasizes that causation does not influence Newton's third law, stating that it is arbitrary which force is considered the action or reaction.
  • Another participant introduces the idea that the inertia of the body and changes in potential energy are relevant to the discussion, although others clarify that these are not forces in the context of Newton's third law.
  • Mathematical expressions are introduced to clarify the relationship between potential energy and force, with a focus on the gradient of potential energy.
  • There is a mention of the mechanics of walking, including the role of ligaments and muscle actions in propelling the body forward.

Areas of Agreement / Disagreement

Participants express differing views on the relevance and interpretation of internal forces, the application of Newton's third law, and the definitions of forces in relation to potential energy and inertia. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some statements rely on specific definitions of forces and may not account for all assumptions involved in the discussion of internal versus external forces. The mathematical relationships presented may depend on the context of the physical scenario being analyzed.

annamal
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TL;DR
Suppose a person is walking on the ground without slipping. For the free body diagram of just the person, only the frictional force is drawn in the horizontal direction. The force exerted by the leg muscles to generate a force against the ground is considered an internal force. What would be the equal and opposite force of the force generated by the leg muscles since that force is an internal force of the free body diagram of the person?
Suppose a person is walking on the ground without slipping. For the free body diagram of just the person, only the frictional force is drawn in the horizontal direction. The force exerted by the leg muscles to generate a force against the ground is considered an internal force. What would be the equal and opposite forces of the force generated by the leg muscles since that force is an internal force of the free body diagram of the person?

Screenshot 2023-04-28 at 3.18.57 PM.png
 
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  1. The force of the leg muscle onto the foot is an internal force and not of interest.
  2. The force of the foot onto the ground is a force onto the ground and not directly of interest.
  3. The reaction force of the ground on foot is of interest and is exactly equal and opposite the force (2) above
 
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hutchphd said:
  1. The force of the leg muscle onto the foot is an internal force and not of interest.
  2. The force of the foot onto the ground is a force onto the ground and not directly of interest.
  3. The reaction force of the ground on foot is of interest and is exactly equal and opposite the force (2) above
Ok, so I guess I am asking what force causes the force of the leg muscle onto the foot and since it is an internal force, what is the reaction force to that force?
 
It is the force of the foot on the muscle (also an internal force and again probably not of interest). The reaction force of A on B is always the force of B on A. Always. $$\vec F_{AB}=- \vec F_{BA}$$
 
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annamal said:
Ok, so I guess I am asking what force causes the force of the leg muscle onto the foot and since it is an internal force, what is the reaction force to that force?
Additionally to what @hutchphd wrote: Causation plays no role in Newton's 3rd Law. It is arbitrary which of the two forces you consider reaction and which action.
 
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annamal said:
What would be the equal and opposite force of the force generated by the leg muscles since that force is an internal force of the free body diagram of the person?
Would that be the inertia of the body to be moved forward plus the changes in potential energy of the center of mass?
Walking uphill, for example, requires greater muscular work and higher friction force of the foot against the slope than walking on a flat surface.

The internal forces come from contractions of certain muscles, which form a triangle respect to two bones.
Those internal forces induce a moment in the leg simultaneously with a moment in the upper body to fall forward, completing one balanced step.

Please, see:
https://en.wikipedia.org/wiki/Leg_mechanism

https://vondesmos.wordpress.com/2016/07/19/a-walking-machine/

tumblr_m2y9knsz7n1rsz0ajo1_500.gif

RVC-1.gif
 
annamal said:
What would be the equal and opposite force of the force generated by the leg muscles since that force is an internal force of the free body diagram of the person?
Lnewqban said:
Would that be the inertia of the body to be moved forward plus the changes in potential energy of the center of mass?
If we are talking about equal and opposite force in the sense of Newton's 3rd Law, then no. Newton's 3rd Law is as trivial and simple as stated by @hutchphd in post #4. Also, neither "inertia of the body" nor "change in potential energy" are forces.
 
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Well, by definition the potential's "change with position" is the "force" ;-)). Of course it's better to make precise statements in terms of math,
$$\vec{F}=-\vec{\nabla} V.$$
 
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Lnewqban said:
Lnewqban said:
Would that be the inertia of the body to be moved forward plus the changes in potential energy of the center of mass?View attachment 325679
Nice "walking machine" but no resemblance to human walking.
The first various ligament actions pulled by muscles, first raise 1 heel to tilt while leaning the centre of mass forward, to scissor forward the alternate thigh and propel with the ball of the foot with the raised heel.
 

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