What is weight shift?

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May 24, 2013
12,461
113
So Cal
I respectfully disagree on the ''rear foot getting unweighted'' and all the weight shifting to the front side. I'm more of the weight shifting to/towards the middle during the swing. Again JMHO.

Hmmm...help me understand where you're coming from on this. I think we are actually in agreement.

As I see it, there is a significant difference between a weight shift against the front side vs. a weight shift over the front side. Pujols is the first one. Clemente is the second one. Both have a weight shift to the front side which creates a complete unweighting of the rear foot. Pujols' COM stays in the middle, but his movements cause his COG to shift from rear foot to front foot (I think I'm using the those terms correctly, but maybe not). Clemente's COM and COG shift over the front foot.

EDIT: Pattar has shown that I don't understand COM and COG very well - LOL (see below)
 
Last edited:

rdbass

It wasn't me.
Jun 5, 2010
9,131
83
Not here.
Hmmm...help me understand where you're coming from on this. I think we are actually in agreement.

As I see it, there is a significant difference between a weight shift against the front side vs. a weight shift over the front side. Pujols is the first one. Clemente is the second one. Both have a weight shift to the front side which creates a complete unweighting of the rear foot. Pujols' COM stays in the middle, but his movements cause his COG to shift from rear foot to front foot (I think I'm using the those terms correctly, but maybe not). Clemente's COM and COG shift over the front foot.

This is the only thing I dis agreed with in bold:
I respectfully disagree on the ''rear foot getting unweighted'' and all the weight shifting to the front side.
I don't 'see' rear foot getting unweight.
 
Jun 8, 2016
16,118
113
Hmmm...help me understand where you're coming from on this. I think we are actually in agreement.

As I see it, there is a significant difference between a weight shift against the front side vs. a weight shift over the front side. Pujols is the first one. Clemente is the second one. Both have a weight shift to the front side which creates a complete unweighting of the rear foot. Pujols' COM stays in the middle, but his movements cause his COG to shift from rear foot to front foot (I think I'm using the those terms correctly, but maybe not). Clemente's COM and COG shift over the front foot.

Ok, the engineer/scientist in me is forcing me to state that in a uniform gravity field COG and COM are the same which for all practical purposes is the case here. Also the only way an object can become unweighted is for the magnitude of the gravitational field to go to zero. I think a better expression would be that the reaction force on the rear foot goes to zero. For the Pujols/Clemente comparison I would just say that the COM/COG moves more towards the front foot for Clemente than it does for Pujols. I apologize for being pedantic :p
 
Last edited:
May 24, 2013
12,461
113
So Cal
Ok, the engineer/scientist in me is forcing me to state that in a uniform gravity field COG and COM are the same which for all practical purposes is the case here. Also the only way an object can become unweighted is for the magnitude of the gravitational field to go to zero. I think a better expression would be that the reaction force on the rear foot goes to zero. For the Pujols/Clemente comparison I would just say that the COM/COG moves more towards the front foot for Clemente than it does for Pujols. I apologize for being pedantic :p

Yeah. I had a feeling I was off on my terms. The rest of what you said pretty much went completely over my head, too.
 
Jun 8, 2016
16,118
113
Yeah. I had a feeling I was off on my terms. The rest of what you said pretty much went completely over my head, too.

Basically a foot has the same weight (mass * magnitude of gravitational acceleration) regardless of whether it is on the ground or in the air. The reaction force
of the foot is the force exerted on the foot by the ground which is equal and opposite in direction to force exerted by the foot on the ground, e.g. Newton's third law. This
is what would be measured when force plates are used (there has been some twitter posts about this lately regarding hitting).
When the foot is in the air there is no interaction with the ground and hence no reaction force.
 
May 24, 2013
12,461
113
So Cal
Ok, the engineer/scientist in me is forcing me to state that in a uniform gravity field COG and COM are the same which for all practical purposes is the case here. Also the only way an object can become unweighted is for the magnitude of the gravitational field to go to zero. I think a better expression would be that the reaction force on the rear foot goes to zero. For the Pujols/Clemente comparison I would just say that the COM/COG moves more towards the front foot for Clemente than it does for Pujols. I apologize for being pedantic :p

Okay...so let me try to wrap my dumb head around part of this...

At the point in time where a hitter's rear foot comes off the ground, the correct terminology would be that the rear foot has "zero ground reaction force"?
 
May 24, 2013
12,461
113
So Cal
Basically a foot has the same weight (mass * magnitude of gravitational acceleration) regardless of whether it is on the ground or in the air. The reaction force
of the foot is the force exerted on the foot by the ground which is equal and opposite in direction to force exerted by the foot on the ground, e.g. Newton's third law. This
is what would be measured when force plates are used (there has been some twitter posts about this lately regarding hitting).
When the foot is in the air there is no interaction with the ground and hence no reaction force.

Us dumb guys call that "unweighted". Kids tend to understand that term better, too. "Reduce the reaction force of your rear foot" is going to get a lot of puzzled stares. ;)
 
Jun 8, 2016
16,118
113
Okay...so let me try to wrap my dumb head around part of this...

At the point in time where a hitter's rear foot comes off the ground, the correct terminology would be that the rear foot has "zero ground reaction force"?

You got it.
 

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