Ranking forces

  • #1
apr4.PNG
1. Homework Statement

in picture

Homework Equations


My equations are
netforceyA = -MG + Friction between A and Hand + Friction between A and B
netforceyB = -MG + Friction between B and A

The Attempt at a Solution


Friction between A and B are equivelant due to newtons third law since they are both accelerating upwards at the same rate correct?
I am not understanding this problem very much.. i know that the netforce = 0 for both cases since they are accelerating upwards at same rate (shown by velocity vector in middle of boxes) so the acceleration is 0. I don't know how to figure the rankings after solving the equations for the forces..
[/B]
 

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  • #2
APR42.PNG


So i know that the force of gravity on A is greater than the force of gravity on B (problem says A has more mass).. and i believe that newtons third law will tell us that the frictional forces between the two blocks are equivalent since they are both accelerating upwards at the same rate.. i am not sure how i would rank these two against eachother and i believe that the frictional force between the hand and A is greater than the force of gravity..
 

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  • #3
APR43.PNG


WOW i actually just figured it out using newton laws
 

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  • #5
I'm glad you figured it out, but you should not have abandoned the thread at https://www.physicsforums.com/threads/a-problem-on-ranking-forces.943704/ and posted it afresh. I was helping you there. Indeed, it sounds like you solved it by taking my advice.
Hey sorry about that.. I can delete that other thread, i am now on part 2 and not understanding something..
I have
netforceyA = -MG + Friction between A and Hand + Friction between A and B > 0
netforceyB = -MG + Friction between B and A > 0

Friction between B and A > MG
FFAB = FFBA because newton 3rd law so FFAB = FFBA > FGBE

Friction between A and Hand > MG - Friction between A and B

Since accelerating upwards the frictional force beats gravity so FFAH > FGAE
i think my ranking is wrong after FGAE, But i am unsure why


con6.PNG
 

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  • #6
haruspex
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netforceyB = -MG +
The masses are different. To avoid confusion, use m for the mass of B. (And it is g everywhere, not G.)
Friction between A and B
Friction between B and A
FFAB = FFBA
Careful about directions. The friction between A and B cannot be pushing both up. One problem is that "between" does not specify which way around. Better to write "frictional force of A on B" etc.
It is not entirely clear, but I would assume the question intends that "FFAB" etc. are only supposed to represent the magnitudes of the forces.
 
  • #7
The masses are different. To avoid confusion, use m for the mass of B. (And it is g everywhere, not G.)



Careful about directions. The friction between A and B cannot be pushing both up. One problem is that "between" does not specify which way around. Better to write "frictional force of A on B" etc.
It is not entirely clear, but I would assume the question intends that "FFAB" etc. are only supposed to represent the magnitudes of the forces.
Hi, yes, it is magnitude ranking i should have mentioned that.. The program is giving a hint tho to check the force of gravity on A..Not sure what I'm doing wrong with A's MG
 
  • #8
Ok i figured out that i had everything correct but i just had to eliminate FGAE because there was not enough info to actually give a ranking for it (i think)
 
  • #9
Ok i figured out that i had everything correct but i just had to eliminate FGAE because there was not enough info to actually give a ranking for it (i think)
it makes sense to me right now because using newtons laws i was totally confused about how to prove FGAE ranking in this picture given increased acceleration..it seemed like there was no way to prove it. not sure
 
  • #10
haruspex
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it makes sense to me right now because using newtons laws i was totally confused about how to prove FGAE ranking in this picture given increased acceleration..it seemed like there was no way to prove it. not sure
How do you deduce that FGAE>FFAB?
 

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