# Ranking forces

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..
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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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WOW i actually just figured it out using newton laws

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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

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haruspex
Homework Helper
Gold Member
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.

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

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)

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

haruspex