3 blocks no friction forces question

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Homework Help Overview

The discussion revolves around a physics problem involving three blocks with no friction, focusing on the forces acting between them and their resulting accelerations. The subject area includes dynamics and Newton's laws of motion.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the forces acting on the blocks and their interactions, questioning the calculations of forces based on Newton's third law. There is discussion about the resultant forces acting on the blocks and how they relate to each other.

Discussion Status

Some participants have offered insights into the forces acting on the blocks, while others are questioning the calculations and interpretations of the forces involved. There is an ongoing exploration of the relationships between the blocks and the forces they exert on one another.

Contextual Notes

There appears to be confusion regarding the application of Newton's laws and the calculations of forces, as well as discrepancies with the homework system's feedback. The original poster acknowledges their calculations may be incorrect, prompting further discussion.

cmwilli
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Homework Statement


[PLAIN]http://img834.imageshack.us/img834/6920/captureqt.jpg


Homework Equations



f=ma

The Attempt at a Solution



Fsum=Fl + Fr=(m1+m2+m3)a
26+-8=(2+8+4)a
a=18/14=1.28571

f=ma
f=4(1.28571)=5.14268N

I know this is incorrect. The force of 4kg block pushes on the 8kg the same as the 8kg pushes on the 4kg, but the way I calculated it doesn't work with that but I know of no other way.
 
Last edited by a moderator:
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You are right that the resultant force acting on the 4 kg block is F=4a=5.143 N. See the figure: What forces act on this block?

ehild
 
The homework systems seems to disagree.

Call the blocks from left to right A,B,C

The way the teacher explained it in class, the force of C acting on B is the same as the force of B acting on C. But the way I calculated it Fc=4a but Fb=8a
 
Look at the 4 kg mass. There are two forces acting on it: the force from the 8kg mass (F8) ponting to the right, and Fr=8 N, pointing to the left. The resultant force points to the right, and it is F8-Fr=4*a. So F8=4a+Fr.

ehild
 
Is the correct answer then 13.14268N?
 
Yes, the 8 kg mass exerts 13.14 N on the 4 kg one, and a force of the same magnitude and opposite direction is exerted by the 4 kg mass on the 8 kg one.

ehild
 

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