How Do Newton's 2nd and 3rd Laws Apply to a System with Friction?

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Homework Help Overview

The discussion revolves around applying Newton's 2nd and 3rd Laws to a system involving two blocks on a horizontal surface with friction and a hanging mass. The problem includes drawing free-body diagrams, identifying action-reaction pairs, and applying Newton's laws symbolically, while also considering the effects of friction.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss their ability to create free-body diagrams and identify forces but express uncertainty about calculating tensions and accelerations. Questions arise regarding the relationship between the blocks' accelerations and the role of friction in their motion.

Discussion Status

Some participants have provided insights into the problem setup and the implications of friction on the motion of the blocks. There is an ongoing exploration of how to approach the calculations and the relationships between the forces acting on the blocks.

Contextual Notes

Participants are navigating the constraints of the problem, including the requirement to express certain answers symbolically and the specific coefficients of friction provided. There is a noted lack of clarity on how the blocks will accelerate differently due to friction.

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Consider the figure below where the coefficient of kinetic friction between the m1= 2.0-kg and
the m2 = 3.0-kg blocks is 0.30. The hanging mass is given to be m3 =10.0-kg and the horizontal
surface supporting the blocks along with the pulleys are frictionless.http://img812.imageshack.us/img812/4836/33515317.jpg

Uploaded with ImageShack.us
(a) Draw the free-body diagram of each block.
(b) Identify the action-reaction pairs of forces between the 2.0-kg and the 3.0-kg blocks.
(c) Apply Newtonʹs second law to each block. Use symbols only.
(d) Determine the magnitude of the acceleration of each block.
 
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what part(s) can't you do?

R.
 
I can do the free body diagram for entire system and for individual box. It means I can do (a) and (b) But I can't do the next step to calculate (c) or (d)
Is the acceleration is the same for two boxes and calculated by (T2- total friction forces of two boxes)/(m1+m2) ? How to find T1?
T1 - friction force of smaller box = (mass of smaller box) x (acceleration of system) ?
I think friction of smaller box =( its weight (or its normal force) x coefficient) because the net internal contacted forces is zero, so they don't count for the normal force. is it true?
Thanks
 
Last edited:
c) doesn't require calculations, as it specifically asks to write the answer in symbols only.

Part d) is entirely down to tensions, weights and coefficient of friction.
For a start, think about what would actually happen when the bodies start moving. it should be easy enough to extrapolate the acceleration of the 10kg body.
From there you can work backwards using forces.R.
 
When the system starts moving ,the 10kg mass goes downwards pull the 3kg mass to the right. In the same time, the 2kg mass is pulled to the left.
But basically I don't know why each box will move with different accelerations.
 
Because there is friction between the 2kg and 3kg masses. (with a specific coefficient of friction that was given in the question).
 

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