How to I find what needs to be the mass of an object in a friction problem?

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    Friction Mass
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Homework Help Overview

The discussion revolves around a friction problem involving two masses, mA and mB, with given coefficients of static and kinetic friction. The original poster seeks to determine the minimum mass of mA required to prevent motion and the mass needed for constant speed movement.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the relationship between tension in the rope and the forces acting on the masses. Questions arise regarding the setup of free body diagrams (FBDs) and the roles of static versus kinetic friction.

Discussion Status

Participants are actively questioning the setup of equations and the implications of equilibrium conditions. Some guidance has been offered regarding the interpretation of forces, but clarity on certain aspects, such as the meaning of acceleration in the context of the problem, remains unresolved.

Contextual Notes

There is uncertainty regarding the values of forces and the definitions of terms used in the equations. The original poster expresses confusion about the tension in the rope and the implications of the system being at rest or in motion.

Mustachio
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Homework Statement



In the figure the coefficient of static friction between mass mA and the table is 0.50, whereas the coefficient of kinetic friction is 0.42.
(a) What minimum value of mA will keep the system from starting to move?
(b) What value of mA will keep the system moving at constant speed?

3HZVZ.gif



Homework Equations


So I know that

Fs <= uN
Fk = uN

But I don't know what to do when I lack the Newtons.

Can anyone give me a hint on where to start?


The Attempt at a Solution

 
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If the system isn't moving, what must be the tension in the rope?
 
Would it be 0 because it'd be at equilibrium when not moving?
 
If someone asked you to support that hanging mass mB by holding the upper end of the rope, what force would your arm have to exert? It would not be zero.
 
A force greater or equal to that of the force that is pulling down mb?
 
A force of how many Newtons?
 
Can I multiply 2 * 9.8(m/s^2) to get the mkg/s^2 = N unit?
 
Mustachio said:
Can I multiply 2 * 9.8(m/s^2) to get the mkg/s^2 = N unit?
Yes.
 
So I made 2 FBDs.

...... ^ N
......|
......|
mSn...mKn...|.... T
<---<--------|---------->
......|
......|
......| Ma(g)
...... vT
|
|
o
|
|
2g

And I derived 2 sets of equations.For the first FBD I got
X: T - mkN - msN = (Ma)a
Y: N - (Ma)g = 0

Second one I got

Y: T - 2g = 2a.

Do I have the problem set it up right?
 
  • #10
Mustachio said:
For the first FBD I got
X: T - mkN - msN = (Ma)a
What do mkN and msN stand for? If those are supposed to be the friction forces, realize that only one acts at any given time; it's either static or kinetic friction.

Y: N - (Ma)g = 0
OK.

Second one I got

Y: T - 2g = 2a.
OK. (What's a?)
Do I have the problem set it up right?
To solve the problem, answer the question I asked before:
If the system isn't moving--or is moving at constant speed--what must be the tension in the rope?
 
  • #11
I don't understand the question. :\
 
  • #12
Mustachio said:
I don't understand the question. :\
What does your equation for MB tell you? (What's the acceleration?)
 

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