Solve Newton's Laws to Find Mc for Static Position of MD

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Homework Help Overview

The discussion revolves around a physics problem involving Newton's Laws, specifically focusing on determining the mass of object C (Mc) required to keep object D (MD) in a static position on a frictionless surface. The problem includes various masses and a static friction coefficient between objects B and D.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the conditions of the problem, including the frictionless nature of the surface and the static friction coefficient. There are attempts to clarify what is meant by "static position" and the implications of the entire system's motion. Some participants suggest identifying the maximum acceleration that object D can tolerate without being displaced.

Discussion Status

There is an ongoing exploration of the problem with participants confirming their understanding and attempting to identify the correct approach to find the mass of C. Some guidance has been offered regarding the need to analyze the maximum acceleration and the relationship between the masses involved.

Contextual Notes

Participants are working under the constraints of the problem as presented, including the specific values for masses and the static friction coefficient. There is a noted confusion regarding the interpretation of the static condition and the frictionless surface.

sedaw
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http://img237.imageshack.us/img237/5499/22gj5.jpg

the floor frictionless
between D and C : MUs = 0.2
mA=5kg
mB=10kg
mD=2kg

what is the value for Mc keepin MD in static position ?


my effort :
http://img389.imageshack.us/img389/9196/hwscan00010zf3.png

the answer in the book 1.333<m<10.5 not as my .

what is wrong ?


TNX !:smile:
 
Last edited by a moderator:
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somebody ?

PLZ ...
 
I don't understand the problem. What's frictionless? The friction coefficient is given for what surfaces? What is being held static? (If the entire system is static, the acceleration would be zero.)
 
Doc Al said:
I don't understand the problem. What's frictionless? The friction coefficient is given for what surfaces? What is being held static? (If the entire system is static, the acceleration would be zero.)


NO ! the entire system is not static .
the surface that object B placed on is frictionless .
The friction coefficient between the objects B and D is 0.2 .
need to determine the value of the mass of object C that hold object D static .
 
Last edited:
sedaw said:
the floor frictionless
between D and C : MUs = 0.2
mA=5kg
mB=10kg
mD=2kg

what is the value for Mc keepin MD in static position ?

the answer in the book 1.333<m<10.5 not as my .

I believe I understand the problem. The surface between B and the floor it's on is frictionless. The surface between D and B however has a static coefficient of .2.

What they are asking, if I am reading your effort correctly, is to identify the domain of values for the mass of C, such that whatever motion ensues, D does not get displaced on the top of B.

That said I would suggest a simpler approach. First identify what acceleration D will tolerate without being displaced relative to B.

Then using that value construct the two cases: one where C is greater than A, that will cause the system to accelerate TOWARD C, the tipping point as it were of the BD contact. And the other case where the system will be set in motion accelerating TOWARD A without disturbing D.

Solving these 2 cases then will set the limit of the domain for the mass of C.
 
yes... that is right !
i think this is what i did .
 
sedaw said:
yes... that is right !
i think this is what i did .

Without checking your algebra for what you did do, I got the answers you gave from the book.
 
Now it makes sense! :wink: :cool:

(I also confirmed the book's answers.)
 
  • #10
The first step is to find the maximum acceleration, as LowlyPion suggests. What did you get for that? (No need for complicated calculations for that, since friction is the only relevant force on the top block.)
 
  • #11
look at my effort this is exactly what i did , i know that i need to find the max acceleration .

where is my mistake ?
 
  • #12
Your work looks OK to me, but you didn't finish. You have M_c in terms of M_total, but M_total contains M_c. Solve for both values of M_c!
 

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