Dynamics: Minimum Force Problem

  • Thread starter JM92
  • Start date
  • #1
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Hi, nice to meet everybody here at physicsforums.com! I'm JM92 and I have a physics problem I need help with.

Homework Statement



Find the smallest value of the mass M for which the system will remain at rest given that the coefficient of static friction is 0.100
[PLAIN]http://img695.imageshack.us/img695/6328/physics2.jpg [Broken]

Homework Equations



Fnet = ma
∑F(Horizontal) = 0
∑F(Vertical) = 0

Ff = μ・FN
Fgx = Fg・sinθ
Fgy = Fg・cosθ

The Attempt at a Solution


[PLAIN]http://img3.imageshack.us/img3/4139/physics11.jpg [Broken]
I retyped and redrew everything with MS paint.. Hopefully that made it easier to read and not harder..:smile:

PS. The correct answer is 4.98kg.

Thank you for your help!!

EDIT: Sorry, the thread title should be called "Minimum Mass Problem"
 
Last edited by a moderator:

Answers and Replies

  • #2
verty
Homework Helper
2,164
198
You've drawn Ff in the wrong place. M is the mass of the second block; if it is any smaller, the first block will slide down the incline.

How small can M be?
 
  • #3
9
0
You've drawn Ff in the wrong place. M is the mass of the second block; if it is any smaller, the first block will slide down the incline.

How small can M be?

Ohh! I was looking at it backwards, then. With Ff in the opposite direction, I got the correct answer. Thank you very much!
 

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