• Support PF! Buy your school textbooks, materials and every day products Here!

Static friction and maximum/minimum force

  • Thread starter Poetria
  • Start date
  • #1
183
4

Homework Statement



A block of mass m is at rest on an inclined plane of angle θ with respect to the horizontal. There is friction between the block and the inclined surface (μk and μs).

Now, assume that the value of θ is less than the value in part A) (so that the block will remain stationary if no other forces act on it). What is the maximum magnitude of a horizontal Force F that(with respect to ground) can be applied without causing the block to move up the incline?



Homework Equations




The Attempt at a Solution



I have found a similar thread here:
https://www.physicsforums.com/threads/block-wedge-problem.698343/#post-4424430
and here: https://www.physicsforums.com/threads/box-on-an-incline-with-friction-min-and-max-forces-involved.619902/

I understand everything except one significant detail. I have to choose between several options:
mg(sinθ−μ*cosθ)/(cosθ+μ*sinθ)


or mg(sinθ+μ*cosθ)/(cosθ+μ*sinθ)

Friction is static. I don't understand the numerator. Why is there -/+μ*sinθ or -/+μ*cosθ?? Could you give me a hint?

 

Answers and Replies

  • #2
248
26
Why don't you draw a Free Body Diagram of the block on the incline
 
  • #3
183
4
Oh, you are right. I did it. I understand it now. Silly me.
Many thanks. :)
Block and wedge.png
 
  • #4
248
26
Oh, you are right. I did it. I understand it now. Silly me.
Many thanks. :)
You did it yourself mate! Cheers!
 
  • #5
  • #6
Could you please explain that Fcos(theta) bit?
Nevermind, I've understood it now.
 

Related Threads on Static friction and maximum/minimum force

  • Last Post
Replies
12
Views
5K
Replies
2
Views
825
Replies
2
Views
4K
Replies
10
Views
6K
  • Last Post
Replies
1
Views
3K
Replies
4
Views
4K
Replies
3
Views
6K
  • Last Post
Replies
1
Views
6K
  • Last Post
Replies
5
Views
18K
Replies
7
Views
2K
Top