Need help with friction forces please

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Homework Help Overview

The discussion revolves around a physics problem involving two blocks, m1 and m2, where m1 is on top of m2. The problem focuses on understanding the forces at play, particularly the frictional force between the two blocks and how it affects the maximum force, P, that can be applied to m2 without causing m1 to move. The subject area is friction and dynamics.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the relationship between the applied force P, the frictional force, and the accelerations of the blocks. There are attempts to clarify the definitions of forces involved and how they relate to each other. Questions arise regarding the frictionless nature of the ground and the implications for the forces acting on the blocks.

Discussion Status

The discussion is active, with participants providing insights into the relationships between the forces and suggesting the use of free-body diagrams. Some participants express confusion over the definitions and calculations, while others attempt to clarify the equations governing the system. There is no explicit consensus, but several lines of reasoning are being explored.

Contextual Notes

Participants note the urgency of the problem due to an impending exam, which may influence the clarity of their reasoning. There are also indications of mixed-up terminology and concepts that are being addressed throughout the discussion.

mewmew
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Hi, I was giving a problem by my teacher that will most likely be on the exam. A block, m1, is ontop of another block, m2 and there is a constant friction, mu, between the two blocks. Between block m2 and the ground there is no friction. What is the largest force, P, that can be put on m2 and have m1 stay in its same position. I have F=m1a and F=m2a, F = (mu)N1 = (mu)m1g. It seems really easy but for some reason I am having trouble with it. Thanks

Edit: My exam is in an hour so I was typing fast without thinking.
 
Last edited:
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F is NOT N1!
It is the frictional force between m1 and m2!
Hence, you must solve the system when F equals MAXIMAL STATIC FRICTION,
F= (mu)N1.
 
I also don't understand the P-F ? isn't the ground with block 2 frictionless?
 
Sorry, I was typing faster than I was thinking so I got a few things mixed up, I think it should be correct now.
 
Ok draw a freebody diagram on each block,

On the block on the ground you have the contact force exerted by the block on top, and the force p, plus its weight.

On the block on the top, you got the friction force, plus the same magnitude for the contact force exerted by block on the ground, and its weight.

You know if the friction force of the block on the top is equal to the acceleration caused by the P force, then the block won't fall.

Try to solve from there.
 
So, is the following correct?

F = mu(m1g)

P=m2a
F=m1a

I could then solve one for a, and get for example a = f/m1 and then plug that into P = m1a and get P = m2(mu(m1g))/m1 ? It doesn't really seem like a good answer, but that's all I can figure out.
 
No, you have:
P-F=m2a (1)
F=m1a (2)
So, adding (1) and (2):
P=(m1+m2)a(3)
Since F is maximal static friction, we get a=(mu)g, or:
P=(m1+m2)(mu)g
 

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