Friction - minimum value of force for movement

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voliseq
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Homework Statement



Determine the minimum value of the force F for which the systems start moving. Assume M1 = 1kg, M2 = 2kg, Us = 0.3.
Solution: F = 9; F = 6N

Two pulley-and-block diagrams side by side: on the left, block M1 sits on block M2 with M2 on a surface and a force F pulling left; on the right, M1 sits on top of M2 with a pulley at the wall. Labels M1, M2 and F.

My problem is that i don't know the forces acting on wheel.

Homework Equations





The Attempt at a Solution


M1 :
F - T = 0
M1g - N = 0
M2:
Fr - T = 0
M1g+M2g - N(from mass 1 and mass 2) = 0
 

Discussion

voliseq said:

Homework Statement



Determine the minimum value of the force F for which the systems start moving. Assume M1 = 1kg, M2 = 2kg, Us = 0.3.
Solution: F = 9; F = 6N

Two diagrams of block M1 on block M2 with a force F pulling M1 left, M1 connected by a rope over a pulley to a wall; the right diagram shows M2 on a lower surface, with M2 attached to the wall pulley.

My problem is that i don't know the forces acting on wheel.

Homework Equations



The Attempt at a Solution


M1 :
F - T = 0
M1g - N = 0
M2:
Fr - T = 0
M1g+M2g - N(from mass 1 and mass 2) = 0
Hello voliseq. Welcome to PF !

Which wheel (or wheels) ?
 
Hi Sammy, i mean those wheels attached to M1 (on first picture) and M2(on second picture).
 
voliseq said:
Hi Sammy, i mean those wheels attached to M1 (on first picture) and M2(on second picture).
The only forces acting on the wheels are normal (vertical) forces. The wheels are free to rotate, so the horizontal forces acting on them are zero.

Chet
 
Another problem

Thanks Chest it works, now i have got another problem to solve,
1. Here is the problem

Diagram of two rods AB and BC hinged at the top A, forming a V with a cylinder of weight W resting between them; forces P act horizontally at A and C, with D on AB, B at the base, and a 30° angle marked.


2. And here is my attempt at solution

Handwritten free-body diagram on grid paper showing three rod-and-coordinate setups: a rod from point O with axes labelled Fx, Fy, Fz; a hemisphere with force arrows labelled M, N and F at its contact points; and a rod from point O on the right with axes labelled Fx, Fy, Fz.


I think something is wrong here as i can't get correct result
 
The members AB and BC need to be in equilibrium. In terms of P, what is the normal force exerted by the cylinder on AB? What is the normal force exerted by AB on the cylinder?

Chet
 
Chestermiller said:
The members AB and BC need to be in equilibrium. In terms of P, what is the normal force exerted by the cylinder on AB? What is the normal force exerted by AB on the cylinder?

Chet

N = Pcos30 ?
 

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