Contact force on two blocks: 'Block on a table'?

Question

http://img.photobucket.com/albums/v638/ravenatic20/Blockonatable.jpg
Block B in the image weighs 711 N. The coefficient of static friction between block and table is 0.25; assume that the cord between B and the knot is horizontal. Find the maximum weight of block A for which the system will be stationary, and explain your answer. Thanks

Answer 1

Suppose:

T: Tension in the rope connected to wall
Wa: Tension in the vertical rope connected to A
Wb: Weight of block B
Tb: Tension in horizontal rope connected to B
Fc: Friction coefficient
Ff: Friction force

Explanations:

1-Ff(max.)=Wb*Fc
2-The maximum force happens when the Tb is equal to maximum Ff. This is the moment that block B is ready to move.
3- Just by looking to diagram you will find out that the only vertical forces are Wa and T*Sin (30). Considering the equilibrium condition: Wa=T*Sin(30)
4-Again, by looking to the diagram you will find out that the only horizontal forces are Tb and T*Cos(30). Considering the equilibrium condition: Tb=T*Cos(30)
5-Finally, it is obvious that Tb=Ff

Calculations:

A) Ff=711*.025=177.75N
Tb=Ff
Tb=177.75N

B) According to #2 above:
Wa=T Sin(30)

C) According to #3 above:
Tb=T*Cos(30)
177.75=T*Cos(30)


Now divide B by C:

(Wa/177.75)=Tan (30)

Wa=177.75*Tan(30)=102.62N

The maximum weight of block A for which the system remains stationary is 102.62N.

Answer 2

The force keeping Block A where it is is the friction force F acting on Block B in direct opposition to the horizontal pull of Block A

So we have: horizontal force by Block A = W(A) cos(30 deg) = W(B)k = 711 (.25) = horizontal force by Block B ; W(A) and W(B) are the respective weights of blocks A and B, k is the coefficient of static friction. On rearranging terms, we have:

W(A) = 711 (.25)/cos(30 deg) = 711 (.25)/.86

The key to this problem is to recognize that the friction force acts in equal but opposite direction of the tug from Block A along the horizontal string so that the result is no motion of A or B.

Answer 3

would not the contact stress between both, it truly is vertical (when you consider that F is horizontal), count on which block is on proper of the different? U would opt for to describe the placement somewhat better thoroughly :>}

Answer 4

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