Engineering mechanics friction question HELP PLEASE

[weihong892]

engineering mechanics friction question! HELP PLEASE!

I need help for this question, please help me. i can't get the right ans. ):

6.2) Block B rests on block A and is attached by a horizontal rope BC to the
wall as shown in Figure-2. What force P is necessary to cause impending motion
of A? The coefficient of friction between A and B is 0.25 and between A and the
floor is 0.33. A has a mass of 14 kg and B has a mass of 9 kg. ( P= 102.7 N)

 

[PhanthomJay]

Welcome to PF! If you show an attempt at your work (which is a requirement in this forum), someone will be better able to assist, to see where you may have gone wrong. Be sure to calculate normal forces correctly.

 

[weihong892]

okay, but i don't know my step are correct anot, but i can't get the ans.

sum of x = 0

sum of x = -Px - (9 x 9.81 x 0.25) + c + (23 x 9.81 x 0.33)
= -Px - 22.0725 + c + 74.4579
= -Px + C + 52.3854
Px = C + 52.3854

Sum of y = 0
Sum of y = Py + Nb + Na - (9.81 x 9) - (14 x 9.81)
= Py + 88.29 + 225.63 - 88.29 - 137.34
= 88.29 + Py

That all i do, and i am stuck, correct me if i am wrong. please.

 

[PhanthomJay]

You are not drawing Free Body Diagrams (FBD's) correctly, and you have signage errors. You might want to start off by first isolating the top block B, using a FBD to note all forces acting on it. By doing this, and looking in the y direction, the normal force on B acts up, and is equal to the 88 N you calculated correctly.

Now draw a FBD of the bottom block A. Which way does Nb act on A? (think of Newton's third law). What other forces act on A? (the tension force c is not one of them).
Now use Newton 1 in the y direction and x direction to solve for P, looking at Block A only. Note that friction forces oppose the direction of relative motion between surfaces.

 

[weihong892]

i still have problem drawing the FBD of block a. ): how many forces are there?

 

[weihong892]

nvm, i got the answer already. but i got a part i dun understand. is Fb pointing to the left? why issit pointing to the left but not the right?

 

[PhanthomJay]

If by Fb you mean the force of friction between A and B, well, if Block A wants to move left, then the friction force of B on A must point right, (the same as the direction of the friction force of the floor on A).
If you're looking at Block B, however, Newton 3 tells you that the friction force of A on B must point_____?

You haven't shown how you arrived at the correct answer.