Pulley Problem with External Force?

[Lori]

Homework Statement

Homework Equations

Seen above

The Attempt at a Solution

Why is the force acting on block B = N + P + W =0? ?
Shouldn't it be B = -P-W+N?
So that P = -W+N=0

 

[TSny]

Note that the equation in the figure is a vector equation:

The three force vectors add to zero. But you are also correct that the magnitudes of these forces obey -P-W+N = 0.

 

[Lori]

Note that the equation in the figure is a vector equation:
View attachment 214382

The three force vectors add to zero. But you are also correct that the magnitudes of these forces obey -P-W+N = 0.

I realized after that! But, I'm still confused on how they got that P =150?

 

[haruspex]

I realized after that! But, I'm still confused on how they got that P =150?

The μ_s factor was omitted. It should read 11g=(P+12g)μ_s.

 

[Lori]

The μ_s factor was omitted. It should read 11g=(P+12g)μ_s.

I'm still not sure how they calculated P=150 :/ Would appreciate it if anyone explained this one to me because I literally don't know how they added up and got 150. I tried multiplying the static friction too

Nevermind, i worked out the problem and figured thatP + W = F_n
F_ƒ= μ_s×F_n
T=ƒ
T=W_a = 107.8=ƒ

F_n = F_ƒ/F_ƒs = 107.8/0.4 = 269.5
P = 269.5 - W_b = 269.5-117.6=251.9 N

 

[TSny]

I'm still not sure how they calculated P=150 :/ Would appreciate it if anyone explained this one to me because I literally don't know how they added up and got 150. I tried multiplying the static friction too

As haruspex noted, the equation 11g = P + 12g as given in the figure in post #1 is not correct; rather it should be 11g=(P+12g)μ_s.

Are you having trouble seeing where this equation comes from, or are you having trouble seeing how to solve this equation for P?

 

[Lori]

As haruspex noted, the equation 11g = P + 12g as given in the figure in post #1 is not correct; rather it should be 11g=(P+12g)μ_s.

Are you having trouble seeing where this equation comes from, or are you having trouble seeing how to solve this equation for P?

Yes, i had trouble with where the equation was coming from, so i just redid it and found the answer myself! let me know if i did it correct cause my physics isn't really strong , so sometimes i doubt myself.!

 

[TSny]

Nevermind, i worked out the problem and figured thatP + Wb = Fn
Ff = fs*Fn
T=f
T=Wa = 107.8=f

Fn = Ff/fs = 107.8/0.4 = 269.5
P = 269.5 - Wb = 269.5-117.6=251.9 N

Yes, that's it. Good.

(You use fs for μ_s. You can use the formatting toolbar to access Greek letters, subscripts, etc. Click on the symbol Σ at then end of the toolbar to access Greek letters and other math symbols.)

 

[Lori]

Yes, that's it. Good.

(You use fs for μ_s. You can use the formatting toolbar to access Greek letters, subscripts, etc. Click on the symbol Σ at then end of the toolbar to access Greek letters and other math symbols.)
View attachment 214402

Thanks! I fixed it! I was actually wondering how to do that XD