Friction - signs of the forces correct?

[masterflex]

Friction -- signs of the forces correct?

Homework Statement

I have a question about part c). I've attached the picture of the problem. It's a typical friction problem on an inclined plane, and I get the wrong answer so there's something about my setup that's wrong.

Homework Equations

The Attempt at a Solution

I show my work in an attachment. I get P=127, but the answer is P=80. What am I doing wrong. How are the direction of my vectors? Thanks for the help.

 

[Doc Al]

Your direction of friction seems wrong. Which way do you think friction acts? Up or down the incline?

 

[masterflex]

Here's where I think the direction of friction is:

1) when motion down the incline is impending, friction points up the incline (Part c).

2) when motion up the incline is impending, friction points down the incline (Part a).

 

[Doc Al]

OK, you seem to have the correct direction for friction. I solved it, and got the expected answer. Perhaps you have an arithmetic error.

 

[masterflex]

How'd you get the right answer? I checked it at least 15 times and kept getting the wrong answer over and over and over again. Maybe another one of my forces is pointing in the wrong direction? If you could be so kind as to look over my work, it is attached in the original post -- I also drew a diagram :) Thank you.

 

[Doc Al]

Found the problem

OK... I found your error. Recheck your number 388. That should be 800 sin(25) = 338.1

 

[masterflex]

omg. I can't believe that. I spent like 2 hours analyzing it and reading the book again to see where I went wrong. You're awesome :)