# Homework Help: Probably an easy problem for you guys, but hard for me

1. Oct 26, 2009

### redgreenpaper

1. The problem statement, all variables and given/known data

hi there, im having some trouble with my physics homework and i was wondering if anyone can help me. i have no clue what to do. these are the questions

1. what does the slope of the graph "maximum force of static friction vs normal force" mean? (maximum force of static friction is on the y axis and normal force is on the x axis)

2. use free body diagrams to derive the equation "tanΘ=Us" (Us is static friction)

2. Relevant equations

3. The attempt at a solution

2. Oct 26, 2009

### Delphi51

That graph must pass through the origin, so slope = Ff/Fn.
Look through your list of formulas for one containing the two forces and solve it for Ff/Fn. You'll see what it equals.

I'm not familiar with your #2. What is the situation? What is the angle between?

3. Oct 26, 2009

### Andrew Mason

The slope, which is just the ratio of the maximum static friction force to normal force, gives the proportionality constant for static friction force, $\mu_s$ as in:

$$F_{sf} = \mu_sN$$

In the above equation the force of static friction is the maximum force that friction can exert - ie. a force larger than this will cause motion.
This sounds like a mass on an inclined plane which is tilted until the mass starts to slide. That is the angle, Θ that you use. What is the normal force at that point? What is the force along the surface of the inclined plane? How is that force related to the force of static friction? (think of the forces just before the mass starts to move).

AM

4. Oct 26, 2009

### redgreenpaper

the situation is a shoe on a wooden board that is 1.85 m long. then it is being lifted up until the shoe starts to move.

5. Oct 26, 2009

### redgreenpaper

6. Oct 26, 2009

### Delphi51

Red, you can do it yourself. I said

"That graph must pass through the origin, so slope = Ff/Fn.
Look through your list of formulas for one containing the two forces and solve it for Ff/Fn. You'll see what it equals."

and Andrew looked up the formula for you, so there is very little left to do!

7. Oct 26, 2009

### redgreenpaper

i cant read andrews formulas. its so small and black and white i cant read it. also, im not that good at physics. im only getting a 70sumthing average :(

8. Oct 26, 2009

### redgreenpaper

ZOMG I THINK I GOT IT.

since slope = Ff/Fn, i can fix up the equation of Ff=uFn to get u=Ff/Fn. am i right?

9. Oct 26, 2009

### Delphi51

Got it!
Now sketch the shoe on the ramp and draw in the force of gravity vector arrow. Separate that into a part down the ramp and a part into the ramp. Write expressions for those two components. Same for the force of friction.

10. Oct 26, 2009

### redgreenpaper

thanks alot delphi