# Homework Help: Static friction problem for person to walk up plank

1. Jun 18, 2012

### Hyperfluxe

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

2. Relevant equations
See my work.

3. The attempt at a solution

2. Jun 18, 2012

### Xisune

You used the wrong trig function, you multiply all forces by sin40 except for friction going up, that's when you use cos40 .

3. Jun 18, 2012

### Hyperfluxe

Shouldn't the weights be multiplied by cos(40) because they are vertically downwards, so the moment arm would be the horizontal distance?

4. Jun 18, 2012

### Xisune

You're right, my mistake.

The answer I get still doesn't match yours, but I can barely see the text, do you have a clearer picture of the solution?

5. Jun 18, 2012

### Hyperfluxe

Yeah my bad, the quality is horrendous. Here's a way better one:

6. Jun 18, 2012

### Xisune

You used the wrong equation during the final step. Set 165.22s + 70.81 equal to equation 2. That should get you the correct answer.

7. Jun 18, 2012

### Hyperfluxe

Wow that was such a silly mistake haha. Thanks for pointing it out! I got 1.00m as the answer.