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.