Finding the static friction force

[gijungkim]

Homework Statement

A 60-kg person rides down an icy hill of 20° slope while standing on a 3.0-kg flat-bottomed bathroom scale. Assume there is no frictional force between the bottom of the scale and the hill. The static friction force the scale exerts on the person is

Homework Equations

The Attempt at a Solution

I got 211 but the answer is 0 where did I do wrong?

 

[TSny]

You have left out a force on the person that contributes to the acceleration of the person down the slope.

You have assumed that the force of static friction on the person is $F_f =\mu N$. But this equation is for the maximum possible static friction force. You cannot assume that the static friction force on the person is at it's maximum possible value. In fact, you will discover that $F_f =0$.

So, set up Newton's second law for the motion of the person down the slope making sure you include all forces acting on the person.

 

[paisiello2]

Not sure how you got 211 but if we agree the person-sled system is accelerating gsin20 then every part of the system must also be accelerating the same?

 

[gijungkim]

You have left out a force on the person that contributes to the acceleration of the person down the slope.

You have assumed that the force of static friction on the person is $F_f =\mu N$. But this equation is for the maximum possible static friction force. You cannot assume that the static friction force on the person is at it's maximum possible value. In fact, you will discover that $F_f =0$.

So, set up Newton's second law for the motion of the person down the slope making sure you include all forces acting on the person.

Oh I got it now! Thank you so much!