1. The problem statement, all variables and given/known data The surfer (70kg) is standing over a board which has an angle of 10º with the horizontal. A wave acelerates him to the right at 1.0m/s^2. There is a force that prevents the surfer from sliding over the board. 2. Relevant equations Calculate that force. What's the origin of this force? 3. The attempt at a solution What I did was to analize the surfer only and since there's horizontal aceleration to the right then there's a wave force in horizontal direction to the right. So I just analized it in axis x given the faact that at axis y it doesn't move. So the equations: http://eva.fing.edu.uy/filter/tex/pix.php/fa2415fb159333cacb5c7a42d79502d8.png http://eva.fing.edu.uy/filter/tex/pix.php/093b650ee4c9116ddcd68135cf37b616.png Then I calculated the weight at axis x: http://eva.fing.edu.uy/filter/tex/pix.php/857f0c71f94a74d9cb6e58fc3e611cfe.png So the friction force has to be the difference among these two and in the direction of the wave force and it would be 50N but the option is not in the moddle multiplechoice I'm doing. Thanks!!