clear
format compact

% stress tensor
S = [100 100 160; 100 0 -150; 160 -150 -60];

% unit normal vector
q = [1/2.0; 1/2.0; 1/sqrt(2.0)];

% tension vector and its norm
p = S*q;
pn = norm(p)

% the angle (in degrees) between the direction of the tension and the normal
alpha = acosd(p'*q/pn)

% tangential component of the tension vector
ptang = pn*sind(alpha)

% tangential component of the tension vector
pnorm = pn*cosd(alpha)