function im=PointForce(cr,a,phi)
% Calculate the stress on a cylinder of radius a with a point force at phi. 
% Usage: im=PointForce(r,a,phi)
%

% revision history:
% 08/04/05 Mark D. Shattuck <mds> ipf.m  
% 01/30/06 mds added abs(cr)
% 04/30/07 mds made w a true measure of width.
% 10/26/08 mds modify for PointForce function

r=abs(cr);
th=angle(cr);
im=(r.^2-a^2).^2/2/pi/a.*(a-r.*cos(phi-th))./(r.^2+a^2-2*r*a.*cos(phi-th)).^2.*(r<a);