To calculate the friction force when an applied force acts at an angle on a flat surface, it is essential to break the applied force into its components. The static frictional force is determined by the equation u_sF_n, where u_s is the coefficient of static friction and F_n is the normal force. The normal force is influenced by the object's weight and the vertical component of the applied force, which can either add to or subtract from the weight depending on the angle of application. Therefore, the frictional force is always opposite to the direction of the applied force. This analysis shows that scenarios where the vertical component of the applied force reduces the normal force result in lower friction, making movement easier.