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The force is normal to the surface only if no friction is present. If we have some sort of sticking between the thrown object (e.g. ball) and the surface (e.g. earth), there will be a non-normal component of the force. Just think of an ball which slides (w/o friction) on the surface of the earth. As there is no friction, there can not be a force. Now go back to the original question. Let's consider the motion of the ball as a superposition of two motions: The vertical motion, which is in normal direction (say y-direction), and the horizontal motion (say in x-direction). If we consider the horizontal motion to be frictionless, there can not be a force in x-direction. Hence, we are left with a normal force, which reflects the ball. Once you include friction to the problem, you will get a superposition of two forces. From that you can derive the direction of the total force. In general it will not be along the moving direction of the ball.