(1) In the game, the player turns the oncoming football head back, and the direction of the football changes, which is the result of force, indicating that force can change the motion state of objects;

(2) The athlete's head will feel pain, which means that when the athlete's head exerts force on the ball, the athlete's head is also affected by the ball's force on his head, which means that the forces between objects are mutual;

(3) When an object is stationary or moving in a straight line at a constant speed, the force is balanced; When the pop-up football flies in the air, it moves at variable speed along the arc, and the force is unbalanced.

So, the answer is: exercise; Mutual; unstable