Third Law of Motion - Show Me The Physics - showmethephysics.com
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Alex changes the direction of the wheel's angle of rotation by applying an unbalanced force. By applying a force to the wheel, she also receives a force. This force causes her to rotate on the chair, thus demonstrating Newton's 3rd law of motion. Newton's 3rd law states that when object A applies a force on object B, object B applies an equal force on object A but in the opposite direction.
Also angular momentum is conserved is this demonstration. See • Conservation of Angular Momentum for a good explanation.