The reaction force to holding your hand out a window of a moving car is the air pushing back against your hand. As you move your hand through the air, you are pushing the air out of the way. The air then pushes back against your hand, trying to stop it from moving. This force is what you feel when you hold your hand out the window of a moving car.

The amount of force that the air exerts on your hand depends on how fast the car is moving and how big your hand is. The faster the car is moving, the more air you have to push out of the way, and the harder the air will push back against your hand. The bigger your hand is, the more surface area the air has to push against, and the harder the air will push back.

You can feel the same force when you put your hand in a stream of water. The water pushes back against your hand, trying to stop it from moving. The faster the water is flowing, the more force it will exert on your hand.

This force is also what keeps airplanes in the air. The wings of an airplane are designed to push the air down. The air then pushes back up against the wings, lifting the airplane into the air.