Action Reaction Pairs

THRUST ACTION/REACTION PAIR

DRAG ACTION/REACTION PAIR

As you drive a car, your tires push against the road. The road pushes back with equal force. This wheel/road force-pair is always equal and opposite. See Newton's 3rd Lawfor more information about action/reaction pairs.

Meanwhile, air hits the front of the car. The car pushes against the air with an equal and opposite force. This air/car force-pair is always equal and opposite. See additional information about Newton's 3rd Law.

The car moves forward because the wheel/road force pair is greater than the air/car force pair. In other words, when thrust is greater than drag, the car accelerates according to Newton's 2nd Law. Drag increases as the speed increases. Once drag equals thrust, the velocity of the car becomes constant. If the engine quits and thrust decreases, drag becomes the greater force, which slows the car down.

Force pairs are always equal. However, this does not mean that drag automatically equals thrust. In the case of an airplane, thrust equals drag only when it flies level at constant velocity.