The Invisible Hand Beneath the Waves: Newton's Third Law Drives Underwater Propulsion

Imagine struggling to swim forward with flippers, moving only sluggishly through the water. Or recall that moment at the pool’s edge when you pushed off hard backward—and your body surged forward instantly. Behind these simple phenomena lies the universal force that drives submarines, submersibles, and torpedoes through the depths: Newton’s Third Law in action.

Revealed centuries ago by Sir Isaac Newton, this physical law is profound yet elegantly simple: When one object exerts a force on a second object, the second object simultaneously exerts a force equal in magnitude and opposite in direction on the first object. In short: "You push me, and I push you back."

We witness this law everywhere:Stand on a skateboard and hurl a heavy ball backward—you’ll find yourself gliding forward. Why? By exerting a backward force on the ball (action), it exerts an equal forward force (reaction) on you.

A rocket engine blasts high-speed gases downward (action), and those expelled gases thrust the rocket upward (reaction), breaking free from gravity’s grip.

Even squeezing the trigger of a garden hose, jetting water backward (action), makes the hose kick forward against your hand—a tangible "recoil" (reaction). The harder the spray, the stronger the push.

Underwater propellers—whether massive submarine screws, compact submersible thrusters, or diver-held "seabobs"—operate on the exact same principle as throwing a ball, rocket exhaust, or water-jet recoil. Their genius lies in harnessing an abundant, surrounding medium: water.

The core mission of any thruster—be it spinning propeller blades or a waterjet’s powerful pump—is to forcefully and rapidly accelerate water backward (or in a chosen direction). This is the thruster’s action force on the water. By Newton’s Third Law, as the water is driven backward, it must exert an equal and opposite force on the thruster (and whatever it’s attached to)—the thrust that propels the craft forward. It’s essentially an oversized water gun firing relentlessly backward: the thruster continuously hurls water aft, and the water responds with an unyielding forward push.

Newton’s Third Law grants not just forward motion but remarkable agility. Need to reverse? Simply flip the thruster to push water forward—the water’s reaction force now drives you backward. Want to turn? Steer the jet’s direction (via rotating nozzles or asymmetric thruster output) so the reaction force points sideways, pivoting the craft like paddling only one side of a canoe. To hover? Angle thrusters downward to push water beneath you—their upward reaction force counters gravity or currents, locking the vessel in place. So when a submarine slips beneath the waves or an ROV darts through coral, it’s no mystery: basic physics powers these marvels.

These sophisticated machines are, at heart, engaged in an intelligent dance with water: they compel currents to rush in chosen directions, and water responds with equal, opposing force, thrusting them steadfastly toward every corner of the ocean’s depths. This "invisible hand," born of the water itself, is humanity’s engine for exploring and conquering the deep blue realms—a triumphant elegance of physical law manifest in engineering.