Newton's Third Law: Action and Reaction
Just as pushing hard against a wall sends your body backward, whenever you push against the universe, the universe pushes right back with the exact same strength.
Definition A fundamental principle of physics stating that whenever one object exerts a force on another, the second object simultaneously exerts an equal force in the exact opposite direction on the first. In nature, forces never exist in isolation; they always occur in matched pairs.
What Happens When You Push a Friend on Ice?
Picture yourself standing side by side with a friend on a slick ice rink wearing skates. If you reach out and shove your friend's back with all your might, does only your friend slide forward? In reality, you will glide backward just as surely as your friend shoots forward.
Even though you only intended to push your friend, a backward force acts on your hands and feet as well. At the exact instant you exert a force (action) on your friend, their body pushes back against your hands with the same intensity in the opposite direction (reaction).
In this way, all forces in our world always appear as interactions between two objects. Just as you cannot clap with one hand without both palms striking each other, a solitary force cannot exist alone in nature.
This is also how we walk across the ground. When your feet push backward against the ground, the ground pushes your body forward, allowing you to advance.
How Do Rockets Fly Through the Vacuum of Space?
How can a massive rocket propel itself through the empty vacuum of space, where there is no air to push against and no solid ground to kick off from? Many people assume rockets push off the surrounding air or atmosphere behind them, but that is not how it works at all.
Instead of pushing external matter, a rocket burns fuel stored in its tanks and blasts exhaust gas backward at tremendous speeds. The moment the rocket pushes the gas backward, the expelled gas forcefully pushes the rocket body forward.
Swimming in a pool works the same way. When you push water backward with your hands and feet, a reaction force from the water pushes your body forward.
Think about what happens if you release an inflated balloon without tying the neck. As air rushes out backward, the balloon zips forward in the opposite direction. Even in empty space, as long as you have mass to eject, you can move forward anywhere.
Looking a Bit More Closely
To be more precise, although action and reaction forces are identical in magnitude and opposite in direction, they do not cancel each other out. That is because they act on two completely different objects.
When you push a wall, the action force acts on the wall, while the reaction force acts on your hands. Unlike balanced forces, where two equal and opposite forces act on a single object to keep it stationary, action and reaction involve two objects exchanging one force each.
Furthermore, when a heavy dump truck and a tiny compact car collide head-on, the force each vehicle exerts on the other is perfectly identical. Many people assume the massive truck hits harder, but the forces are equal down to the last decimal.
Why does the compact car get crushed so much more severely? Because the much lighter car undergoes a far more drastic change in velocity (acceleration). When subjected to the same force, a lighter mass experiences a much more violent impact and change in motion.
π€ Common misconceptions
When a massive truck collides with a compact car, the truck exerts a greater force on the car.
At the moment of impact, both vehicles exert the exact same force on each other. The compact car suffers more damage simply because its smaller mass undergoes a much greater acceleration and change in velocity.
Because action and reaction forces are equal and opposite, they must cancel out and prevent anything from moving.
The two forces do not act on the same object; they act on two different objects. Therefore, they do not cancel each other out, and each object can accelerate independently.
π§Ί Where you meet it
Forces in nature always exist in pairs: the moment you exert a force on an object, that object exerts an equal and opposite force back on you.