Newton's Third Law
Why nothing in the universe can push without being pushed back.
How this lesson works
Every slide has a colored label so you know what your brain is doing:
- READ — the idea explained
- LOOK — diagrams and a simulation you control
- DO — experiments with stuff from your house
- CHECK — questions that tell you why
Use the buttons at the bottom, or the arrow keys on a computer.
First, what is a force?
A force is a push or a pull. You cannot see a force, but you can always see what it does: it starts something moving, stops it, speeds it up, slows it down, or changes its shape.
Forces are measured in newtons (N), named after Isaac Newton. Holding a small apple takes about 1 newton.
The big surprise of this week: a force is never a solo act. You cannot push on something without that thing pushing back on you. Forces always show up in pairs.
Press your hand hard against your desk right now. Your hand pushes the desk — and you can feel the desk pushing back into your skin. That squished feeling is the second force.
Students often say the desk "blocks" the force instead of "pushes back." Blocking sounds passive. Ask them: if the desk did nothing, why does your hand hurt?
One push, two arrows
A skateboarder pushes the wall. Watch where the arrows point — and notice that each arrow lands on a different object.
The pink arrow acts on the wall. The blue arrow acts on the skater. Same strength, opposite directions, two different objects, at the exact same moment.
The wall is bolted to a whole building, so it does not visibly move. The skater is not. That is why only one of them rolls away.
The law itself
Newton's Third Law: For every action there is an equal and opposite reaction.
Rewrite it in real words
That sentence is 300 years old. Here is what it actually means:
"When object A pushes object B, object B pushes back on object A just as hard, in the opposite direction, at the same time."
Three rules hiding inside the law
- Equal size. If you push the wall with 50 N, the wall pushes you with exactly 50 N.
- Opposite direction. Forward ↔ backward. Down ↔ up.
- Two different objects. One force lands on you, the other lands on the thing you touched.
"Action first, then reaction" is wrong. There is no delay and no first. The two forces are born and die at the same instant — they are two halves of one interaction.
Quick check
Tap an answer. You will get the reason right away.
Three pairs you already know
Every one of these works the same way: something gets pushed one way, so the pusher gets pushed the other way.
Swimming
Try swimming without pushing any water backward. You will stay exactly where you are.
Rocket launch
A rocket does not push against the air — there is no air in space. It pushes against its own exhaust gas.
Walking
On ice your shoe cannot grip, so it cannot push the ground backward — and the ground cannot push you forward. That is why you slip.
Equal forces, very different results
Here is the question that trips almost everyone up. In a demolition derby, a speeding car slams into a car that is sitting still. The moving car bounces back a little. The stalled car flips over twice.
So the moving car hit harder, right? No. The two cars pushed on each other with exactly the same size force.
Equal forces do not mean equal results. What changes the result is mass — how much stuff an object is made of. The same push moves a light object a lot and a heavy object a little.
Think of a bowling ball and a ping-pong ball. Give each the exact same shove. The ping-pong ball flies across the room; the bowling ball barely rolls. Same force, different mass, different result.
In the derby, the moving car was also carrying the energy. Its motion is what got transferred into the stalled car and flipped it — but the forces during that crash were a perfectly matched pair.
"The bigger/faster object pushes harder" is the single most common wrong answer on this topic. Separate the two words on the board: force (always equal) vs. effect (depends on mass).
Quick check
Experiment 1: Balloon Rocket
15 minBuild a rocket that flies across your room using nothing but air.
Materials
- 1 long balloon
- 1 drinking straw
- String, 15–20 ft
- Tape
- 2 chairs
- A binder clip or clothespin
Steps
- Thread the string through the straw. Tie each end to a chair and pull the string tight across the room.
- Blow up the balloon. Do not tie it — pinch the neck shut or use a binder clip.
- Tape the balloon to the straw so the balloon's opening points toward one chair.
- Slide the straw to that end of the string. Count down and let go of the neck.
- Run it three times. Try a half-full balloon and a completely full balloon.
The air rushes out of the balloon toward the back. Which direction will the balloon travel — the same direction as the air, or the opposite? And will the fuller balloon travel farther? Say your answer out loud first.
What actually happened
The balloon travels the opposite way from the escaping air. The stretched rubber squeezes the air and pushes it out the back — that is the action. The air pushes forward on the balloon just as hard — that is the reaction, and it is what carries your rocket down the string.
The fuller balloon goes farther because it pushes out more air for a longer time, so the reaction force keeps pushing longer.
This is exactly how a real rocket works. Swap rubber for a rocket engine and air for burning fuel, and you have a NASA launch at Wallops Island.
Experiment 2: The Push-Off Test
10 minTwo people, one push, two very different rides.
Materials
- 2 rolling office chairs (or 2 skateboards)
- A smooth floor with clear space
- A partner
- Masking tape
- A tape measure
Steps
- Put a strip of tape on the floor. Sit facing your partner with both chairs touching that line. Feet up!
- Put your palms against your partner's palms.
- Only one of you pushes. The other person just keeps arms stiff and does nothing.
- Measure how far each chair rolled from the tape line.
- Repeat, but this time the other person is the only one pushing.
- Now load one chair with a heavy backpack or have the heavier partner sit in it. Push again and measure.
If only one person pushes, will the person who did nothing move at all? Will the person who pushed stay put? And when one chair is much heavier, which chair rolls farther?
What actually happened
You both move — every single time. It does not matter who "did the pushing." The moment your palms touch and one person pushes, two forces exist: you push your partner away, and your partner pushes you away just as hard.
When one chair is heavier, both chairs still feel the same size force, but the heavier one rolls a shorter distance. Same force, more mass, smaller effect — the derby rule again.
Why the chairs matter: wheels remove almost all the friction. Standing on a normal floor, the ground's grip hides the effect.
Pair or not a pair?
A real Newton's Third Law pair needs two forces on two different objects. If both forces are squeezing the same object, it is something else. Tap your choice for each one.
Gravity down + table up on the same book is balanced forces (Newton's First Law), not a Third Law pair. The test students can run themselves: "Are these two forces touching two different objects?"
Push-Off Lab
Two carts sit still, back to back, with a squeezed spring between them. Release the spring and they shove each other apart. Change the mass of the blue cart and watch what stays the same — and what does not.
The force numbers never split apart, no matter what you set the mass to. Only the speeds change. That is Newton's Third Law and mass working together.
The spring pushes each cart for about half a second, which works out to 12 N on each side. Set the blue cart to 8 kg. It is four times heavier than the red cart, so it crawls away at one quarter of the red cart's speed — while both carts feel exactly 12 N.
Quick check
Word bank
Tap a card to flip it. Tap again to hide the meaning and quiz yourself.
Four traps to avoid
These are the four wrong ideas that show up most often on tests. Read each one and check yourself honestly.
✗ "The heavier thing pushes harder."
A semi-truck and a mosquito hitting the windshield push on each other with the same force. The mosquito loses because of its tiny mass, not because it was pushed harder.
✗ "The reaction happens after the action."
There is no waiting. Both forces exist at the same instant, and both end at the same instant.
✗ "Equal and opposite forces cancel out, so nothing should move."
Forces only cancel when they act on the same object. Third Law forces act on two different objects, so each object gets pushed for real.
✗ "A rocket pushes against the air."
Then rockets would not work in space — and they clearly do. A rocket pushes on its own exhaust gas.
Ask: "A book rests on a table. Name the reaction to Earth's gravity pulling the book down." Correct answer: the book pulls Earth up with the same force. Almost every class first answers "the table pushes up," which is a different pair.
Your day, powered by pairs
Everything below is Newton's Third Law running in an ordinary American week.
Football tackle
A linebacker hits a running back and gets knocked back too. Both players feel the same force — the lighter one goes flying.
Pushing off the pool wall
You push the wall backward with your feet; the wall shoots you forward. Free speed at every swim meet.
Fire hose
The hose throws water forward, so the water shoves the hose backward. That is why firefighters brace in teams.
Basketball
The ball pushes the court down; the court pushes the ball back up. A flat ball loses that bounce.
Trampoline
You push the mat down, the stretched mat pushes you up — harder than you pushed feels like, because the springs add stored energy.
Rowing on the Potomac
The oar pushes water backward, the water pushes the boat forward.
Car tires
Tires push the road backward; the road pushes the car forward. On ice, no grip means no push means no go.
Bumper cars
Both cars jolt on every hit, even when only one driver was moving. Equal forces, two riders who feel it.
Pick one thing you did today that used a force pair, and name both halves out loud: "I pushed ____, and ____ pushed me."
Final quiz
8 questions. Each one explains itself as soon as you answer.
What you now know
- A force is a push or pull, measured in newtons.
- Forces always come in pairs: equal in size, opposite in direction, on two different objects, at the same time.
- Equal forces do not create equal effects — mass decides how much something moves.
- Rockets, swimming, walking, and every collision run on this one rule.
Next up → Topic 2: Kinetic and Potential Energy
In the derby crash, we said energy moved from one car to the other. But what is energy, where was it hiding before the crash, and where does it go afterward? Next lesson you will ride a roller coaster to find out — and you will meet the law that says energy can never be created or destroyed.