Rocket walks across the driveway in slow motion, one careful step at a time.
"Each step, my foot pushes backward on the ground," he says. "So the ground pushes me forward. That is how walking works!"
Raven watches his shoes. "What do you notice on the wet patch? Your foot slid and you did not move forward."
"No grip, no push back," Rocket says. "The ground could not push me."
Nova hovers over a toy car on the step. "Cars work the same way," she says. "So do rockets, in a way that surprises most people."
Rocket grins. "Rockets push on the air behind them. Right?"
Nova dims her glow, which is her way of saying not quite. "Read today's lesson," she says.
A physics text lists everyday examples of the third law. A person walking exerts a force backward on the floor.
The floor exerts a reaction force forward on the person. That forward push is what moves her.
A car speeds up because the ground pushes forward on the wheels, in reaction to the wheels pushing backward on the ground.
You can see the wheels pushing backward when tires spin on gravel and throw stones backward.
Helicopters push air down and feel an upward push. Birds and airplanes push on air in the direction opposite to the push they need.
| Mover | What it pushes on | Which way it pushes | The push back |
|---|---|---|---|
| A walker | the floor | backward | floor pushes the walker forward |
| A car | the ground | backward with its wheels | ground pushes the car forward |
| A helicopter | the air | downward | air pushes the helicopter up |
| A rocket | its own exhaust gas | backward, out the back | gas pushes the rocket forward |
Many people think rockets fly by pushing on the ground or on the air behind them. A physics text calls this a common misconception.
Rockets move forward by expelling gas backward at high velocity. The rocket pushes a large backward force on the gas.
The gas therefore exerts a large reaction force forward on the rocket. This reaction force is called thrust.
NASA engineers say the same thing: thrust is produced according to Newton's third law of motion.
Because rockets push on their own gas, they work in space, where there is no air at all. They actually work better in a vacuum.
| Statement | True or false? |
|---|---|
| A rocket flies by pushing on the air behind it. | ? |
| Rockets work in space where there is no air. | ? |
| A car moves because the ground pushes forward on its wheels. | ? |
| A wet, slippery floor gives a walker more push back. | ? |
Well reasoned. Tomorrow is Field Lab: you build a balloon rocket on a string and time its run.