Meaning & use
Newton's third law
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Meaning
nounsingularphysics
The principle in physics that when one object pushes on another, the second pushes back on the first with the same strength in the opposite direction.
Grammar: A named law, so it takes no article before the possessive: Newton's third law explains rocket motion.
Patterns
- according to Newton's third law
- Newton's third law of motion
Newton's third law explains why a rocket moves forward as gas shoots backward.
When you push on a wall, Newton's third law says the wall pushes back on your hand.
The swimmer moves ahead because, by Newton's third law, the water pushes against her as she pushes it.
Common combinations
- according to Newton's third law
- According to Newton's third law, the gun recoils when the bullet is fired.
- illustrate Newton's third law
- A balloon flying around the room illustrates Newton's third law.
- apply Newton's third law
- Engineers apply Newton's third law when designing jet engines.
- action and reaction
- Every action and reaction pair obeys Newton's third law.
- Newton's third law of motion
- Students often learn Newton's third law of motion with a skateboard demonstration.
Synonyms and how they differ
- law of action and reaction
- This is the descriptive name for the same idea, while Newton's third law names the scientist behind it.
- third law of motion
- This is the shorter formal form and leaves out the name, but means the same principle.
Word family
Commonly confused: Newton's third law or Newton's first law?
Newton's third law vs Newton's first law
The first law is about inertia, meaning objects keep moving or stay still unless a force acts, and it is a different principle from action and reaction.
Practice
Which situation best illustrates Newton's third law?
Choose one answer.
Pushing water backward makes the water push the boat forward, which is an action and reaction pair. The ball illustrates the first law and the truck the second law.
Make it your own
Write a sentence using “Newton's third law”, then check it and compare it with the examples above.