Ages 12–15 · Key Stages 3–4 · Science · 6 minutes
Newton’s Three Laws of Motion Explained
Newton’s three laws of motion explained through Apollo 11, sports, force diagrams, and a worked acceleration problem.
Key terms
- Net force
- The combined effect of all forces acting on an object, including their directions.
- Inertia
- The tendency to resist a change in motion.
- Acceleration
- A change in velocity, including speeding up, slowing down, or turning.
- Thrust
- The force on a rocket caused by expelling exhaust gas in the opposite direction.
- Force pair
- Two simultaneous, equal and opposite forces acting on different objects.
Transcript
How can a rocket push itself upward when there is nothing solid to push against? Watch Apollo eleven leave Florida in nineteen sixty nine. The answer starts with a book. In sixteen eighty seven, Isaac Newton published a book called the Principia, setting out three laws of motion. More than two centuries later, those laws help explain this launch. We will freeze the action, follow the forces, and solve one small problem. By the end, you will know why a rocket can work even where there is no air.
First law: motion does not change by itself. Imagine this rocket before its engines fire. It stays still, with the launch pad supporting it against gravity. The forces balance. Their combined effect, called the net force, is zero. Now imagine a puck gliding with almost no friction. It keeps moving. Zero net force does not always mean zero speed. It means no change in velocity: no speeding up, slowing down, or turning. This tendency to resist a change in motion is called inertia.
Here is the everyday trap. You kick a soccer ball, and eventually it stops. Did it run out of force? No. Your foot stopped pushing when contact ended. Friction and air resistance then changed the ball's motion. Take those opposing forces away, and a moving ball would keep going straight at a constant speed. A net force is needed to change motion, not to keep motion going. That is why our rocket needs a net upward force to start rising from rest.
Second law: how much does motion change? Once the rocket is clear of the pad, thrust pushes up and weight pulls down. While it rises through air, drag also acts downward. Add the forces with their directions to find the net force. Upward net force means upward acceleration. Acceleration means a change in velocity, not simply going fast. A speeding ball, a slowing ball, and a ball turning a corner are all accelerating. For a constant mass, Newton's rule is: net force equals mass times acceleration.
Let's make that useful with a small cart. These are example numbers, not Apollo measurements. Its mass is two kilograms. A pull of ten newtons acts right, while friction of four newtons acts left. First, subtract: ten minus four gives six newtons to the right. Then divide net force by mass. Six divided by two gives an acceleration of three meters per second squared, to the right. Starting from rest with this constant acceleration, after one second the cart moves at three meters per second. The direction belongs in your answer.
Now pause and try this: same cart, twice the net force. What happens to its acceleration? It doubles. Keep the net force the same but double the mass, and acceleration halves. A loaded cart is harder to accelerate than an empty one. Mass measures inertia. Back at the launch, a bigger engine force is not the whole calculation. As propellant leaves, the rocket’s mass changes too. You must account for the opposing forces and the mass being accelerated. Looking only at thrust is like counting your team's points and ignoring the other team's score.
Third law: forces come in pairs. A sprinter pushes backward on the track. The track pushes forward on the sprinter. The forces have equal size and opposite directions, and they happen at the same time. When you kick a ball, your foot pushes on the ball and the ball pushes back on your foot. Equal forces do not require equal accelerations: the second law tells us mass matters, and these objects have different masses. Most important, each force acts on a different object. That is why the pair does not cancel on the ball.
Now our launch mystery opens up. The engine pushes exhaust gas downward. The gas pushes the rocket upward. These are the third law pair: force on gas, force on rocket. The rocket does not have to push against the ground or against air. It carries the propellant it needs, including an oxidizer, which lets the fuel burn without outside air, so its engine can work in a vacuum. But do not pair thrust with weight. Both act on the rocket. The partner of Earth's pull on the rocket is the rocket's gravitational pull on Earth.
Put all three laws back into the launch. Before liftoff, balanced forces keep the rocket at rest. Expelling gas creates thrust. When the upward thrust exceeds weight at liftoff, the rocket starts accelerating upward. Its acceleration depends on net force and mass. Later, if an engine shuts down, the spacecraft does not suddenly stop. With thrust gone, it keeps moving; gravity still changes its velocity and path. In orbit, gravity continually bends that path around Earth. Newton's first law never meant that space has no gravity.
Here are the phrases to use when you explain motion. First: zero net force means constant velocity, including staying at rest. Second: net force equals mass times acceleration. Third: equal and opposite forces act on different objects. For a calculation, choose your object, draw its forces, combine them with direction, and divide by mass. For a force pair, name both objects. A book from sixteen eighty seven helps us read a rocket launch today. Next time you see a kick, a sprint, or a launch, ask: which object, which forces, and what changes?
Sources
Every fact in the video, and where it comes from.
- 1loc.gov/item/2021667054
- Newton published the Principia in 1687, presenting his three laws of motion.
- 2nasa.gov/image-article/apollo-11-launches-into-history
- Apollo 11 launched on a Saturn V from Kennedy Space Center on July 16, 1969.
- 3openstax.org/books/physics/pages/4-2-newtons-first-law-of-m…
- With zero net external force, an object remains at rest or moves at constant speed in a straight line. Inertia is resistance to a change in velocity; mass measures inertia.
- Friction and air resistance can slow a moving ball; a low-friction puck illustrates approximately constant horizontal velocity.
- 4grc.nasa.gov/WWW/k-12/TRC/Rockets/rocket_principles.html
- A stationary rocket is supported against gravity; upward liftoff requires upward thrust exceeding weight, with drag also opposing upward flight.
- 5openstax.org/books/university-physics-volume-1/pages/5-3-ne…
- Acceleration is a change in velocity, including speed changes or turning; it points along the net external force. For constant mass, F_net = ma.
- At fixed mass, doubling net force doubles acceleration; at fixed net force, doubling mass halves acceleration.
- 6openstax.org/books/physics/pages/4-3-newtons-second-law-of-…
- In an invented example, a 2 kg cart with 10 N rightward and 4 N leftward has net force 6 N rightward and acceleration 3 m/s² rightward. After 1 s from rest under constant acceleration its speed is 3 m/s.
- Applying a = F_net / m: for the same 2 kg cart, 12 N net gives 6 m/s²; for 4 kg with 6 N net, acceleration is 1.5 m/s². These are illustrative, not Apollo measurements.
- 7openstax.org/books/physics/pages/4-4-newtons-third-law-of-m…
- Interaction forces are simultaneous, equal in magnitude, opposite in direction and act on different objects; they need not produce equal accelerations.
- A runner pushes backward on the ground and the ground pushes forward on the runner; a kicked ball and foot exert equal opposite forces during contact.
- 8www1.grc.nasa.gov/beginners-guide-to-aeronautics/rocket-thrust
- A rocket pushes exhaust backward; exhaust pushes the rocket forward. Rockets carry propellant including oxidizer and can work in vacuum without pushing against air.
- 9openstax.org/books/university-physics-volume-1/pages/5-5-ne…
- Weight is Earth’s gravitational force on the rocket; its third-law partner is the rocket’s gravitational force on Earth, not thrust.
- 10science.nasa.gov/learn/basics-of-space-flight/chapter3-4
- After engine cutoff a spacecraft keeps moving; Earth’s gravity still changes its velocity. An orbit is a curved free-fall path, not absence of gravity.
- 11www1.grc.nasa.gov/beginners-guide-to-aeronautics/ideal-rocket-eq…
- Rocket mass decreases as propellant is expelled.
Image and sound credits
- launch.jpg: Liftoff of the Apollo 11 lunar landing mission, NASA S69-39959, public domain (US government work). https://images.nasa.gov/details/S69-39959
- engines.jpg: Saturn Apollo Program, NASA 9903405, public domain (US government work). https://images.nasa.gov/details/9903405
- engine-test.jpg: Saturn Apollo Program, NASA 6413912, public domain (US government work). https://images.nasa.gov/details/6413912
- orbit.jpg: Apollo 9 Lunar Module in lunar landing configuration, NASA as9-21-3181, public domain (US government work). https://images.nasa.gov/details/as9-21-3181
- pad.jpg: Saturn Apollo Program, NASA 6900846, public domain. https://images.nasa.gov/details/6900846
- rise.jpg: Saturn Apollo Program, NASA 6901001, public domain. https://images.nasa.gov/details/6901001
- principia.jpg: “Royal Society - Isaac Newton’s Philosophiae Naturalis Principia Mathematica manuscript 1” by Photographed at the Royal Society, London, with the kind permission of the Royal Society Library, as part of WikiProject Royal Society. Photograph by Mike Peel (www.mikepeel.net). (Public domain) https://commons.wikimedia.org/wiki/File:Royal_Society_-_Isaac_Newton%E2%80%99s_Philosophiae_Naturalis_Principia_Mathematica_manuscript_1.jpg
- manuscript.jpg: “Royal Society - Isaac Newton’s Philosophiae Naturalis Principia Mathematica manuscript 4” by Photographed at the Royal Society, London, with the kind permission of the Royal Society Library, as part of WikiProject Royal Society. Photograph by Mike Peel (www.mikepeel.net). (Public domain) https://commons.wikimedia.org/wiki/File:Royal_Society_-_Isaac_Newton%E2%80%99s_Philosophiae_Naturalis_Principia_Mathematica_manuscript_4.jpg
- soccer.jpg: “Guy Man” by Negative Space (CC0) https://stocksnap.io/photo/guy-man-XR731ZM77K
- football.jpg: “U.S. Sailors members Malaysian Armed” by U.S. Forest Service (source) (CC0) https://www.rawpixel.com/image/8738982/photo-image-public-domain-soccer-ball-grass
- sprint.jpg: “2013 IPC Athletics World Championships - W 200m T46” by TwoWings, CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) https://commons.wikimedia.org/wiki/File:2013_IPC_Athletics_World_Championships_-_W_200m_T46.jpg
- cart.jpg: “Mini grocery toy pushcart” by Me (Elsa Versailles) (Public domain) https://commons.wikimedia.org/wiki/File:Mini_grocery_toy_pushcart.jpg
- puck.jpg: Ice-hockey puck 2 by Santeri Viinamäki, CC BY 4.0, https://commons.wikimedia.org/wiki/File:Ice-hockey_puck_2.JPG (https://creativecommons.org/licenses/by/4.0).
- Paper textures: ambientCG (CC0).
- Sound effects: Freesound contributors (CC0), studio kit.
- Music: studio collage bed, generated with ACE-Step. Voice: Google Gemini TTS.
- newton.jpg: Isaac Newton, etching by L. Cars after F. Boucher, Wellcome Collection, CC BY 4.0. https://commons.wikimedia.org/w/index.php?curid=36410231 (https://creativecommons.org/licenses/by/4.0/)
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