Motion
From a falling apple to a speeding rocket, understanding how things move is the key to understanding the entire universe. In this lesson, we will explore the invisible forces that push, pull, start, and stop everything around us.
Is Everything Actually Moving?
Yes, you are moving incredibly fast! Even when you feel perfectly still, the Earth is spinning at roughly 1,600 kilometres per hour. At the same time, our entire solar system is hurtling around the outer edges of the galaxy at 220 kilometres per second!
A force! A force is simply a physical push or a pull. Bill Nye the Science Guy explains that unless an outside force pushes or pulls on an object, it will just keep doing exactly what it is already doing.
Put Your Instincts to the Test
Think about what you already know about gravity, weight, and motion. Pick an answer for each question, then see if your instincts were right.
The coin drops straight down into the glass. The coin has inertia. Because you only pushed the card, there was no sideways force acting directly on the coin. The coin stays perfectly still horizontally whilst the card vanishes, allowing gravity to pull it straight down.
The mass stays exactly the same. Mass is the amount of physical stuff an object is made of. Bill Nye the Science Guy demonstrates that even if a scale reads zero because the apple is in freefall, it is still a physical apple and it still has the exact same amount of inertia.
They move off together at a much slower speed. This is the law of action and reaction. Because the moving car suddenly has to push twice as much mass, the resulting motion is cut to a fraction of the original speed.
Understanding the Science
Tap each card to reveal the fundamental laws of physics that govern everything around us.
Key Concepts
The Universe in Motion
Tap to learn moreIt might feel like the ground beneath your feet is completely still, but everything in the universe is in a constant state of motion. The Earth spins on its axis, orbits the sun, and the entire Milky Way galaxy is expanding rapidly through space. Understanding how things move is the most fundamental part of understanding how the universe actually works.
Pushes and Pulls
Tap to learn moreAn object can never start moving, stop moving, or change direction all by itself. To change motion, you absolutely must have a force. A force is defined very simply as a physical push or a pull. Whether it is an engine pushing a car forward, or gravity pulling an anvil towards the floor, forces are the only things that create action.
Newton's First Law
Tap to learn moreThe scientist Isaac Newton realised that motion is lazy. Newton's First Law states that an object at rest will stay perfectly at rest, and an object in motion will stay in motion, unless acted upon by an outside force. If you push a frictionless cart along a perfect air track, it will theoretically slide at the exact same speed forever until it hits a wall.
Inertia and Mass
Tap to learn moreThis laziness of motion is called inertia, and it is a fundamental property of matter. The amount of inertia an object has depends entirely on its mass (how much physical stuff is packed inside it). A heavy bowling ball has much more mass than a light golf ball, which means the bowling ball has much more inertia and is therefore much harder to move.
Newton's Second Law
Tap to learn moreBecause heavy objects are harder to move, you have to push them harder to speed them up. Newton's Second Law explains that to move a mass, you need a force, written mathematically as F=ma (Force equals mass times acceleration). If you want to make an object accelerate twice as fast, you have to push it with exactly twice as much force.
Gravity
Tap to learn moreWe do not normally see objects sliding around forever because there are constant outside forces acting on them, like friction and gravity. Gravity is the constant downward pull of the Earth. When Bill Nye the Science Guy places a heavy anvil on a floating pad of air, he has to perfectly balance the upward push of the air pressure against the downward pull of gravity.
Newton's Third Law
Tap to learn moreForces never happen alone; they always come in pairs. Newton's Third Law states that for every single action, there is always an equal and opposite reaction. If you push hard against a wall, the wall pushes back against your hands with the exact same amount of force. If two bumper cars crash into each other, they violently push each other apart in opposite directions.
Frames of Reference
Tap to learn moreBecause everything is moving, calculating motion depends entirely on where you are standing. This is called your frame of reference. If you are sitting inside a moving train, the cup of tea on your tray looks like it is sitting perfectly still. But to an observer standing outside watching the train go past, that exact same cup of tea is moving forwards at incredible speed!
Try It: Frictionless Air Track Laboratory
Test the Laws of Motion! Adjust the mass of the two sleek hover-carts, then launch the blue cart. The momentum transfers seamlessly upon impact. Watch the Impact Maths panel to see exactly how much the velocity changed!
Apply Your Knowledge
Let us see if you can connect what you have learned about motion to real-world physics concepts.
Match the Concepts
Click an object to select it, then click the matching description to place it.
Real-World Challenge
Imagine you are a mechanical engineer hired to design the safety restraints for a brand new rollercoaster. The carriage moves incredibly fast, but suddenly hits the brakes at the very end of the track. Based on your knowledge of inertia and mass, what will happen to the passengers, and exactly how should your safety system be built to counter those specific forces?
What Has Changed Since This Episode Aired
This episode first aired in late 1998. Since then, physics and cosmology have taken some massive leaps forward. Here is what we know now.
Updated: Later that exact same year (1998), scientists made a shocking discovery. The universe is not just expanding; the expansion is actually accelerating! A mysterious force called Dark Energy is pushing galaxies apart faster and faster every single day, a mind-bending discovery that won the Nobel Prize in 2011.
Updated: Yes! In 2012, quantum physicists theorised a bizarre new phase of matter called "time crystals", which were successfully created in a lab in 2017. Inside a time crystal, the atoms constantly move and repeat in a pattern without requiring any outside energy or force at all!
Updated: In 2015, the European Space Agency launched the LISA Pathfinder mission. They successfully created the most perfect state of rest ever achieved by humanity. They shielded floating metal masses from absolutely all outside forces (including the microscopic pressure of sunlight) to test the laws of gravity with unbelievable precision.
Test Your Understanding
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Results
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Reflection
Think about the last time you rode in a car or a bus that suddenly slammed on the brakes. Based on Newton's First Law and inertia, what did your body naturally want to do, and how did the safety restraints provide the necessary force to keep you safe?
Episode Discussion
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