- Age:6+
- Time:10 min
- Difficulty:Easy
- Mess level:Low
- Supervision:No
What is inertia exactly?
People used to think that objects have a natural tendency to stop. If you want them to continue moving, you have to apply some kind of force again. And that is something that we can observe in everyday life as well. If we throw a ball, it won’t just keep on moving forever. It will fall down on the ground. Everything will stop eventually.
But that notion was challenged in the 1600s by Galileo, and later, Newton and Descartes. Galileo was studying planetal orbits and noticed something strange. Objects will actually continue to move unless some force causes them to stop. This discovery helped to explain how is it possible that we don’t feel any motion even though Earth revolves around the Sun. We are moving together with Earth so from our perspective, it looks like it stands still.
Sometimes we can observe a similar effect on a smooth ride. If we don’t see or hear anything, it would be hard to say if we are actually moving or standing. That new insight was called the Law of Inertia.
Inertia is defined as the object's tendency to continue doing what it was doing - if it was moving to continue to move and if it was resting to continue resting. This second part does align with our everyday life - objects don’t start moving by themselves. But we all know objects eventually stop. So how can this be true?
But here comes the caveat - they will continue what they were doing unless acted upon by some net force. The net force is just the sum of all the forces that act on an object. Our environment interacts with us in various ways and there are a lot of acting forces. Friction and gravity being the most obvious.
Objects that rest will continue to rest and objects that move will continue to move unless they are influenced by some outside force.
When we roll a ball it will come to a stop because of the friction. If there was no friction it would continue to move (unless acted upon by some other unbalanced force). We can notice that by comparing surfaces with different levels of friction. For example, ground and grass produce much more friction when the ball rolls, so the ball will stop sooner. Ice, on the other hand, would cause the ball to roll much, much longer. If there was no friction at all, like in a vacuum, the ball would never stop. And that’s actually true in the space!
All objects resist change, but the bigger they are (the more mass they have), the harder it is to change. We know that intuitively. We know that it’s harder to move (and stop!) a huge boulder than a tennis ball.
Inertia in everyday life
You feel inertia all the time. When a car brakes suddenly, your body keeps moving forward - that's inertia, and it's exactly why seatbelts exist. The same thing lurches you sideways when a bus turns a corner, and it's the secret behind the classic tablecloth trick: yank the cloth fast enough and the dishes, resisting the change, barely move.
What is motion and properties that describe a movement
Motion is one of the key topics in physics. And no wonder, since it’s fair to say that everything in our universe is constantly in motion. Science of motion answers so many important questions. Like where you are, where you have been and where are you going.
Motion can be one-dimensional, for example, a car going in a straight line. Or it can be multidimensional - like submarine who can also move up and down.
We can define motion as a change in the position of some object in a certain amount of time. Besides distance and time, there are some other properties that describe motion.
When talking about a change in position, there are two important terms: (The total length of the path travelled, no matter the direction.) and (The straight-line change in position, including its direction.).
Distance is a scalar quantity. It tells us the total amount of travel from the start position to the end position. All changes in position are added together, no matter the direction. If we walk 50m west and then 20m back east, our total distance will be 70m.
Displacement is a vector quantity. It takes into consideration direction, so in the same example, our total displacement will be 30m west. If the object travels in a straight line in just one direction, distance and displacement will be of the same amount. We should always preserve direction when talking about displacement though!
A similar case is with (How fast an object moves - distance over time, with no direction.) (scalar) and (Speed in a specific direction - a vector quantity.) (vector). Speed tells us how fast the object is moving, and its formula is the distance over time. Velocity is similar to speed, but with direction.
Unlike speed which is interested in the total distance you traveled in a certain time, velocity cares for displacement (change in position). If there is no displacement, for example, you move left 10 steps and then right 10 steps, you still moved with some non-zero speed. However, your velocity will be zero.
Speed, Velocity and Acceleration - often used in the same context, but it is important to know the difference.
The last important term is acceleration. Acceleration is the change in velocity and we perceive it as speeding up or slowing down. Since velocity also includes direction, even change in direction is considered acceleration. What’s tricky about acceleration is that we often confuse it with velocity or speed. If something is moving very fast, it has to have a high acceleration, right? No, actually. The only thing that matters is if there is a change in velocity (speed or direction).
Principles of Newton’s laws of motion
Philosophiae Naturalis Principia Mathematica was published in 1687. It’s Isaac Newton’s life work and one of the most influential books in the history of mankind. It proposed fundamental laws of motion that are still in use today. Usually called Newton’s Laws of Motion, they changed the way we perceive the world and are the foundation of classical mechanics (sometimes even called Newtonian Physics).
There are 3 of them, usually called just First, Second and Third Newton’s law. Good news, you already know everything about the First law. Newton took Galileo’s Law of inertia and rephrased it. To be fair, he did give credit to Galileo. It states that "A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force."
The Second Law of Motion describes what happens to an object when it is acted upon by an external force. It’s defined as "The force acting on an object is equal to the mass of that object times its acceleration." You probably know famous formula F = m * a, which describes this law. Even the First law can be derived from this formula, it’s the special case when acceleration is 0. No change in velocity means there was no force.
Force equals mass and acceleration of an object. That is the formula for calculating the second Newton Law.
The Third Law of Motion states, "For every action, there is an equal and opposite reaction." It’s also descriptively known as Law of action and reaction. When there is an interaction between bodies, they apply symmetrical forces to each other. Same in magnitude, but opposite in direction. A good example is launching the spacecraft and you can even test it out for yourself by building a homemade rocket using vinegar and baking soda, or by making a rocket using the matches.
Materials needed for demonstrating inertia
All we need is a big enough Cup and Bead Chain for this simple Inertia demonstration.
- Bigger empty cup, ideally transparent
- Chain of beads (we used simple beaded Christmas garland)
Instructions for demonstrating inertia
We have a video on how to demonstrate inertia at the start of the article or continue reading instructions below if you prefer step by step text guide.
- Put the chain of beads in the cup slowly so it doesn’t get entangled.
- Raise the cup from the ground and pull slightly on the chain’s end.
- Enjoy the show! :)
When you pull the end of the bead chain out of the cup, what will it do?
Make your prediction, then tap an answer to check!
Explanation of the Inertia Experiment - Why does it jump upwards?
What is this sorcery? How are beads defying gravity?
The phenomenon of “chain fountain” caught the public's attention when it was showcased in a YouTube video by Stephen Mould in 2013. It caused lots of attention and research in the physics community. The key is in the structure of the chain and the fact that beads are connected to each other.
When pulled upward, links connecting the beads rotate, and beads bump into each other. This produces a huge downward force on the opposite end of the chain. That, as we know from Third Newton’s law means that the opposite force of equal magnitude is created. So our seemingly small pull generates huge push from the other side of the chain which causes beads to go upward.
What will you develop and learn
- What is inertia
- Properties of motion
- Newton’s laws of motion
- That science is fun! :)
Key takeaways
- Inertia is an object's resistance to any change in its motion - resting objects stay at rest, moving objects keep moving.
- Things only speed up, slow down, or change direction when a net force (like friction or gravity) acts on them.
- The more mass an object has, the more inertia it has, and the harder it is to start or stop.
- This is Newton's first law of motion, first uncovered by Galileo.
- In the bead-chain "fountain", inertia plus Newton's third law makes the chain leap upward as you pull it from the cup.
Frequently Asked Questions
What is inertia in simple terms?
Inertia is an object's tendency to keep doing what it's already doing. If it's still, it stays still; if it's moving, it keeps moving in a straight line at the same speed - until some force acts on it to change that.
What is Newton's first law of motion?
Newton's first law states that a body at rest stays at rest, and a body in motion stays in motion, unless acted upon by an external force. It's really just a restatement of Galileo's law of inertia, which Newton credited.
If inertia keeps things moving, why do rolling objects stop?
Because a force is acting on them - usually friction. A ball rolling on grass stops quickly because of high friction, rolls much farther on ice, and in a frictionless vacuum (like space) it would never stop at all.
Does a heavier object have more inertia?
Yes. Inertia depends on mass, so the more mass an object has, the more it resists changes in motion. That's why it's much harder to push or stop a boulder than a tennis ball.
What is the difference between speed and velocity?
Speed tells you only how fast something moves (distance over time). Velocity tells you how fast and in which direction. If you walk 10 steps left then 10 steps right, you moved with some speed but your overall velocity is zero, because your position didn't change.
Why does the bead chain jump up out of the cup?
It's called the "chain fountain". As the falling part of the chain pulls the beads, the connected links push back against the pile in the cup. By Newton's third law that push has an equal, opposite upward reaction, which flings the chain up into an arch before it falls.
Is the bead chain inertia experiment safe for kids?
Yes, it's completely safe - no heat, sharp edges, or chemicals. Just use a bead chain or garland with no loose small parts for very young children, and enjoy watching the fountain.
If you liked this activity and are interested in more physics experiments, we recommend exploring heat conduction or try crushing can with air pressure. Sounds cool? It really is. If you want to just sit and think in peace, we have you covered. Hanoi tower is an ancient puzzle that will surely challenge your logical thinking (and patience!). Learn how to make your own Hanoi tower and even how to solve it.
