Sir Isaac Newton was a mathematician and scientist who lived in England from the mid 1600s to the early 1700s. He is famous for being the first person to explain how gravity and the laws of motion worked. Now, let’s be clear – Newton did not invent gravity, but he did come up with a way to explain it to everyone else so that they could understand how and why it worked. He also explained the rules about how and why things move. These rules related to gravity and movement are part of the science of physics.
Physics is the study of matter and motion, and all the things that affect how and why things move, including mechanics, heat, light, electricity, sound, even down to the nucleus of atoms. In 1687, Newton wrote a book called Principia Mathematica (which is Latin for “Mathematical Principles”). In that book, Newton outlined the three laws that govern how everything in the universe moves.
The Coin Tower experiment demonstrates the “Law of Inertia” or the First Law of Motion: An object at rest (one that isn’t moving at all) will stay still and an object that is moving will keep moving at the same speed, until that object interacts with something else that causes a change. Inertia just means that nothing is changing. The “something” that causes a change is called a force. A force can make an object begin to move, speed up, slow down, or stop altogether – altering that object’s inertia.
The Experiment
Supplies: 12 coins of the same size (quarters and nickels work best), a dinner knife
What to do: Stack the coins in a single, even, and straight stack. You may need to move the stack so that it is near the edge of the table. With a quick motion, use the blade of the dinner knife to sweep the bottom coin from the stack. The blade should stay flat against the surface of the table so that you are only hitting the bottom coin. Continue doing this until you run out of coins or the stack collapses. How many coins were you able to sweep away before the stack collapsed?
What is happening: When you stack the coins, they are “at rest” – not moving – and they don’t want to move. Gravity is the force that pulls everything toward the center of the Earth. The force of gravity is helping the coins to not move. A second force, friction, is also helping the coins to not move. Any time two objects rub up against each other, it causes friction. Friction is sort of like one object trying to convince the second object not to move. It is the force that makes it hard to drag a heavy box or piece of furniture across the floor.
When you hit the bottom coin with the knife, the inertia (motion) of the knife is greater than the forces of friction and gravity holding that one coin in place, but it is not so great that it overcomes the forces holding all the coins in place. The bottom coin is impacted by the force of the motion of the knife, but the remainder of the stack is still at rest, so those coins stay at rest and gravity pulls them down, dropping them into the place where the bottom coin used to be.
Take It Further
Try this experiment, swiping the knife in the same direction each time. Then repeat it again, this time swiping the knife back and forth, alternating to the left, then the right. How did the two stacks compare while you were doing the experiment?
Links
To see the Coin Tower experiment in action, watch this video from Poppy Does Science.
Rader’s Physics4Kids.com has a web page that explains Newton’s Laws of Motion in greater detail. It includes a great video of NASA scientists explaining the laws and demonstrating them in motion on the International Space Station!