Tag Archives: Mass

Center of Mass Fork Experiment

Center of mass (COM) it’s easy balancing act or a trick to try at home! All you need is 2 forks, a quarter, a cup, and some patience.

putting fork trick together (1)

Why does this look so weird? It all has to do with center of mass. COM is hard to define, but almost everyone has a great intuition for it! You find the COM of objects when you’re balancing things. Once an object is balanced, wherever you are holding up that object is where it’s COM is. You could probably guess where the center of mass is for a lot of symmetric objects. For example, a ruler’s COM is in the middle. But if you add some extra weight to the end, it’s COM will shift! 

rulers center changing (1)

A fork is pretty asymmetic, and that shows in it’s COM. 

Center of Mass Experiment

If you try to balance 2 forks on your fingers, you probably won’t win. This is where things get weird– the center of mass does not necessarily need to be within the object! For 2 forks, it ends up being just right outside of it. 

2 forks cm full (1)

Which makes the “trick” work. If you stick something, like a quarter, between the 2, now the center of mass of this collection of objects is now on the quarter. You will intuitively find the exact place on the quarter where the center of mass lies, when you achieve balance on something like the edge of a cup (pro tip: the more rigid the edge of the glass is, the better. We used the bottom of the cup here because that was less rounded, and hence stuck better, than the top of the cup). There you have it! Rather than thinking of it as a trick, think of it as you showing a weird property of physics in a simple way!

A Planet Size Comparison

A planet is an astronomical body orbiting a star or stellar remnant that: is massive enough to be rounded by its own gravity, not massive enough to do fusion, and has cleared its neighboring region of planetesimals.

Our star (the sun) has 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Though these planets share a common place in the Universe, they are vastly different in composition, temperature, distance from the sun, and size.

planet

But how different are these sizes? To demonstrate this we can use a 1 pound chunk of clay. Roll it out into as symmetrical a log as you can. Cut it into 10 equal pieces.

Jupiter, the largest planet in the solar system, takes seven of those pieces, 70% of the solar system by mass (excluding the sun). Saturn, the second largest planet, will take two of the remaining chunks, 20% of the mass of the solar system. This means that the last 10% of mass of the solar system is the six remaining planets.

planet roll

Roll out the next chunk and cut it into ten more pieces. Uranus and Neptune are the next largest planets, the get 4 and 5 pieces respectively. The last four planets are the inner rocky planets. Earth and Venus are considered to be “sister planets”, they are roughly the same size and will get 5 and 4 pieces of the remaining clay. The last tiny chunk should be rolled out and cut into three pieces this time. Since Mars is larger than Mercury, it will get two of the pieces, and Mercury will get the last one.

planet model

There you go! A clay model of the planets in our solar system by mass. Try to test your friends, family, students, or teachers to see if they can get the scale right.

Written by: Mimi Garai

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