How did gravity experiments start?
Galileo was actually one of the first people to do science experiments on gravity using the tallest tower he could find – the Tower of Pisa in Italy. At least, that’s what we’ve been told. But Galileo, who always wrote everything down, never mentioned in his notes of dropping things off the tower. What he did document, however, was rolling things down ramps (also called ‘inclines’). Remember that at this time in history, most people were still answering questions by simply arguing about them instead of doing any scientific studies!
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Galileo soon figured out that objects could be the same shape and different weights (think of a golf ball and a ping pong ball), and they will still fall the same. It was only how they interacted with the air that caused the fall rate to change. By studying ramps (and not just dropping things), he could measure how long things took to drop using not a stopwatch but a water clock (imagine having a sink that regularly dripped once per second). He quickly learned how to find the acceleration (which older kids are going to do during a few experiments later) and that the higher you dropped the ball, the bigger the impact. But we still don’t know why.
The rest of this article is for advanced students…
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Newton used Galileo’s work along with lots of cool mathematics to figure out that the Moon constantly falling around the Earth. Think of it this way – if you throw a ball toward the sunset, it will fall 16 feet during the first second. If you throw the ball faster and faster, it will still fall 16 feet during the first second. If you shot a bullet horizontally, it will also fall 16 feet during the first second. It doesn’t matter how fast you get that bullet to travel – it will always fall 16 feet during that first second.
What if you could shoot the bullet fast enough so that the rate the Earth curves away from the bullet at the same rate that it falls? If you shoot the bullet at 5 miles per second, it will fall at the same rate that the Earth is curving away from it… and that’s how we get objects into orbit. And that’s also what scientists mean when they say that ‘the Moon is falling around the Earth’.
Problems with Gravity
Things with gravity were going along just fine, until we started looking at the planets. It turns out that Mercury’s orbit doesn’t follow the math the way that it should – meaning that scientists can describe the orbits of the planets using complicated math equations… all except for Mercury.
And for a while, scientists actually thought there was another planet between the sun and Mercury that accounted for the trouble they were having (they even named this undiscovered planet Vulcan!). It turned out to be a problem with the math itself, and it took years before Einstein came along and tweaked the equations around so it made more mathematical sense.
Einstein changed the way we see the universe by viewing it from a totally different point of view that gravity is nothing more than geometry (more on that when you get to college!). There are still problems with gravity, though. The math that scientists use to describe gravity breaks down when they try to describe black holes – things go to infinity and zero at the same time, and in math, that’s usually bad news.
One of the things that scientist do know about gravity is that it is not instantaneous. It appears that it does take time for gravity to travel, the same way that it takes time for sound waves to travel to your ears. (Ever notice how you see the lightening before you hear the thunder?) Meaning that if you swapped our sun for a tiny star (with much less gravitational pull), the Earth would not know about it for about 8 minutes. The best guess we have is that gravity propagates at the speed of light.
Is gravity strong?
Another thing that we know is that gravity is the weakest of the four fundamental forces on small scales (we’re talking about the size of atoms here). On large scales, however, it’s the force that keeps the planets in orbit, galaxies in orbit, and everything slinging around each other as they should. Think about when you stick a magnet to the fridge: the magnetic forces are keeping the magnet up (well, most of the time anyway!) and overcoming the gravitational field effects from the planet.
Here’s an interesting thought experiment I came across not too long ago: if you could construct a human body that held together by only gravity, it would take a single breath to shatter the entire body. That’s how weak gravity is on a small scale. On a large scale, though the strong forces that keep the atoms together get so weak that gravity can take over. When you get to scales larger than an atom, the forces that bind the nucleus together get suddenly weaker.
Scientists are really bothered that they cannot figure out the how and why gravity works. They suspect that there are these little particles (gravitons) that shuttle information back and forth about gravity, but they are still invisible to us.
For advanced students, we have a more advanced version of the topic in the form of a textbook download. These files are also in PDF form and include exercises in addition to solid physics content. Note that these downloads are written for upper level science students, so if math isn’t your focus right now, just skip over it and read on. (You’ll find these also posted in the main reading section for each lesson.) Here’s the textbook download for gravity: Gravity Lesson textbook download.
Click here to get started with the experiments!
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