Gravity is the reason behind books being dropped and suitcases feeling heavy. It’s also the reason our atmosphere sticks around and oceans staying put on the surface of the earth. Gravity is what pulls it all together, and we’re going to look deeper into what this one-way attractive force is all about.
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Galileo was actually one of the first people to do science experiment on gravity.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).
Whenever I teach a class about gravity, I’ll drop something (usually something large). After the heads whip around, I ask the hard question: “Why did it fall?”
You already know the answer – gravity. But why? Why does gravity pull things down, not up? And when did people first start noticing that we stick to the surface of the planet and not float up into the sky? No one can tell you why gravity is – that’s just the way the universe is wired. Gravitation is a natural thing that happens when you have mass.
Would it sound strange to you if I said that gravity propagates at the speed of light? If we suddenly made the sun disappear, the Earth’s orbit wouldn’t be instantaneously affected… it would take time for that information to travel to the earth. What does that mean? By the end of this section, you’ll be able to tell me about it. Let’s get started!
So far, saying the force of gravity is pretty comfortable. When you throw a ball high in the air, the force of gravity slows it down and as it falls back to the earth the force of gravity speeds the ball up. The force of gravity causes an acceleration during this flight, and is called the acceleration of gravity. The acceleration of gravity g is the acceleration experienced by an object when the only force acting on it is the force of gravity. This value of g is the same no matter how massive the object is. It’s always 9.81 m/s2.
Johannes Kepler, a German mathematician and astronomer in the 1600s, was one of the key players of his time in astronomy. Among his best discoveries was the development of three laws of planetary orbits. He worked for Tycho Brahe, who had logged huge volumes of astronomical data, which was later passed onto to Kepler. Kepler took this information to design and develop his ideas about the movements of the planets around the Sun. We’re going to go into deeper discussion about Kepler’s Laws in the next section, but here they are in a nutshell:
- The Law of Orbits: All planets move in elliptical orbits, with the sun at one focus.
- The Law of Areas: A line joining the planet to the sun sweeps out equal areas in equal times.
- The Law of Periods: The square of the period of any planet is proportional tot he cube of the semi-major axis of its orbit.
Did you notice that while Kepler’s Laws describe the motion of the planets around the sun, they don’t say why these paths are there? Kepler only hinted at an interaction between the sun and the planets to drive their motion, but not between the planets themselves, and it really was only a teensy hint.
Newton wasn’t satisfied with this explanation. He was determined to figure out the cause for the elliptical motion, especially since it wasn’t a circle or a straight line (remember Newton’s First Law: Objects in motion tend to stay in motion unless acted upon by an unbalanced force?) And circular motion needs centripetal force to keep the object following a curved path. So what force was keeping the planets in an ellipse around the sun?
Click here to go to next lesson on Inverse Square Law.
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