Einstein once said: “I was sitting in a chair in the patent office at Bern when all of the sudden a thought occurred to me: If a person falls freely, he will not feel his own weight. I was startled. This simple thought made a deep impression on me. It impelled me toward a theory of gravitation.”

This led Einstein to develop his general theory of relativity, which interprets gravity not as a force but as the curvature of space and time. This topic is out of the scope for our lesson here, but you can explore more about it in this lesson.

The fundamental principle for relativity is the principle of equivalence, which says that if you were locked up in a box, you wouldn’t tell the difference between being in a gravitational field and accelerating (with an acceleration value equal to g) in a rocket.

The same thing is also true if you were either locked in a box, floating in outer space or in an elevator shaft experiencing free-fall. Any experiments you could do in either of those cases wouldn’t be able to tell you what was really happening outside your box. The way a ball drops is exactly the same in either case, and you would not be able to tell if you were falling in an elevator shaft or drifting in space.

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Have you ever seen an icicle? They usually grown downward toward the center of the earth (the direction of free-fall). But icicles on a car wheel grow radially outward because the wheel spins and flings the water toward the outside of the wheel where it freezes into spikes. The icicles can’t tell if the wheel is rotating and that’s why they grow radially, or it’s at rest at the gravitational field is in the radial direction!

Here’s a questions for you: these two astronauts (below) are inside the space station, which is currently in orbit around the earth. Which astronaut is upside down? Can you tell?

The principle of equivalence has some consequences! Navigation systems for ships, airplanes, missiles and submarines rely on acceleration information to calculate their velocity and position. However, the instruments that measure acceleration also react with unexpected variations in the earth’s gravitational field, and there’s no direct way to separate these two effect to avoid errors.

Highlights for Kepler’s Laws:

• 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.

Yay! You completed this set of lessons on circular motion! Now it’s time for you to work your own physics problems!

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