Homework 2, Astr 305G, Spring 2008

Due: Th, February 14

1. Consider Copernicus's model for the solar system. The Sun is at the center, then we have the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn in order of increasing distance from the Sun. We see the planets because they reflect sun light. Which planets will show large difference in phases, much like our moon (so we cannot always see the full disk of the planet), and which ones won't? Explain with a simple diagram.

2. Newton taught us that what we call our "weight" in the US and measure in units of pounds is actually the result of the force of gravity between the Earth and our body. Most of the world uses the concept of "mass" expressed in (kilo)grams and we frequently interchange the two concepts and convert one to the other (220 lbs = 100 kg). However, these are in fact two different things. Your mass does not depend on your location in the universe, your weight does. An astronaut will have 6 times less weight on the Moon because the force of gravity on the surface of the moon on a particular object is 6 times weaker than the force of gravity on the surface of the earth on that same object.  The astronaut would still have the same mass though.

Now, this is a question about "weight" and "weightlessness". We have all seen pictures of the astronauts floating in space inside their space ship. This state of "weightlessness" brings with it several challenges, including how to eat your eggs and bacon and how to drink your morning cup of coffee.

What exactly is meant with weightlessness? What conditions will cause someone to be "weightless"? Think about this carefully. The astronauts are still very close to Earth and its gravitational field, yet they are "weightless", how can this be?