10/16/09

We started off class today by reviewing the concept of the two rooms. There are two sets of rooms, the first of which has one room in the middle of deep space and the other falling with the acceleration of g, or approximately 9.8 m/s^2. From the inside of these two rooms, there isn't an experiment that can be performed to distinguish the two. For example, if you release a ball in the air in either elevator, it will appear to stay in one place. In the deep space room, this is because it is truly weightless. For the falling room, this occurs because the ball accelerates downward at the same rate as the room and so appears to not move in relation to the room and everything in the room. The other set of rooms has one sitting on the surface of the Earth and the other in a deep-space rocket with an acceleration of 9.8 m/s^2. Einstein believed that these two rooms should be indistinguishable from the inside as well (for example, a ball dropped in the room on the Earth will accelerate downward at 9.8 m/s^2. The ball in the rocket room would attempt to keep a constant velocity due to its inertia but would appear to accelerate downward because the room is accelerating constantly so the floor is actually striking the ball. The ball drops on the surface of the Earth due to its gravitational mass while the ball in the rocket appears to fall due to its inertial mass). However, if he was to go by this theory, then light would need to behave the same in both rooms as well. A laser shined in the rocket room would attempt to move in a straight line, but since the room is accelerating constantly, it actually strikes a point on the wall slightly below where it would strike if the room had a constant velocity. Therefore, the laser would need to behave the same on the surface of the Earth, bending below a completly straight path. Because of this, Einstein believed that gravity must have some affect on light, a new idea for his time.

On a similar note, we also discussed the basics of Einstein's theory of general relativity. Before Einstein's time, Newton's theories regarding gravity were widely accepted; Newton stated that the force that makes things fall and the force that keeps things in orbit were one and the same, the force we know as gravity. However, not understanding how gravity worked, he more or less saw gravity as a mysterious force that large objects used to grab smaller objects (i.e. the Sun grabs the Earth and keeps it in orbit). However, Einstein proposed a new theory in the early 1900s and found a way to understand gravity. He thought of the first four dimensions as a single plane of "space-time fabric." Large, heavy objects, such as planets or stars would create dips in this space-time fabric which would alter the paths of things moving through the dip. Therefore, the force associated with gravity is really caused by objects being influenced by dips in the space-fabric. The following picture shows the Sun bending the space-time fabric and how this affects the surrounding planets:

As you can see in this picture, dips in the space-time fabric can not only change the paths of planets, but it can also influence the path of light. Today, this phenomenom is seen when we are able to observe stars that should be physically blocked by a large object, say the Sun. The following picture shows the light from a star bending around the Sun:

Point A in this picture is the actual position of the star. But because light bends around the Sun, we perceive the position of the star to be Point B, which would be the actual position of the star if the light was traveling in a straight line.

Finally, we also talked about frictional forces. Unlike normal forces which act perpendicular to a surface, frictional forces act parallel. However, these two kinds of forces are strongly related; the size of the frictional force depends on the size of the normal force (i.e. the harder you press two surfaces together, the greater the amount of frictional force). The general term "friction" can be divided into two different types of said force: static friction and kinetic friction. Static friction occurs when two surfaces have no relative motion, for example if a block is on a slanted surface but doesn't move. When surfaces have relative motion, kinetic friction occurs. You can see this when you slide a block along a tabletop. Friction is often quantified by using something called the "coefficient of friction." This is defined as the frictional force between two surfaces at any given moment divided by the normal force at that moment. The higher this number, the more frictional force for a smaller normal force (i.e. a big frictional force). After the friction talk, we split into groups and performed experiments to find the factors that affect or don't affect frictional force (we tested factors like mass, normal force, surface area, velocity, etc.).

These are the things we did in class today. Next scribe is Eliza.