Physics 12 Mr. Jean January 4th, 2012

The plan: Video Clip of the day Chapter 14 – 16 Quiz outline Doppler Effect for waves (particles of light) Modern Physics –Relative motion (Review) –Length contraction formula. –Videos to help make this make sense.

Quiz Outline: Chapter #14: –Electrostatics forces –Describing fields –Fields & potential energy –Coulomb’s Law –Point charges etc…

Quiz Outline: Chapter #15: –Electric potential difference –Electric circuits Series Parallel –Resistance in a wire

Quiz Outline: Chapter #16: –Electricity and Magnetism –Electric Engines AC & DC –Magnets AC & DC –Current flows with forces, Ie: Right hand rules

Format: Part A: –True and False Roughly 20 points Part B: –Multiple choice questions Roughly 20 points worth Part C: –Long answer questions Roughly 30 points worth

DOPPLER EFFECT Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves

When a source of waves and an observer of waves are getting closer together, the observer of the waves “sees” a frequency for the waves that is higher than the emitted frequency. When a source of waves and an observer of waves are getting farther apart, the observer of the waves “sees” a frequency for the waves that is lower than the emitted frequency.

All waves exhibit the Doppler effect. A particularly interesting example is used by astronomers to determine if light emitting objects (such as stars) are getting closer to us or farther away. On average most stars are moving farther away, and their light spectra are “red shifted.”

Stationary Observer: No change Observer moving towards source: Observer moving away source:

Key points to relativity: Observations are relative to motion. The speed of light never changes. It always travels the same distance per second relative to the motion. Observers must see light at a constant velocity. Time on a moving object slows down in comparison to a stationary observer.

A Light second is a light second (no matter what!):

Both spaceships fire their lasers at the same time. The bottom spaceship is moving quickly from left to right.

The lasers travel outwards at the same speed. The speed of light. We would see both beams of light travelling side by side at the same speed.

The crew of the stationary ship sees that the light travels 1 light second per for each 12 light seconds. What about the moving ship?

If we agree that the speed of light is a constant then, we must disagree with the time scales and distance scales.

The problem with Time Dilation: Unfortunately time dilation does not do enough to explain this. We need to show how distance is also influenced by speed.

Since the bottom spaceship is moving time dilation does occur; however the spaceship also becomes squished due to length contraction. The spaceship is physically squished compared to the stationary observer.

Only 9 seconds actually pass on the moving spaceships clock due to time dilation. Due to length contraction our ruler is no longer accurate.

With length contraction we now see that the lasers distance travelled was 9 light seconds per second. Relative to the people on the moving space, light is travelling at light speed.

Length Contraction: Two observers that move at a constant speed with respect to each other, disagree on the time that it takes for an event to occur. They also disagree on the length of an object or the distance between two points.

Suppose that a spaceship travels from planet A to planet B. Observers on A and B both measure the distance between the planets as L 0. We shall show that the astronaut measures a different distance between the planets.

From the observers' point of view, the spacecraft, or anything else that is moving, also contracts. Are you serious?!… (yes)

Spaceship Moving at the 10 % the Speed of Light

Spaceship Moving at the 85 % the Speed of Light

Spaceship Moving at the 99 % the Speed of Light

Spaceship Moving at the % the Speed of Light

Highly Variable Activity: Option #1: –Start cheat sheet for Physics Quiz Option #2: –Study for AP Chemistry Test after school. Option #3 –A combination of option #1 & #2 Option #4 –A combination of option #1, #2 & #3. –Why would you read this?