Physics 12 Mr. Jean January 3rd, 2012

The plan: Video Clip of the day Modern Physics Relative motion Light is a problem Einstein’s change to thinking Time Dilation Returning Projects

Welcome Back: Things to look forward to in 2012 December 21st, 2012…..

Total Solar Eclipse It doesn’t happen that often !! Next year, on November 13th, the Earth will witness a total solar eclipse

Closest known near miss: A NASA team has discovered that next year will be our closest asteroid miss EVER. The beast in question is named ‘433 Eros‘ and has been haunting our skies since its discovery in 1898. Come Jan 31st, if you’re looking in the right direction, you should be able to get a decent look.

Space Travel for a price: Scheduled to kick off in 2012 is COMMERCIAL SPACE TRAVEL. Yes, that’s right. Come October 2012, tickets will be on sale for you to get your very own space cadet feeling. However, at a cool $200,000.

And of course: Physics 12 - Modern Physics Relativity Special Relativity Atomic Physics Nuclear Physics Physics 12 – Quiz Chapter #14 to #16 Modern Physics formal testing will be found on exam. Exams!!! (woohoo?)

Relative Motion: In order to understand Albert Einstein's "Theory of Relativity", one must first understand the concept of relative motion. In the diagram following, the velocities of objects are relative to some other object.

Worst Husband ever award: Last year I travelled to Singapore. It was an incredible experience; more importantly I threw an object quicker than I will ever throw again. Somewhere between NYC and Munich, Germany at 30,000 feet in the air we were traveling nearly 850km/h. I decided to throw a piece of paper at my wife.

Worst Husband ever award: Let’s say I only threw the ball of paper at 40km/h. Using relativity, a stationary observer would see the piece of paper traveling at 890km/h. I pointed this out to my wife. I thought this was hilarious… Her comment “You’re an idiot!”.

Michelson’s Experiment: A very important experiment was performed in the 1880's by Michelson and Morley. They designed a device called an interferometer which was supposed to find small differences in the speed of light.

Earth’s Speed: Situation A: Situation B: Earth is travelling towards the light sources with a velocity of 3.0 x 104m/s Situation B: Earth is travelling away the light sources with a velocity of 3.0 x 104m/s

If relativity applied to all then: Situation A: Light Speed + 3.0 x 104m/s = speed of light observed Situation B: Light Speed - 3.0 x 104m/s = speed of light observed

The problem: There was NO difference between the observations of light’s speed. With no variation in light speed this would suggest that the Earth, the star, and the galaxy was not moving. This was known to be incorrect.

Einstein suggested two postulates that changed physics. Postulate 1 : Absolute motion is not detectable. Postulate 2 : The speed of light is constant. ***Remember Einstein’s life evolved around the movement of light and understanding how light worked!!! Nothing could go faster, and light always traveled at the same speed.

The first postulate is often stated in another form The first postulate is often stated in another form. That is; the laws of physics are the same for all observers that move at a constant velocity. The second postulate says that; no matter what the relative velocity is between the source of light and the observer of that light, the speed of light (c) will always be measured to be 3.00x108 m/s.

What does this mean? Time is also relative to speed. As soon as there is motion in a system time is no longer a constant. Objects size must change at velocities to account for a constant light speed. This is an effect called length contraction.

Consider the following: Let’s us a simple clock to explain time dilatation. Our clock will be a piston travelling upwards and downwards. The piston its self is traveling at light speed (We’ll discuss why soon) The piston hits it’s highest peak at time zero. A second later the piston is at it’s lowest position and so on….

Stationary Observer As the piston travels upwards and downwards the time is constant. No change provided the clock and the observer are travelling at the same rate of speed.

Time Dilation: Now let’s consider a situation where you are on a spaceship with that clock sitting in your ship. No matter how fast you travel, relative to you the clock will still be constant with your time since it is travelling at the same rate of speed as you.

Here’s where things get interesting:

Time measurement for you in the spaceship is the time it takes for the piston to complete one cycle.

Time measurement for observer is the triangular distance travelled by the piston.

The faster you travel: The greater the gap in velocity between you inside the spaceship and the observer.

The faster you travel: The greater the relative time differential.

It shows that the time an observer measures for an event on the spaceship, depends on the speed of the spaceship relative to the observer. The time t is larger than the time to. This phenomenon is called time dilation. It is important to note that the times measured in both instances are made using identical clocks!

V (spaceship) t (observer) 0.010 c _____ 0.100 c _____ 0.500 c _____ 3) The phenomenon of time dilation depends on the velocity of the spaceship relative to the observer. Find the time t (to 3 decimals) for an event on the spaceship as measured by an observer, if the time to for the event is 1.00 seconds. V (spaceship) t (observer) 0.010 c _____ 0.100 c _____ 0.500 c _____ 0.900 c _____ 0.990 c _____

How long would Mr. Jean’s best class ever take? Situation: Teaching a 10 minute physics class which is stationary. My relative speed to them would be 0.9999999999c

Returning Projects: We will continue into Chapter 17 next day. Topics to follow: Length Contraction Mass and Energy Relationship