IB Physics 12

Mr. Jean

January 6th, 2014

The plan:

• Video Clip of the day– http://www.youtube.com/watch?v

=jKNv87CSUB0

• Modern Physics (Option D & H)– Relative motion– Light is a problem

• Einstein’s change to thinking• Time Dilation• Returning Projects

The next 8 days of class:

• Topic D (for SL)• Topic H (for HL)

• Hamper– Chapter 13

• Page 456 - 478

• Giancoli: – Chapter 26

• Pages 726 - 748

Key concepts:

• Speed of light

• Time travel / time dilatation

• Length contraction

• Mass energy relationship

• Basic particle physics

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!”.

Relative Motion:

• http://www.youtube.com/watch?v=PKBXNVzpCJY

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: – Earth is travelling towards the light sources

with a velocity of 3.0 x 108m/s

• Situation B: – Earth is travelling away the light sources with

a velocity of 3.0 x 108m/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.

• Postulate 1 : Absolute motion is not detectable.

• Postulate 2 : The speed of light is constant.

Einstein suggested two postulates that changed physics.

***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. 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!

Time Dilation Demonstration:

• http://www.youtube.com/watch?v=KHjpBjgIMVk

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 _____

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:

• Topics to follow: – Length Contraction – Mass and Energy Relationship