ib physics 12 mr. jean january 6 th, 2014. the plan: video clip of the day – ub0
TRANSCRIPT
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IB Physics 12
Mr. Jean
January 6th, 2014
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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
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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
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Key concepts:
• Speed of light
• Time travel / time dilatation
• Length contraction
• Mass energy relationship
• Basic particle physics
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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.
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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.
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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!”.
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Relative Motion:
• http://www.youtube.com/watch?v=PKBXNVzpCJY
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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.
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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
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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
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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.
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• 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.
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• 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.
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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.
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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….
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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.
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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.
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Here’s where things get interesting:
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• Time measurement for you in the spaceship is the time it takes for the piston to complete one cycle.
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• Time measurement for observer is the triangular distance travelled by the piston.
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The faster you travel:
• The greater the gap in velocity between you inside the spaceship and the observer.
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The faster you travel:
• The greater the relative time differential.
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• 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!
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Time Dilation Demonstration:
• http://www.youtube.com/watch?v=KHjpBjgIMVk
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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 _____
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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 _____
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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
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Returning Projects:
• Topics to follow: – Length Contraction – Mass and Energy Relationship