Physics 106, Summer 2012

Reading: Chapter 26 (All)

Did you complete the reading assignment?

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Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links? Yes No

1) Objects in all reference frames follow the same laws of physics.

2) The speed of light is the same in all reference frames.

3) The theory of special relativity requires several consequences to exist that will blow your mind if you think about them too hard.

Einstein's postulates of the theory of special relativity state that: 1) Objects in all reference frames must follow the same laws of physics, and 2) The speed of light is the same in all reference frames, regardless of their motion. With these two postulates he formed the theory of special relativity that is partially responsible for physics as we know it today. The way that he determined the theory of relativity was by making up thought experiments and running them through his mind to see what consequences they would bring to the world.

Exercise 1

To get into the mind of Einstein and to be able to see and understand what he was thinking when he came up with the theory of special relativity, go to: http://physics.ucsc.edu/~snof/Tutorial/index.html

The different thought experiments will slowly walk you through the make-believe experiments that Einstein thought of in determining the the consequences of special relativity including time dilation, length contraction, simultaneity, and relativistic momentum. Some of the activities will require you to play the simulation before continuing on to the next part. Also note that the value of gamma and the velocity of the objects are displayed at the top of the lesson box.

Time in a moving reference frame appears to be:

Faster Slower The same

The factor that speed affects everything in the theory of special relativity is gamma (γ) was determined by what?

A direct relationship The quadratic equation The Pythagorean theorem Calculus

According to the theory of relativity, when you throw a ball, what is moving after you let go of the ball?

You The ball No one can ever really know

How is the problem from the question above resolved by the theory of relativity?

Objects moving close to the speed of light shrink in length. Objects moving close to the speed of light expand in length. Objects moving close to the speed of light progress through time more slowly. Objects moving close to the speed of light progress through time more quickly.

If you are on Earth and a friend is in a space ship leaving earth at a speed near the speed of light and a huge explosion happens in Earth's atmosphere (your reference frame) halfway between you and your friend, who will see the explosion first?

You Your friend You will both see it at the same time.

Space-time diagrams can help us understand the consequence of relativity for objects moving close to the speed of light. In a space time diagram time is on which axis?

x-axis y-axis

When observers sees a collision in a different, relativistic rest frame, they will observe the initial and final momentum to be different. What consequence of special relativity does this lead to?

The law of conservation of momentum does not apply at relativistic speeds. The observation is skewed by time dilation and length contraction, so the law of conservation of momentum only appears to be violated at relativistic speeds. The mass of objects moving at relativistic speeds increases. The mass of objects moving at relativistic speeds decreases.

Exercise 2

Now that you understand the basics of the theory of relativity, try out the relativistic rollercoaster and take note of the consequences of the special theory of relativity that you see. Which consequences did you notice?

Time dilation, simultaneity, and relativistic mass are all really hard to tell in this setting. The most notable consequences that can be seen are length contraction and color shifting, which is a consequence of the doppler effect for light when moving at relativistic speeds.

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Was there anything that you didn't understand in the reading assignment? What was confusing to you?

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