1. Principle of Relativity
Frame of Reference: a portion of a power point found on open source Purdue University Physics.
Relativity: how things appear from different frames of reference.
Principle of Relativity
The laws of physics are the same in any inertial frame of reference

2. The forward motion of an adjacent bus can give you the impression that your own bus is moving backward.

3. Inertial Frame of Reference
For Newtonian Mechanics to work you must be in an inertial frame
How do you know if you are in an inertial frame?
Not when in a non-inertial frame you feel forces or accelerations
Must add fictitious forces for “physics” to work. (centifugal forces)
Newton choose “at rest respect to the distance stars”

4. A motorboat and a piece of wood move downstream
A motorboat and a piece of wood move downstream. The boat moves a distance dbe relative to the earth while the wood moves a distance dwe.

5. A motorboat moving upstream loses ground to the stream’s current if it cannot travel fast enough relative to the water (vbw).

6. A B C v
This truck platform serves as the “rest” frame for Observers A and B, but it moves with a speed v towards Observer C, who stands on the roadside.

7. Question 1
Observer A throws a baseball to Observer B at a speed of 30 m/s; B catches the ball. The truck platform moves in the same direction with a speed of 20 m/s.
A. For Observers B & C, the ball has the same speed.
B. For Observers B & C, the ball travels the same distance.
C. For Observers B & C, the ball is in the air for the same amount of time.
D. Observers A & B both say the ball has a speed of 50 m/s.
E. Observer C says the ball had a speed of 10 m/s. A B C v

8. Question 2
Observer A blows a whistle. The speed of sound is 340 m/s in air; no wind is blowing. The truck has a speed of 20 m/s.
For the sound wave moving in the direction of C
A. B & C agree that the whistle’s sound travels at 340 m/s.
B. B&C agree that the whistle’s sound travels at 360 m/s.
C. B says the speed is 360 m/s, while C says 340 m/s.
D. B says the speed is 340 m/s, while C says 320 m/s.
E. B says the speed is 320 m/s, while C says 340 m/s. A B C v

9. Faster than Sound
If the observer moves faster than sounds then the sounds wave never catches up to the observer.
But note that something physical happens when one goes faster than sounds. A shock wave is formed.