AST201 Exam Notes
Lecture 2
-We can only measure motion relative to give and frame of reference
Ex: a plane is moving 900 km/h .A passenger throws a ball going 10 km/hr:
1. How fast is the ball moving to someone on the ground?
900+10 = 910 KM/H
2. How fast is the ball moving relative to the passenger?
10 km/hr
3. Who is right?
Both of them. It's all relative
Now doing the same thing with light:
1. Lights always travels at c (in a vacuum)
2. No object with mass can exceed the speed of light
Note: . all observers measure the same speed of light. It's invariant which means it's constant.
Always c
Foundations of special relativity
1. Speed of light is invariant
2. We can only measure speeds of objects relative to one another
Ex : Perspective of a guy throwing a ball up IN the train
- he says the ball went up 1 m, came down 1 m in one second for speed of 2m/sec
Perspective of someone standing outside looking at same situation
- he says the ball went diagonally 2metres came down diagonally 2metres in one second
for speed of 4M/S
What do observers agree on?
- The time it takes for event to happen
How about with light?
- The speed of the light ray
Note: Since the distance = Speed x time and because the two observers MUST agree about the
speed of light they must disagree about the time
Time flows at different rates for any two people in a motion relative to one another
If someone is moving relative to you:
-their clock appears to run SLOWER (time dilation)
-their lengths will all appear SHORTER to you (length contraction)
Lecture 2
-We can only measure motion relative to give and frame of reference
Ex: a plane is moving 900 km/h .A passenger throws a ball going 10 km/hr:
1. How fast is the ball moving to someone on the ground?
900+10 = 910 KM/H
2. How fast is the ball moving relative to the passenger?
10 km/hr
3. Who is right?
Both of them. It's all relative
Now doing the same thing with light:
1. Lights always travels at c (in a vacuum)
2. No object with mass can exceed the speed of light
Note: . all observers measure the same speed of light. It's invariant which means it's constant.
Always c
Foundations of special relativity
1. Speed of light is invariant
2. We can only measure speeds of objects relative to one another
Ex : Perspective of a guy throwing a ball up IN the train
- he says the ball went up 1 m, came down 1 m in one second for speed of 2m/sec
Perspective of someone standing outside looking at same situation
- he says the ball went diagonally 2metres came down diagonally 2metres in one second
for speed of 4M/S
What do observers agree on?
- The time it takes for event to happen
How about with light?
- The speed of the light ray
Note: Since the distance = Speed x time and because the two observers MUST agree about the
speed of light they must disagree about the time
Time flows at different rates for any two people in a motion relative to one another
If someone is moving relative to you:
-their clock appears to run SLOWER (time dilation)
-their lengths will all appear SHORTER to you (length contraction)
