1. What’s your point of view?
Reference Frames
What’s your point of view?

2. What is a Reference Frame?
Definition: A Reference Frame is a particular location that someone is making observations from.
A reference frame is simply where you are, and how things look to you.

3. What Reference Frames Do
Reference frames can change what you see when making scientific observations.
If what you see appears to be different, then your measurements will be different too…

4. For Example You are sitting next to a road watching a car pass by…
What object is moving?

5. For Example You are sitting next to a road watching a car pass by…
From outside the car, it looks like the car is moving toward the trees. (Earth’s reference frame)
From inside the car, it looks like the trees are coming toward the car and the car is still. (Car’s reference frame)

6. Another Example You have the misfortune to fall from a tree…
What are the reference frames? What does the motion look like for each?

7. Another Example You have the misfortune to fall from a tree…
Your reference frame: The ground is coming up to hit you.
Earth’s reference frame: You are falling down.

8. A numerical example
From the Earth’s reference frame, you observe someone riding a bicycle at 25 m/s throw a baseball, which travels at 45 m/s according to you.
Remember: This is how things look from Earth’s reference frame (sitting still).
What does the person on the bicycle see?
45 m/s
25m/s

9. A numerical example
From the Earth’s reference frame, you observe someone riding a bicycle at 25 m/s throw a baseball, which travels at 45 m/s according to you.
If the ball is going 45 m/s, it’s only going 20 m/s faster than the bicyclist.
So to him, it looks like the ball’s velocity is 20 m/s, and he’s sitting still.
45 m/s
25m/s

10. A numerical example
From the Earth’s reference frame, you observe someone riding a bicycle at 25 m/s throw a baseball, which travels at 45 m/s according to you.
Earth’s Ref. Frame Rider’s Ref. Frame
Rider= 25 m/s Rider= 0 m/s
Ball = 45 m/s Ball = 20 m/s
45 m/s
25m/s

11. Sample Problem Start by drawing a diagram of the situation…
An escalator moves passengers down into a subway station at -5 m/s. A guy who can’t afford to get a stair climber is running up the down escalator at 3.5 m/s. What is his velocity relative to the Earth?
Start by drawing a diagram of the situation…

12. Sample Problem
An escalator moves passengers down into a subway station at -5 m/s. A guy who can’t afford to get a stair climber is running up the down escalator at 3.5 m/s. What is his velocity relative to the Earth?
3.5 m/s
-5 m/s

13. Sample Problem
An escalator moves passengers down into a subway station at -5 m/s. A guy who can’t afford to get a stair climber is running up the down escalator at 3.5 m/s. What is his velocity relative to the Earth?
The escalator is carrying the person at -5m/s.
The person is walking at 3.5 m/s.
3.5 m/s
-5 m/s

14. Sample Problem -2.5 m/s DOWN the escalator
An escalator moves passengers down into a subway station at -5 m/s. A guy who can’t afford to get a stair climber is running up the down escalator at 3.5 m/s. What is his velocity relative to the Earth?
The escalator is carrying the person at -5m/s.
The person is walking at 3.5 m/s.
The person’s velocity is -5m/s m/s = -2.5 m/s
-2.5 m/s DOWN the escalator
3.5 m/s
-5 m/s