1. Unit 4 Motion & Forces

2. 1. What is Motion?

3. Suppose you are looking out a window.
You see a car going by.

4. The car is in motion
Vocabulary:
motion
movement

5. But how do you really know the car is in motion?

6. the car is to the right of the tree
Look at the pictures:
the car is to the right of the tree

7. Now after 2 seconds, the car is to the left of the tree
The tree cannot move from one place to another

8. So the car must have moved from one place to another
The car must have been in motion

9. Now look at the following pictures:
Picture 3 and picture 4 are 2 seconds apart

10. Did the car move?
You cannot tell.

11. some other object to which you compare an objects position
You cannot tell whether an object is in motion unless you have a frame of reference
Vocabulary:
Frame of reference
some other object to which you compare an objects position

12. In this case, you have no frame of reference
So you cannot tell whether the goose is in motion

13. An object is in motion only in relation to other objects

14. Have you ever ridden on an airplane at night?
If you have, you know that you do not feel like the plane is moving

15. Unless you can see lights from the window,
You have no frame of reference

16. You know the plane is in motion in relation to Earth
But you feel as though the plane were still

17. Scientists always use a frame of reference to talk about motion

18. For example, when they study the motion of stars in the sky
They use other stars as a frame of reference

19. Only in this way can they know the stars are moving