Unit 4 Motion & Forces

1. What is Motion?

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

The car is in motion Vocabulary: motion movement

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

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

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

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

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

Did the car move? You cannot tell.

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

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

An object is in motion only in relation to other objects

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

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

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

Scientists always use a frame of reference to talk about motion

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

Only in this way can they know the stars are moving