MOVING ON...

Kepler's description of the motions of planets and Galileo's descriptions of the motions of falling bodies, play a crucial role in our understanding of the motion of objects. You can describe a motion if you know where it is going to be at any given time.When we describe how things move, it is important to include the following in the explanation:

1) speed
a) rate = average speed
2) direction
3) changes in speed or direction
a) acceleration = the rate at which velocity changes; tells you how fast the velocity of something is changing
b) velocity = rate at which an object travels

What do you understand intuitively about average speed?
Let's pretend you are on a subway train and the train is traveling east at 85 miles per hour. Suppose you decide to get out of your seat and walk west at 2 miles per hour toward a friend that you think you recognize.

QUESTION:

In what direction are you moving and at what speed? (THINK, THEN CLICK)





















direction and speed

Can you answer this question without a frame of reference? NO

If your frame is the TRAIN, you are walking west at 2 miles per hour.
If your frame is the GROUND, you are cruising east at 83 miles per hour (85 for the train subtract 2 walking).

SO there is no way to measure the motion of an object unless compared to the motion of another. Measurement is relative.