Why is it important to define a frame of reference?

a. An observer cannot be moving.

b. Motion must be defined relative to something.

c. It's impossible to measure motion.

d. A stationary observer is always moving.

Motion must be defined relative to something.

Here's an obvious, everyday example:

- - You're in a passenger jet, going to visit grandma on the

coast for the holidays.

- - You're sitting still in your seat, listening to some 'mp3's,

reading a book, and dozing off.

- - At the same time, people on the ground see you flying over

at almost 500 miles per hour.

Are you moving at 500 mph, or are you not moving at all?

The answer is 'Yes. Both.'. It just depends on who's measuring your speed.

There's no such thing as your "real" speed. Motion is always

relative to something. Different reference = different speed.