a). is true. <span>All motion is relative to a frame of reference. That's a fancy way of saying that whenever you talk about a distance, an acceleration, or a speed, they're always compared to something. Many surprising things come out of this:
-- There's no such thing as "how fast is it <u>really</u> moving". -- There's no such thing as "how high is it <u>really</u>". -- There's no such thing as "<u>really</u> moving" or "<u>really</u> at rest".
What about quantities where you use distance or motion to calculate them ? Like . . . . .
Potential energy . . . . . (mass) x (gravity) x (<u>height</u>)
Kinetic energy . . . . . (1/2) (mass) (<u>speed</u>)²
Momentum . . . . . (mass) x (<u>speed</u>) .
Yep, that's right. Those things are all relative to a frame of reference too; different observers can get different answers,and they're all correct ... for their own frame of reference.
Simple example:
You're flying in a passenger jet to visit your grandma.
In the frame of reference of somebody on the ground, you're moving at 400 miles per hour.
In the frame of reference of the pilot or the person sitting next to you, you are at rest, and you can listen to your pod or read a book ... (or maybe you're <em>heavily</em> at rest and taking a nap :-).)