a). is true. 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 really moving".
-- There's no such thing as "how high is it really".
-- There's no such thing as "really moving" or "really at rest".
What about quantities where you use distance or motion to calculate them ?
Like . . . . .
Potential energy . . . . . (mass) x (gravity) x (height)
Kinetic energy . . . . . (1/2) (mass) (speed)²
Momentum . . . . . (mass) x (speed) .
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 heavily at rest and taking a nap :-).)
