Physics - relative...

There is one of the basical terms in physics I don't understand. Relative motion. I come over this very often when I read physics theory and not understanding makes it impossible to get the whole picture. So what exactly is relative motion? Is there anything else called "relative"..something?

Please provide me with some examples too!

Isn't that when something looks like its moving, but its relative to your perspective? Such as the ground looks like its moving when you are in a car driving.

Yeah. Basically, Einstein postulated that all things are relative. Time is relative, length/distance is relative, etc.

I don't have much time, relatively, tonight, so here's a quick example of your "relative motion" question.

First of all, motion is simply moving through space (space-time, but space, nonetheless). If you are out in some giant void, you will never know if you have moved or not because you have no reference point from which to calculate your distance. Of course, you can COUNT, but that doesn't help you at all. Baring equilibriums from the fluids in your body, you won't even know if you have started to spin.

Now, for a very classical example (as opposed to quantum) let us say that one car is coming at another car head on. The speed of Car1 is V1, and the speed of Car2 is V2.

Now, here's the pic:

C10==0 -->V1 V-relative V2<-- 0==0C2

The two velocities add vectorially (meaning directionally) and you can look at it this way:

The RELATIVE Velocity (motion--distance per unit of time) of the system is V1-V2. Now, let us say the |V1|=4m/s and |V2|=7m/s, where | | denotes magnitude (for us, absolute value) without direction.

So, with the DIRECTIONS (TRUE velocities have both a magnitude and a direction, otherwise they're just speeds), we have

V1=+4m/s and V2=-7m/s (there are many ways of notation for direction and this is the simplest for our case--just positive and negative signs).

So, 4m/s-(-7m/s)=11m/s==>the Relative Velocity (or perCEIVED velocity) of the system.

You will find that, as you drive along, you can imagine yourself stopped (NOT IN MOTION) and the vehicle going by you will seem to be going faster than it really is (in relation to the road).

Well, this is fun, but I have a paper to write tonight :0)

Bye!