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Mainly started special relativity and learning about the inertial frames of reference. I think I understand that, but here's what I don't get.

If from one inertial frame of reference moving at a constant 100 MPH, I toss a ball at a speed of 10 MPH, from my frame of reference the ball has moved at a speed of 10 MPH.

A viewer from a different inertial frame of reference moving at 20 MPH in the same direction as the other reference would witness the same ball traveling at a speed of 90 MPH.

But the speed of light is said to be constant.

So, does that mean that if I am in an inertial reference frame traveling 1/2 the speed of light, and an outside viewer is on a "stationary" reference frame (not sure if such a thing exists, but for simplicities sake..) and I shine a flashlight...

From MY reference frame, traveling 1/2 the speed of light, I "witness" the electromagnetic waves emitted from a flashlight traveling at 3 X 10^8 m/s, correct?

But if someone from a stationary reference framed witnessed my frame of reference traveling at 1/2 speed of light AND witnessed the flashlight, would they also "witness" the electromagnetic waves emitted from the flashlight at a speed of 3X10^8 regardless of the speed of the source (flashlight-1/2 the speed of light)?

Is that what it means for C to be constant? Both viewers see it at C, even if one viewer is traveling with the lightsource at 1/2 the speed of light and the other is stationary, they both witness the light from the flashlight at the same speed?