# Special relativity

1. Aug 17, 2015

### thecosmos123456

special relativity says speed can be additive and subtractive ,our motion influences the speed , i did not get this part .how can our motion influence the speed of other object ?and how can speed be additive and subtractive

2. Aug 17, 2015

### Staff: Mentor

What do you mean by this? It's not clear what your question is.

3. Aug 18, 2015

### Mordred

The only thing I can think of, as the question isn't very clear. Is how you measure the speed of an object is influenced by the speed of the observer. Ie if your on a train moving east at 100 km per hour and look at a car moving at the same speed and direction. The car will appear to not be moving.

So the speed is additive and subtractive compared to your momentum and direction.
The speed of the object itself isn't influenced but how you measure the speed relative to your motion is influenced.

However that's just a guess, if your dealing with relativistic apparent velocity then it gets more complicated.

Last edited: Aug 18, 2015
4. Aug 21, 2015

### HallsofIvy

Staff Emeritus
You still haven't said what you mean by "speed can be additive and subtractive, our motion influences speed", but you seem to have got it exactly backwards.

If you mean by that that we can only observe or measure speeds of other objects relative to our own, that goes back to Galileo (and what is called "Gallilean relativity" as opposed to "Einsteinian relativity"). Gallileo said that if A observes B moving at velocity v (i.e B's velocity relative to A) and B observes C moving at velocity u (i.e C's velocity relative to B) then A will observe C moving at velocity v+ u (i.e. C's velocity relative to A).

In special relativity, while those speeds will change with respect to change of reference, they are not "additive". In the scenario above, A will observe C's velocity (C's velocity relative to A) to be $$\frac{u+ v}{1+ \frac{uv}{c^2}}$$