Question

I have a question about the (less than) speed-of-light limit for objects with mass: Suppose there are 3 objects: A, B and C. I'm an observer inside object A. Objects B and C move away from me, in the same direction. Object B is moving at 90% speed-of-light (relative to me / object A). Object B is moving at 20% speed-of-light from me.

From object B, the relative speed of object C would appear as 70% the speed of light right? Now, can object C still accelerate to 90% the speed of light, as measured from object B? Wouldn't that exceed the 100% speed-of-light as measured from object A?

Thanks.

Answer 1

First of all, You asked the question wrongly.
You are in A and lets consider it as 0c
B is moving away from you with 90c
C is moving away in direction of B away from you at 20c
NOW
From C, the relative speed of object B would appear as 70c...
Here you are asking if C can accelerate to 90c as measured from B.
Now we know dat C is a universal constant and is never relative.
Now lets jump from objekt A(0c) to Objekt B(90c), when C(20c) goes to C(90c) then it will be equal to B(90c). It never will be relative, because speed of light is universal constant.
In this case, you also have to include the TIME, MASS and LENGTH factor.

Answer 2

Thanks for your reply.

Yes I made a mistake: I meant C to have a speed of 90c, not 20c, measured from A.
So, from observer in object A:
B: 20c
C: 90c

The same question with this correction: From object B I'm expecting to see object C move away at 70c.

According to this, an observer inside object B therefore may still see object C accelerate to for example 90c.

When that happens, how would this be measured at point A? Object C has accelerated to 90c (+20c), as seen from object B. From object A the speed of object C measured 90c already, so it's obviously impossible to have 110c...

Answer 3

dear, i already told u that SPEED OF LIGHT IS A UNIVERSAL CONSTANT.... No relativity is concerned at such case.... when something reaches speed of light.. its speed wil b constant i.e speed of light... so it will b equal to c, but never greater than c

Answer 4

Thanks for your enlightening answer ahmad...

Answer 5

I found the explanation in The Einstein velocity relation additions: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html#c1