# Q & A: absolute velocity?

Q:
people say that we cant find the speed of object with out a reference,i have derived an expression,but can somebody tell me whether it is right or not(i doubt about logic behind the expression because i am professionally not into physics and i know basics that's it) lets take speed a frame of reference(let it be earth) as "ve" and mass "m",its intial velocity is zero with reference to earth and true velocity is "ve".we will set that into motion with velocity "v0" with respect to earth with the help of energy "e" so e=0.5*m(ve+vo)^2-0.5*m(ve)^2 =>ve=(2e-vo)/2*vo*m do anybody have comments in this also tell me where i can find the relativity theory concepts to study and latest updates of quantam mechanics
- A.U.Mahesh (age 27)
Bangalore,Karnataka,India
A:
The general approach you're using assumes that the kinetic energy is defined independently of the choice of reference frame. Without that assumption, there are too many unknowns. However, there is no separate independent definition of energy. There is no way to pick any one object and say that it is the one that's really at rest.
(There also seems to be an algebra error, but that's not important.)

Mike W.

(published on 05/08/2011)

## Follow-Up #1: relative speed

Q:
So,there is no way finding the speed of object with out a reference.And also Albert Einstein say that objects can't get a speed more than or equal to speed of light.Then with which frame of reference he is talking about the speed of object----------- I have another Q.--a person "v" is traveling in straight path with speed c/4 from point "a" to "b" separated by great distance."v" drops two light beams "k" and "l" on "a" and "b" respectively.now-- what is the speed of beam "k" for observer at "a"-- what is the speed of beam "k" for "v"-- what is the speed of beam "l" for observer at "b"-- what is the speed of beam "l" for "v"
- A.U.Mahesh (age 27)
Bangalore,Karnataka,India
A:
Einstein was talking about any frame of reference. How can that be?  Let's confine our attention to motion in a single dimension. The velocity v as seen in reference frame 2 is not u+w, where u is the velocity of reference frame 1 in RF 2 and w is the velocity of the object in RF 1. That's what you'd expect if distance and time intervals were the same in each RF, but they aren't. Special Relativity includes the rules for how those change. The actual value of v is (u+w)/(1+uw/c2). Plug in any values of u, w < c, and you get  v < c. If u or w equals c, so does v.

Mike W.

(published on 05/09/2011)

## Follow-Up #2: are there relativistic invariants?

Q:
Universe got to be kidding with us.Event "time" itself is not independent of frame of reference right.I wonder ,is there any parameter that is independent of RF.And why mass increases with velocity. The equation u had given in last conversation is very helpful in understanding "why a absolute velocity of object is always less than c".
- A.U.Mahesh (age 27)
Bangalore,Karnataka,India
A:
Sure, there are lot's of things that don't change in different reference frames. We call them "invariants". Einstein got so fed up with hearing people talk nonsense about relativity that he tried to rename it "Invariants theory" but by them the name had stuck.

Sticking to Special Relativity (because I don't know GR well enough to discuss it properly) here's some invariants:

1) c
2. d2-c2t2, where d is distance and t is time interval ((1) is just a special case of this.).
3. E2-p2c2

None of those seem intuitively like they should be invariant. (2) sort of replaces t, which seems like it should be invariant but isn't. (3) sort of replaces the inertial mass m, which seems like it should be invariant but isn't.

As for why inertial mass (defined as what you have to multiply v by to get p) increases as |v| increases, there's a very simple argument based on symmetry and conservation of p. However, it's just a little long for these Q&A.

Mike W.

(published on 05/11/2011)