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Relativistic velocity addition

Either light is an exception, or the classical velocity addition formula only works at low velocities. The latter is the case. The correct formula for one-dimensional relativistic velocity addition    is

u = v+u 1 + v u c 2 , size 12{ ital "u=" { { ital "v+u'"} over {1+ { {v` ital "u'"} over {c rSup { size 8{2} } } } } } } {}

where v is the relative velocity between two observers, u is the velocity of an object relative to one observer, and u is the velocity relative to the other observer. (For ease of visualization, we often choose to measure u in our reference frame, while someone moving at v relative to us measures u .) Note that the term v u c 2 becomes very small at low velocities, and u = v+u 1 + v u c 2 gives a result very close to classical velocity addition. As before, we see that classical velocity addition is an excellent approximation to the correct relativistic formula for small velocities. No wonder that it seems correct in our experience.

Showing that the speed of light towards an observer is constant (in a vacuum): the speed of light is the speed of light

Suppose a spaceship heading directly towards the Earth at half the speed of light sends a signal to us on a laser-produced beam of light. Given that the light leaves the ship at speed c size 12{c} {} as observed from the ship, calculate the speed at which it approaches the Earth.

A spacecraft is heading towards earth v equals zero point five zero zero times c. A laser beam from the ship travels towards the Earth with velocity c as shown by a vector. A second spaceship traveling away from the Earth. The velocity of the second ship and second laser are the same as the first, but in the opposite direction.


Because the light and the spaceship are moving at relativistic speeds, we cannot use simple velocity addition. Instead, we can determine the speed at which the light approaches the Earth using relativistic velocity addition.


  1. Identify the knowns. v= 0 . 500 c ; u = c
  2. Identify the unknown. u size 12{u} {}
  3. Choose the appropriate equation. u = v+u 1 + v u c 2 size 12{ ital "u=" { { ital "v+u'"} over {1+ { {v` ital "u'"} over {c rSup { size 8{2} } } } } } } {}
  4. Plug the knowns into the equation.
    u = v+u 1 + v u c 2 = 0.500 c + c 1 + ( 0.500 c ) ( c ) c 2 = ( 0.500 + 1 ) c 1 + 0.500 c 2 c 2 = 1.500 c 1 + 0.500 = 1.500 c 1.500 = c


Relativistic velocity addition gives the correct result. Light leaves the ship at speed c size 12{c} {} and approaches the Earth at speed c size 12{c} {} . The speed of light is independent of the relative motion of source and observer, whether the observer is on the ship or Earth-bound.

Velocities cannot add to greater than the speed of light, provided that v size 12{v} {} is less than c size 12{c} {} and u does not exceed c . The following example illustrates that relativistic velocity addition is not as symmetric as classical velocity addition.

Comparing the speed of light towards and away from an observer: relativistic package delivery

Suppose the spaceship in the previous example is approaching the Earth at half the speed of light and shoots a canister at a speed of 0.750 c . (a) At what velocity will an Earth-bound observer see the canister if it is shot directly towards the Earth? (b) If it is shot directly away from the Earth? (See [link] .)

In part a, a spaceship is moving towards the earth from left to right with a velocity v equals to zero point five zero times c. The spaceships shoots a canister towards earth with velocity u prime equals zero point seven five times c. A man stands stationary on earth observing. In part b, the spaceship shoots the canister away from earth with same velocity. In both the cases, the velocity of the ship is v equals 0 point five zero times c toward left.


Because the canister and the spaceship are moving at relativistic speeds, we must determine the speed of the canister by an Earth-bound observer using relativistic velocity addition instead of simple velocity addition.

Solution for (a)

  1. Identify the knowns. v= 0.500 c ; u = 0 . 750 c size 12{u rSup { size 8{'} } = - 0 "." "750"c} {}
  2. Identify the unknown. u size 12{u} {}
  3. Choose the appropriate equation. u= v+u 1 + v u c 2
  4. Plug the knowns into the equation.
    u = v+u 1 + v u c 2 = 0.500 c + 0.750 c 1 + ( 0.500 c ) ( 0.750 c ) c 2 = 1.250 c 1 + 0.375 = 0.909 c

Solution for (b)

  1. Identify the knowns. v = 0.500 c ; u = 0.750 c
  2. Identify the unknown. u
  3. Choose the appropriate equation. u = v+u 1 + v u c 2
  4. Plug the knowns into the equation.
    u = v+u 1 + v u c 2 = 0.500 c + ( 0.750 c ) 1 + ( 0.500 c ) ( 0.750 c ) c 2 = 0.250 c 1 0.375 = 0.400 c


The minus sign indicates velocity away from the Earth (in the opposite direction from v ), which means the canister is heading towards the Earth in part (a) and away in part (b), as expected. But relativistic velocities do not add as simply as they do classically. In part (a), the canister does approach the Earth faster, but not at the simple sum of 1.250 c . The total velocity is less than you would get classically. And in part (b), the canister moves away from the Earth at a velocity of 0.400 c , which is faster than the −0.250 c size 12{c} {} you would expect classically. The velocities are not even symmetric. In part (a) the canister moves 0.409 c size 12{c} {} faster than the ship relative to the Earth, whereas in part (b) it moves 0.900 c size 12{c} {} slower than the ship.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
How we can toraidal magnetic field
Aditya Reply
How we can create polaidal magnetic field
Mykayuh Reply
Because I'm writing a report and I would like to be really precise for the references
Gre Reply
where did you find the research and the first image (ECG and Blood pressure synchronized)? Thank you!!
Gre Reply
Practice Key Terms 3

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Source:  OpenStax, Physics 101. OpenStax CNX. Jan 07, 2013 Download for free at http://legacy.cnx.org/content/col11479/1.1
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