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  • Compute total energy of a relativistic object.
  • Compute the kinetic energy of a relativistic object.
  • Describe rest energy, and explain how it can be converted to other forms.
  • Explain why massive particles cannot reach C.
This photo shows the outside of the fusion reactor of the National Spherical Torus Experiment at the Princeton Plasma Physics Laboratory. The reactor, which sits in a large room, is connected to numerous tubes and instruments.
The National Spherical Torus Experiment (NSTX) has a fusion reactor in which hydrogen isotopes undergo fusion to produce helium. In this process, a relatively small mass of fuel is converted into a large amount of energy. (credit: Princeton Plasma Physics Laboratory)

A tokamak is a form of experimental fusion reactor, which can change mass to energy. Accomplishing this requires an understanding of relativistic energy. Nuclear reactors are proof of the conservation of relativistic energy.

Conservation of energy is one of the most important laws in physics. Not only does energy have many important forms, but each form can be converted to any other. We know that classically the total amount of energy in a system remains constant. Relativistically, energy is still conserved, provided its definition is altered to include the possibility of mass changing to energy, as in the reactions that occur within a nuclear reactor. Relativistic energy is intentionally defined so that it will be conserved in all inertial frames, just as is the case for relativistic momentum. As a consequence, we learn that several fundamental quantities are related in ways not known in classical physics. All of these relationships are verified by experiment and have fundamental consequences. The altered definition of energy contains some of the most fundamental and spectacular new insights into nature found in recent history.

Total energy and rest energy

The first postulate of relativity states that the laws of physics are the same in all inertial frames. Einstein showed that the law of conservation of energy is valid relativistically, if we define energy to include a relativistic factor.

Total energy

Total energy E size 12{E} {} is defined to be

E = γmc 2 , size 12{E= ital "γmc" rSup { size 8{2} } } {}

where m size 12{m} {} is mass, c size 12{c} {} is the speed of light, γ = 1 1 v 2 c 2 size 12{γ= { {1} over { sqrt {1 - { {v rSup { size 8{2} } } over {c rSup { size 8{2} } } } } } } } {} , and v size 12{v} {} is the velocity of the mass relative to an observer. There are many aspects of the total energy E size 12{E} {} that we will discuss—among them are how kinetic and potential energies are included in E size 12{E} {} , and how E size 12{E} {} is related to relativistic momentum. But first, note that at rest, total energy is not zero. Rather, when v = 0 size 12{v=0} {} , we have γ = 1 size 12{γ=1} {} , and an object has rest energy.

Rest energy

Rest energy is

E 0 = mc 2 .

This is the correct form of Einstein’s most famous equation, which for the first time showed that energy is related to the mass of an object at rest. For example, if energy is stored in the object, its rest mass increases. This also implies that mass can be destroyed to release energy. The implications of these first two equations regarding relativistic energy are so broad that they were not completely recognized for some years after Einstein published them in 1907, nor was the experimental proof that they are correct widely recognized at first. Einstein, it should be noted, did understand and describe the meanings and implications of his theory.

Questions & Answers

how many subject is in physics
Adeshina Reply
the write question should be " How many Topics are in O- Level Physics, or other branches of physics.
how many topic are in physics
yh I need someone to explain something im tryna solve . I'll send the question if u down for it
Tamdy Reply
a ripple tank experiment a vibrating plane is used to generate wrinkles in the water .if the distance between two successive point is 3.5cm and the wave travel a distance of 31.5cm find the frequency of the vibration
the range of objects and phenomena studied in physics is
Bethel Reply
what is Linear motion
Hamza Reply
straight line motion is called linear motion
then what
linear motion is a motion in a line, be it in a straight line or in a non straight line. It is the rate of change of distance.
your are wrong Saeedul
Linear motion is a one-dimensional motion along a straight line, and can therefore be described mathematically using only one spatial dimension
is a one-dimensional motion along a straight line, and can therefore be described mathematically using only one spatial dimensions. 
what is a classical electrodynamics?
what is dynamics
dynamic is the force that stimulates change or progress within the system or process
what is the formula to calculate wavelength of the incident light
David Reply
if a spring is is stiffness of 950nm-1 what work will be done in extending the spring by 60mmp
Hassan Reply
State the forms of energy
Samzy Reply
Word : Mechanical wave Definition : The waves, which need a material medium for their propagation, e.g., Sound waves. \n\nOther Definition: The waves, which need a material medium for their propagation, are called mechanical waves. Mechanical waves are also called elastic waves. Sound waves, water waves are examples of mechanical waves.t Definition: wave consisting of periodic motion of matter; e.g. sound wave or water wave as opposed to electromagnetic wave.h
Clement Reply
what is mechanical wave
Akinpelu Reply
a wave which require material medium for its propagation
The S.I unit for power is what?
Samuel Reply
Am I correct
it can be in kilowatt, megawatt and so
OK that's right
SI.unit of power is.watt=j/c.but kw.and Mw are bigger.umots
What is physics
aish Reply
study of matter and its nature
The word physics comes from a Greek word Physicos which means Nature.The Knowledge of Nature. It is branch of science which deals with the matter and energy and interaction between them.
why in circular motion, a tangential acceleration can change the magnitude of the velocity but not its direction
Syafiqah Reply
because it is balanced by the inward acceleration otherwise known as centripetal acceleration
What is a wave
Mutuma Reply
Tramsmission of energy through a media
is the disturbance that carry materials as propagation from one medium to another
mistakes thanks
find the triple product of (A*B).C given that A =i + 4j, B=2i - 3j and C = i + k
Favour Reply
Difference between north seeking pole and south seeking pole
Stanley Reply
Practice Key Terms 3

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