# H6: Relativistic momentum and energy

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H6: Relativistic momentum and energy
Relativity H6: Relativistic momentum and energy

Two basic physical quantities need to be modified
Length Time Other physical quantities will need to be modified as well

Relativistic momentum
To properly describe the motion of particles within special relativity, Newton’s laws of motion and the definitions of momentum and energy need to be generalized These generalized definitions reduce to the classical ones when the speed is much less than c

Relativisitc momentum
To account for conservation of momentum in all inertial frames, the definition must be modified v is the speed of the particle, m is its mass as measured by an observer at rest with respect to the mass When v << c, the denominator approaches 1 and so p approaches mv

Problem: particle decay
An unstable particle at rest breaks up into two fragments of unequal mass. The mass of the lighter fragment is 2.50 × 10–28 kg, and that of the heavier fragment is 1.67 × 10–27 kg. If the lighter fragment has a speed of 0.893c after the breakup, what is the speed of the heavier fragment?

For the heavier fragment,
An unstable particle at rest breaks up into two fragments of unequal mass. The mass of the lighter fragment is 2.50 × 10–28 kg, and that of the heavier fragment is 1.67 × 10–27 kg. If the lighter fragment has a speed of 0.893c after the breakup, what is the speed of the heavier fragment? Momentum must be conserved, so the momenta of the two fragments must add to zero. Thus, their magnitudes must be equal, or Given: v1 = 0.8 c m1=2.50×10–28 kg m2=1.67×10–27 kg Find: v2 = ? For the heavier fragment, which reduces to and yields

Relativistic energy The definition of kinetic energy requires modification in relativistic mechanics KE = mc2 – mc2 The term mc2 is called the rest energy of the object and is independent of its speed The term mc2 is the total energy, E, of the object and depends on its speed and its rest energy

A particle has energy by virtue of its mass alone
A stationary particle with zero kinetic energy has an energy proportional to its inertial mass E = mc2 The mass of a particle may be completely convertible to energy and pure energy may be converted to particles

Energy and Relativistic Momentum
It is useful to have an expression relating total energy, E, to the relativistic momentum, p E2 = p2c2 + (mc2)2 When the particle is at rest, p = 0 and E = mc2 Massless particles (m = 0) have E = pc This is also used to express masses in energy units mass of an electron = 9.11 x kg = MeV Conversion: 1 u = MeV/c2