E =mc2 - continued

• If this is completely different from Newtonian ideas, how can it be that Newton’s laws were widely used - and still are ?

• The relativistic expression is E = mc2 = ? m0 c2  Now recall:

? =

1

sqrt(1 - v2 /c2 )

Rest Energy

Newton’s Kinetic Energy!

• For small v/c one can show ? is very nearly 1 + (1/2)(v/c)2 (Try this on a calculator!)

• Thus E = mc2 = ? m0 c2 = m0 c2 + (1/2) m0 v2

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