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## Main Question or Discussion Point

I am reading "Relativity" by Einstein right now, and I came across a formula he gave for kinetic energy in accordance with his theory of relativity. He says that "the kinetic energy of a material point of mass m is no longer given by the well-known expression

.5m(v^2)

but by the expression

[m(c^2)]/[sqrt(1-(v^2)/(c^2))]

I'm assuming this expression was found by the Lorentz transformation, but he doesn't show that. He goes on to explain how this formula suggests the impossibility of speeds faster than c, since the fraction approaches infinity as v approaches c.

The reason why I am confused is that the two expressions are extremely different. If the mass is 2 kg and the velocity is 3 m/s, then by the first expression, the KE is 9 J. By the second expression, you get about

180,000,000,000,000,000 J. What am I missing?

(also, what is a material point?)

Thanks for any help on this

.5m(v^2)

but by the expression

[m(c^2)]/[sqrt(1-(v^2)/(c^2))]

I'm assuming this expression was found by the Lorentz transformation, but he doesn't show that. He goes on to explain how this formula suggests the impossibility of speeds faster than c, since the fraction approaches infinity as v approaches c.

The reason why I am confused is that the two expressions are extremely different. If the mass is 2 kg and the velocity is 3 m/s, then by the first expression, the KE is 9 J. By the second expression, you get about

180,000,000,000,000,000 J. What am I missing?

(also, what is a material point?)

Thanks for any help on this

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