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Kinetic Energy = 1/2 m v^{2}

m = mass (kg)

v = speed (m/sec)

example: The KE of a bullet of mass 50 grams moving at 400 m/sec
KE = 1/2 x (50x10^{-3} kg) x (400 m/sec)^{2} = 4000 joules

Q: Which is greater, the kinetic energy of a 100 gram tennis ball
moving at 2 x 10^{8} cm/sec (about 10% of the speed of light)
or a 5 x 10^{6} gram truck moving at 20 cm/sec?