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During the Elseworlds crossover, Flash and Supergirl begin circling the earth at quote unquote "Mach 7" to slow down time. The Mach 7 figure makes no sense from basically any perspective at all, so I'm choosing to ignore it to actually measure how fast Barry and Kara are visibly moving.

At their peak, it takes them 0.16 seconds to circle the Earth once. This is an increase from 0.76 seconds, which is how long it took them to make one circumnavigation 4.8 seconds before reaching their top speed. Using these figures we can find out both their top speed and their maximum acceleration.


v = d/t

  • d = 40,000 km
  • t = 0.76 s

40,000/0.76 = 53,000,000 m/s

v = d/t

  • d = 40,000 km
  • t = 0.16 s

40,000/0.16 = 250,000,000 m/s

vD = vf-vi

  • vf = 250,000,000 m/s
  • vi = 53,000,000 m/s

250,000,000-53,000,000 = 200,000,000 m/s

a = v/t

  • v = 200,000,000 m/s
  • t = 4.8 s

200,000,000/4.8 = 42,000,000 m/s2


Results

  • Barry and Kara Circle the Earth (Top Speed) - 0.83c, Relativistic Class+
  • Barry and Kara Circle the Earth (Acceleration) - 42,000,000 m/s2

Tidbit: This level of acceleration implies that Supergirl could become "faster than a speeding bullet" in around a tenth of a centimeter. Basically just twitching a single one of her toes. After moving an entire footstep's worth of distance she would be moving faster than any man-made earthbound object.

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