# Why exercise makes you gain weight

Let’s say we have a person with a mass of 100 kg who can run at 5 m/s.

Now we can make use of Einstein famous equation of E = MC^{2 }:einstein)

When the person is at rest, the total energy of the person is 100 * (3x10^{8})^{2} or 9x10^{18} J

When the person is running, their Kinetic energy can be defined as ½mv^{2}

K_{e }= (½)*(100)* 5^{2}

K_{e} = 1250 J

Therefore the person total energy is (9x10^{18} + 1250)J

With a little reworking of the equation, we can get Δ m = (E_{f} – E_{o}) / (C^{2})

Δ m = (1250 J / 9x10^{18} m^{2}/s^{2})

Therefore Δ m = 1.3888x10^{-16 }kg

This means the person running has gained mass (even if very small) and therefore Weight.

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