A man of 100kg starts to move on the earth (mass 6.4x10kg), and acquires a speed of 3m/s. With what speed does the earth recoil?
Solution. (Hint: Momentum is conserved in the process)

-4.6875 m/s

Explanation:

Mass of man; m_m = 100 kg

Mass of earth; m_e = 6.4 × 10 kg = 64 kg

Speed of man; v_m = 3 m/s

Since we are told that momentum is conserved, it means that momentum of the man will be equal and opposite to momentum of the Earth.

Thus;

m_m × v_m = -m_e × v_e

100 × 3 = -64 × v_e

v_e = 300/-64

v_e = -4.6875 m/s

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