A man of mass 50 kg starts moving on the earth and acquires a speed of 1.8 m/s. With what speed does the earth recoil? Mass of earth = 6 × 1024 kg.
Mass of man, m1 = 50 kg
Mass of earth, m2 = 6×1024 kg
Speed of man, v1 = 1.8 m/s
Speed of earth = v2
Applying conservation of momentum, m1v1 = m2v2
50×1.8 = 6×1024×v2
v2 = 1.5×10-23 m/s
∴ Speed at which earth recoils = 1.5×1024 m/s