Question:
A man of mass $50 \mathrm{Kg}$ starts moving on the earth and acquires a speed of $1.8 \mathrm{~m} / \mathrm{s}$. With what speed does the earth recoil? Mass of earth $=6 \times 10^{24} \mathrm{Kg}$.
Solution:
Conservation of lin. Momentum (C.O.L.M)
$0=M_{E} V_{E}+M_{M} V_{M}$
M-Man
E-Earth
$V_{E}=\frac{-M_{M} V_{M}}{M_{E}}=\frac{-50 \times 1.4 \times 10^{7}}{6 \times 10^{24}}$
$=-15 \times 10^{-24} \mathrm{~m} / \mathrm{s}$