A man (mass `=50kg`) and his son (mass `=20kg`) are standing on a frictionless surface facing each other , the man pushes his son so that he start moving at a speed of `0.70ms^(-1)` with respect to the man. The speed of the man with respect to the surface is:

Options

A. `0.14 m s^(-1)`
B. `0.47 m s^(-1)`
C. `0.28 ms^(-1)`
D. `0.20 ms^(-1)`

Answer

D

Explanation

`V_c to` velocity of child (Rightward) `" "V_m to` velocity of man (Left ward)
`V_(C//M) = 0.7` (take Rightward as positive)
`V_C - (V_M) = 0.7 " "V_C = 0.7 - V_m`
Conservation of Linear momentum `p_i = p_t`
`0 = -50 V_m + 20(0.7 - V_m) " "V_m = 0.2`.