The kinetic energy of a man is half that of a boy whose mass is half that of the man. When the man speeds up by 5 m/s, his kinetic energy doubles that of the boy’s kinetic energy. What is the initial velocity of the man?(a) 5 m/s(b) 10 m/s(c) 15 m/s(d) 20 m/sI have been asked this question in quiz.This question is from Kinetic Energy in division Work, Energy and Power of Physics – Class 11

The kinetic energy of a man is half that of a boy whose mass is half t

1.	The kinetic energy of a man is half that of a boy whose mass is half that of the man. When the man speeds up by 5 m/s, his kinetic energy doubles that of the boy’s kinetic energy. What is the initial velocity of the man?(a) 5 m/s(b) 10 m/s(c) 15 m/s(d) 20 m/sI have been asked this question in quiz.This question is from Kinetic Energy in division Work, Energy and Power of Physics – Class 11
Answer» Right answer is (a) 5 m/s The explanation: The kinetic energy of MAN (initial) = KE = KE’/2 The kinetic energy of man (FINAL) = 2 x KE’ The kinetic energy of boy = KE’ Mass of man = M Mass of boy = m = M/2 v = Initial VELOCITY of man v’ = Initial velocity of boy (1/2) x M x v^2= (1/2) x m x v’^2 = (1/2) x M/2 x v’^2 Therefore; v = v’/2^0.5 (1/2) x M x (v+5) ^2=2 x (1/2) x m x v’^2 = 2 x (1/2) x M/2 x (2 x v^2) (v+5) ^2 = 4 x v^2 v + 5 = 2 x v v = 5m/s.

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