Question

A boy of mass m kg boards a trolley of mass 2m moving with constant speed u along a horizontal track. Neglecting friction, if the boy jumps vertically up with reference to the trolley to catch hold of a branch of a tree, the speed of trolley after the boy has jumped off is

• A. u
• B. 2w
• C. $\frac { \mathrm { u } } { 2 }$
• D. $\frac { 3 u } { 4 }$

Answer 1

Answer: (A)

u

Answer 2

By conservation of momentum

(m + 2m)u = mu + 2mV

or 3mu = mu + 2mV

or V = u The vertical motion does not affect the horizontal motion. Hence the trolley moves with u.