Question

A man of mass m is standing on a stationary flat car of mass M. The car can move without friction along horizontal rails. The man starts walking with velocity v relative to the car. Work done by him

• A. is greater than $\frac{1}{2}m{v}^2$ if he walks along rails
• B. is less than $\frac{1}{2}m{v}^2$ if he walks along rails
• C. is equal to $\frac{1}{2}m{v}^2$ if he walks normal to rails
• D. can never be less than $\frac{1}{2}m{v}^2$

Answer 1

Answer: (B), (C)

is less than $\frac{1}{2}m{v}^2$ if he walks along rails
is equal to $\frac{1}{2}m{v}^2$ if he walks normal to rails

If he walks along the rails he pushes the car back on gaining velocity $V$ so some so the work done will be less than its gained kinetic energy but, when he walks normal to the rail his total work has converted into kinetic energy so work done will be $\frac{1}{2}{mv}^2$.