Question

A ball kept in a closed box move in the box making collisions with the walls. The box is kept on a smooth surface. The velocity of the center of mass :

• A. of the box remains constant
• B. of the (box+ball) system remains constant
• C. of the ball remains constant
• D. of the ball relative to the box remains constant

Answer 1

Answer: (B)

of the (box+ball) system remains constant

No external force is acting on the (ball+box) system when the ball collides with the wall of the box i.e ${\overrightarrow{F}}_{ext}=0$

Thus acceleration of centre of mass of the system ${\overrightarrow{a}}_{cm}=\frac{{\overrightarrow{F}}_{ext}}{M}=0$

$⟹\Delta \overrightarrow{v}=0$

Thus the velocity of the (ball+box) system remains constant.