Question

In small balls, each of mass m impact elastically each second on a surface with velocity u, then the force experienced by the surface in one second, will be.

• A. $4$mnu
• B. $2$mnu
• C. $1.5$mnu
• D. $0.8$mnu

Answer 1

Answer: (B)

$2$mnu

Force experienced by the surface is equal to change in momentum in one second.
From Newton's second law of motion, the rate of change of momentum of a body is equal to the external force applied on the body and the change in momentum always takes place in the direction of the force.
Momentum of ball before impact$=$mu
Momentum after imact$=-$mu
Change in momentum$=$mu$-(-$mu$)=2$mu
Change in momentum due to n balls in one second is $=2$mnu.
Change of momentum $=$ force experienced per second $=2$mnu.