Question

$n$ small balls, each of mass $m$ impinge elastically each second on a surface with a 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

Answer 2

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 is momentum always takes place in the direction of the force.
Momentum of ball before inpact $=m u$
Momentum after inpact $=-m u$
Change in momentum $=m u-(-m u)=2 \,m u$.
Change in momentum due to $n$ balls in one
second is $=2\, mnu$.
Change of momentum $=$ Force experienced per second
$=2\, mnu$