Question

A block of mass 2 kg is at rest on a horizontal frictionless plane. It is struck horizontally by a water jet releasing water at the rate of 4 kg s–1 and at a speed of 12 m/s. The initial acceleration of the block is

Answer 1

bhai chap ka h bato to batey

Answer 2

The final velocity of mass of water after hitting the metal block is zero.

So the average change in momentum per sec = initial momentum 
#="mass"xx"initial velocity of water"#

#=1kgxx5m"/s"=5kgm"/s"#

As this change in momentum occurs during 1 sec. We may say that the force exerted on the 2kg metal block is
#F="change in momentum"/"time"#
#=(5kgm"/s")/(1s)=5N#

Hence by Newton's law initial acceleration of the block of mass #m=2kg# is given by

#a=F/m=(5N)/(2kg)=2.5m"/"s^2#