Question

A block of metal weighing $2$ kg is resting on a frictionless plane it is struck by a jet releasing water at a rate of $1$ kg/s and at a speed

Answer 1

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

So the average change in momentum per sec = initial momentum
$=\text{mass}\times \text{initial velocity of water}$

$=1kg\times 5m\text{/s}=5kgm\text{/s}$

As this change in momentum occurs during 1 sec. We may say that the force exerted on the 2kg metal block is
$F=\frac{\text{change in momentum}}{\text{time}}$
$=\frac{5kgm\text{/s}}{1s}=5N$

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

$a=\frac{F}{m}=\frac{5N}{2kg}=2.5m\text{/}{s}^2$