Question

How many liters of water flow out of pipe of cross section area ${\text{5c}}{{\text{m}}^{\text{2}}}$ in ${\text{1minute}}$, if the speed of the water in the pipe is ${\text{30cm/s}}$.A. ${\text{9 litres}}$B. ${\text{8 litres}}$C. ${\text{7 litres}}$D. None of the above.

Hint: Firstly calculate the flow of volume per unit time from then we can use unit conversion of ${\text{1litre = 1000c}}{{\text{m}}^{\text{3}}}$ and then multiply it with the given time in order to obtain your final answer.

Speed of water flow is ${\text{30cm/s}}$.
Area of pipe is ${\text{5c}}{{\text{m}}^{\text{2}}}$
So, flow of volume of water per unit time will be ${\text{30\times5= 150c}}{{\text{m}}^{\text{3}}}{\text{/s}}$
As per the unit conversion ${\text{1litre = 1000c}}{{\text{m}}^{\text{3}}}$so,
Volume flow per unit time $\dfrac{{{\text{150}}}}{{{\text{1000}}}}{\text{litre/s}}$${\text{ = 0}}{\text{.15litre/s}}$
And hence the water flows for ${\text{1min = 60sec}}$. So the total water that will pass the cross section is
${\text{ = 0}}{\text{.15}} \times 60{\text{ = 9litres}}$
Always write proper units and remember important conversions, as ${\text{1litre = 1000c}}{{\text{m}}^{\text{3}}}$.