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# CBSE Class 11-science Answered

Water is flowing through horizontal pipe and sir we apply force on tap and open it and water comes out. Inside tap water is at rest and when water cakes from tap it gains velocity means momentum changes. So Newton second law is applicable or not. If not which law is applicable. Please explain.
Asked by Tanmmygarg6 | 06 Sep, 2019, 10:18: AM
Water is at rest inside a pipe when tap is closed but it is under pressure greater than atmospheric pressure.
That is the reason if small leak hole happens in a pipe, water jet is coming out through this hole.

When we open tap, we are not applying force. what we do when we open the tap is,
we expose the pressurised water to atmospheric pressure.

Hence water flow due to pressure difference just like objects fall vertically in gravitational field
due to potential energy difference.

Liquid or gas will flow due to pressure difference between upstream and downstream.

Since the water flow is due to pressure difference, pressure is nothing but force per unit area,
as per newtons first law external force changes the state of motion , hence water flows.

Newtons second law can be applied  to get the change of momentum or speed of water flow or flow rate of water

Newtons first law and second law are related to motion of objects.

First law describes the cause of chage of state of motion. It is qualitative.
First law tells us object at rest will move if it is subjected to external force,
or moving object will change its speed or can be stopped by applying force.

Second law describes the quantitative change of state of motion.
Second law tells us how much will be rate of change of momentum or rate of change of speed.

Hence if any object is moving, you can apply newton's first law and second law
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