In fluid mechanics, what is the value of viscosity for an ideal or perfect fluid?

Introduction / Context:
In fluid mechanics, the idea of an ideal or perfect fluid is very important for building simple mathematical models of real fluids. Real liquids such as water, oil or air always have some internal friction when layers slide past one another, but in many theoretical problems this friction is assumed to be absent. That internal friction is measured by a quantity called viscosity. Understanding what happens to viscosity in the ideal fluid model helps students clearly separate idealised concepts from real world behaviour and also prevents common conceptual mistakes in exam questions.

Given Data / Assumptions:
• We are dealing with an ideal or perfect fluid, not a real fluid like water or oil.
• Viscosity means internal friction between neighbouring layers of the fluid when they move relative to each other.
• There is no need to use any numerical value or formula in this question, it is purely conceptual.
• Pressure and density of the fluid are not being changed in the statement, only the nature of the fluid is idealised.

Concept / Approach:
Viscosity is defined as the property of a fluid that opposes relative motion between its layers. It is responsible for viscous forces or drag forces inside the fluid. Real fluids always have some viscosity. For example, honey has a high viscosity and water has a much lower viscosity. In theory, to simplify the equations of motion, physicists and engineers often imagine an ideal fluid that has no internal friction at all. This means the viscosity coefficient of such a fluid is taken to be exactly zero. This assumption makes the mathematics much easier and is used in many topics, such as ideal fluid flow and Bernoulli equation.

Step-by-Step Solution:
Step 1: Recall the definition of viscosity as a measure of internal friction between layers of a fluid. Step 2: Recall the definition of an ideal or perfect fluid from theory in fluid mechanics. Step 3: An ideal fluid is defined to have no internal friction or viscous resistance. Step 4: If there is no internal friction, the viscosity coefficient must be taken as zero. Step 5: Therefore the correct statement about the viscosity of an ideal fluid is that it is zero.

Verification / Alternative check:
You can verify this idea from the standard equations of fluid dynamics. The Navier Stokes equations for real fluids contain terms that involve viscosity. When we want equations for an ideal fluid, those viscous terms are removed by setting the viscosity coefficient to zero. The result is the Euler equation for ideal fluid flow. This confirms that in all ideal fluid models, viscosity is assumed to vanish. No standard textbook ever defines an ideal fluid with non zero viscosity, so any option that suggests a non zero value is inconsistent with the theoretical definition.

Why Other Options Are Wrong:
Equal to its mass: This is meaningless because viscosity and mass are completely different physical quantities with different units.
Equal to its weight: Weight is a force, while viscosity is a material property, so they cannot be equal in this sense.
Very large but finite: A large viscosity means a very sticky fluid, which is the opposite of the behaviour of an ideal fluid.
Infinite: Infinite viscosity would mean the layers cannot slide at all, which does not match the definition of an ideal, frictionless fluid.

Common Pitfalls:
Students sometimes confuse an ideal fluid with an incompressible fluid or with a superfluid. Incompressible simply means constant density, and superfluids are special quantum states that still have their own specific properties. None of these ideas imply a large value of viscosity for an ideal fluid. Another mistake is to assume that ideal means very good in a loose sense, so some learners think of very low but non zero viscosity. In theory, however, ideal is a precise word and means exactly zero viscosity. Remember that ideal models are simplifications and do not correspond to any actual liquid in everyday life.

Final Answer:
The viscosity of an ideal or perfect fluid is taken to be zero.