Understanding Resistance to Displacement in Substances

The question asks about a substance that encounters no resistance when it is displaced. In physics, particularly in fluid mechanics, the resistance encountered by layers of a substance (like a fluid or gas) when they move relative to each other is called viscosity. Viscosity is essentially a measure of a fluid's internal friction.

What is Resistance to Displacement?

When one part of a substance moves past another, there are forces between the molecules that oppose this motion. This opposition is the resistance to displacement or flow. Substances with high viscosity, like honey, offer more resistance than substances with low viscosity, like water or air.

Examining the Options

Let's consider the properties of the substances mentioned in the options:

• Fluid: A fluid is a substance that continuously deforms (flows) under an applied shear stress. Both liquids and gases are fluids. Real fluids exhibit viscosity, meaning they offer some resistance to displacement.
• Water: Water is a liquid, which is a type of fluid. Water has a relatively low viscosity, but it is not zero. Therefore, water encounters some resistance to displacement.
• Gas: A gas is also a type of fluid. Gases have lower viscosity than most liquids, but they still exhibit viscosity. For example, air resists motion through it (wind resistance).
• Ideal Fluid: An ideal fluid is a theoretical concept. It is defined as a fluid that has zero viscosity and is incompressible (density remains constant). Because an ideal fluid has zero viscosity, it offers absolutely no resistance to displacement or flow.

Identifying the Substance with No Resistance

Based on the definitions, only the theoretical "ideal fluid" is characterized by the complete absence of resistance to displacement (zero viscosity). Real-world substances like water, air, or any other fluid or gas will always have some level of viscosity, however small.

Therefore, a substance that encounters no resistance by displacement is known as an ideal fluid.