What Is Fluid Friction? Key Factors, Applications & Practice Problems
Fluid friction is a force that restricts the movement either within itself or of another medium moving through the liquid. Due to the movement of the molecules inside the fluid, internal friction occurs, and how the fluid interacts with other matter, external conflict occurs. It describes the friction between the layers of the viscous liquid that moves relative to each other. The internal resistance to flow is termed as viscosity. The less-dense the fluid, the greater is the ease for movement or frictional force. Fluid friction is mostly used in water slides so that we can prompt or slide down gently.
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State the Law of Fluid Friction
The laws of fluid friction are:
The rules hold differently for different lubricants.
It is indirectly proportional to the temperature of the lubricant.
The friction force is independent of the substances of the bearing surfaces as well as the load.
What are the Five Examples of Fluid Friction?
Some of the examples of fluid friction are:
The air particles that form up the air. It causes a falling object to slow down.
The lubricants used in hinges.
Submarine moving through water. It is external fluid friction that occurs on it.
When you drop any object in a fluid, the extent of its splash depends on its fluid friction.
The viscosity of honey is another type of example of fluid friction.
Types of Friction
There are five different types of friction:
Dry Friction
It restricts the motion of two concrete surfaces in contact.
Fluid Friction
It resists the movement of viscous fluids relative to each other.
Lubricated Friction
It separates two solid surfaces by a lubricant fluid.
Skin Friction
It is a drag component, resisting the movement of fluid against the surface of the body
What are the Factors Affecting Fluid Friction?
The factors on which fluid friction depends are as follows:
The Speed of the Body:
Speed and friction are directly proportional to each other. The more the rate will be, the more the drag will be. For instance, a body moving with higher velocity will have more drag then a body moving with comparatively lesser speed.
Nature of the Fluid:
The thinner the fluid, the lesser the fluid friction will be, and vice versa. For instance, water is thinner than honey. That is why there is less fluid friction in water than honey.
The Viscosity of Fluid:
Fluid friction is directly proportional to the thickness of the liquid. The more the density, the more the drag will be.
The Shape of the Body:
Other figures feel more drag than the body with a streamlined shape which begins and ends in points like the shape of a fish.
The Temperature:
It is directly proportional to the fluid friction. Hence, with an increase in temperature, the fluid friction increases.
The Surface Area of the Body:
The size of the body which faces the fluid should be as less as possible to feel less drag and so that it can cut through the fluid easily. For instance, a horizontal moving body can cut through the liquid more easily than a body moving vertically.
Solved Examples
Give a reason why objects must have unique shapes to move in fluids.
According to the laws of fluid friction, it is directly proportional to the shape of an object. In order to reduce fluid friction, special forms have been designed to minimize the opposing frictional force acting on them. These unique shapes are termed as streamlined shapes which are pointed at the ends, to cut the fluid smoothly.
What is a Viscous Drag?
Viscosity is defined as “resistance to flow”. Let’s consider pipe flow for the sake of illustrating an example. In fluid mechanics, any fluid directly in contact with a different medium moves at the same speed as that medium ). In the case of pipe flow, the tube is stationary, yielding a fluid velocity along the pipe walls of zero. The fluid velocity gradually increases with increasing distance from the pipe wall towards the center of the pipe, at which the rate is at maximum.
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So, since the flow velocity is 0 at the pipe wall, and V at the center of the flow, there must be some force holding it back, right? That’s viscous drag in action, the magnitude of which determines how much energy is required to maintain a given flow rate.
Fluid Friction - Law, Examples, Types, Factors Affecting and Solved Examples.
The conflict and the friction that is found in between the layers of a viscous fluid is Fluid Friction. Internal friction occurs due to movement of molecules that are inside the fluid and Viscosity is the internal resistance flow, commonly known as the ‘thickness’ of a fluid. External conflict happens when we see how the fluids interact with one another.
Fiction can be of several types:
Dry friction resists the motion of two solid surfaces which come into contact.
Fluid friction occurs between the layers of a viscous fluid that are moving relative to each other.
Lubricated friction When a lubricant fluid separates two solid surfaces, it is said to be a case of Lubricated friction.
Skin friction is a component of drag, and is when a force is trying to resist the motion of a fluid across the whole body surface.
Internal friction is the force which, while going through deformation, tries to resist the motion between the elements that make up a solid material while it undergoes deformation.
There are some factors that affect Fluid Friction
Object Design - Objects with a particular and specific design are helpful in order to be able to reduce the friction by allowing fluid molecules to travel through the body of the objects. Take the example of fish - for the fish to swim , they have a specific and streamlined body.
Object Speed - The higher the speed of the object, the greater will be the friction. Friction and Speed are directly proportional to one another . A body that moves with a higher velocity will definitely have more drag than a body which moves slowly, and has a reduced speed.
Object Size - If the object is large, the resistive force will be greater. A blue whale will encounter more friction when it is compared to a man swimming.
Fluid Nature - Fluids which seem to have a high inner resistance are said to have lots of fluid friction. Things, when dipped in honey, have a harder time flowing through it than water since honey is much thicker than water.
FAQs on Fluid Friction Explained: Laws, Types & Real-World Examples
1. What is fluid friction and how does it occur at a molecular level?
Fluid friction, also known as drag, is the resistive force exerted by a fluid (like a liquid or gas) on an object moving through it, or between layers of the fluid itself. At a molecular level, it occurs due to two main factors: the cohesive forces between the fluid's own molecules (which cause viscosity) and the adhesive forces between the fluid's molecules and the surface of the object. As an object moves, it must push aside fluid molecules, which in turn pull on neighbouring molecules, creating a resistance to the motion.
2. What are some common real-world examples of fluid friction?
Fluid friction is present in many everyday situations. Some key examples include:
Air Resistance: A parachute uses air resistance to slow a person's descent. Similarly, the sleek design of a sports car is to minimise air resistance.
Movement in Water: A swimmer feels the resistance of water while pushing through it. The streamlined body of a fish is a natural adaptation to reduce this drag.
Flow of Liquids: The thick, slow flow of honey from a jar is due to its high internal fluid friction (viscosity), compared to water which flows easily.
Flying: Birds and aeroplanes are designed to manipulate fluid friction, using their wings to generate lift while minimising drag.
3. What are the fundamental laws that govern fluid friction?
While complex, the principles of fluid friction can be summarised by a few key observations:
The frictional force is directly proportional to the area of contact between the fluid and the object's surface.
The force increases significantly with the relative velocity between the object and the fluid.
The force depends on the nature of the fluid, specifically its density and viscosity. A denser or more viscous fluid will exert a greater frictional force.
The force is also dependent on the shape of the object, which is why streamlining is effective.
4. How does fluid friction differ from the friction between two solid surfaces?
There are crucial differences between fluid friction and solid friction. Solid friction is largely independent of the contact area and the relative velocity of the surfaces (at low speeds). In contrast, fluid friction is highly dependent on both the contact area and the relative velocity. As an object speeds up in a fluid, the drag force increases dramatically, which is not typically observed with solid friction. Furthermore, solid friction involves surface irregularities, while fluid friction involves intermolecular forces within the fluid.
5. How does the viscosity of a fluid influence the amount of friction?
Viscosity is a measure of a fluid's internal resistance to flow and is a primary factor in fluid friction. A fluid with high viscosity, like honey or tar, has strong intermolecular forces. This causes its layers to resist sliding past one another, resulting in high internal friction. Conversely, a fluid with low viscosity, like water or air, flows easily because its molecules move more freely, leading to lower internal friction. Therefore, a more viscous fluid will exert a greater drag force on an object moving through it.
6. What is the importance of a streamlined shape in reducing fluid friction?
The importance of a streamlined shape lies in its ability to minimise pressure drag, a major component of fluid friction. A streamlined body, like that of a fish or an aircraft wing, allows the fluid to flow over its surface in smooth, parallel layers (laminar flow). This prevents the formation of a large, turbulent wake behind the object. This turbulent wake creates a low-pressure zone that pulls the object backward, increasing drag. By promoting smooth flow, streamlining significantly reduces this resistance, allowing for greater speed and efficiency.
7. Are there different types of fluid friction?
Yes, fluid friction can be broadly categorised into two types. The first is internal friction, which is the fluid's inherent resistance to flow, defined by its viscosity. This is the friction between adjacent layers of the fluid itself. The second is external friction, more commonly known as drag. This is the force that resists the motion of a solid object as it moves through a fluid. Drag itself is composed of factors like skin friction (due to viscosity) and pressure drag (due to the object's shape).
8. Does fluid friction depend on the speed of the object moving through it?
Yes, absolutely. Unlike kinetic friction between solids, fluid friction is highly dependent on speed. At very low speeds in a smooth (laminar) flow, the drag force is often directly proportional to the velocity. However, as speed increases and the flow becomes turbulent, the drag force becomes proportional to the square of the velocity. This means that doubling the speed of an object like a car or a boat can increase the fluid friction force by a factor of four, making it a critical consideration in high-speed vehicle design.
9. In what ways can fluid friction be deliberately reduced or minimised?
Fluid friction can be minimised through several effective methods based on its governing principles:
Streamlining the Shape: Designing objects with a tapered, smooth shape (like a teardrop) to allow fluid to flow over it with minimal turbulence.
Reducing Viscosity: For internal friction, the viscosity of the fluid can sometimes be lowered, for instance, by heating lubricating oils to make them flow more easily.
Polishing Surfaces: Creating a very smooth surface on the object reduces 'skin friction', which is a component of drag caused by the fluid sticking to the object's surface.
10. Is fluid friction always a disadvantage?
No, fluid friction is not always a disadvantage and is often essential. While it opposes motion and reduces efficiency in vehicles, it is also the principle that allows many things to work. For example, a parachute relies entirely on maximising air friction to ensure a safe landing. The ability to swim or fly is possible only because an organism can push against a fluid (water or air) and use the resulting frictional and reactive forces to propel itself forward. Fluid friction is also critical for the function of automotive brakes that use hydraulic fluid.
