4.6.5 Fluid Friction

We have discussed friction between solid surfaces, but friction also occurs when an object moves through a fluid—a liquid or a gas. If you have ever tried to run in a swimming pool or felt the wind push against you while cycling, you have experienced fluid friction. Understanding this type of friction is essential for explaining the motion of objects through air and water, from a falling raindrop to a speeding car.

Definition

Fluid Friction is the retarding force that a fluid (liquid or gas) exerts on an object moving through it. The moving body exerts a force on the fluid to push it out of the way, and by Newton's third law, the fluid pushes back on the body with an equal and opposite force. This retarding force is called the drag force.

Main Idea

The main idea is that objects moving through a fluid experience a resistive force that opposes their motion. The magnitude of this drag force depends on several factors, including the size, shape, and speed of the object, as well as the properties of the fluid. Understanding fluid friction is crucial for designing efficient vehicles, predicting the motion of falling objects, and explaining phenomena like terminal velocity.

Core Concept

The Nature of Fluid Friction The text states: "When an object moves through a fluid, the fluid exerts a retarding force that tends to reduce the speed of the object. The moving body exerts a force on the fluid to push it out of the way. By Newton's third law, the fluid pushes back on the body with an equal and opposite force. This retarding force experienced by an object moving through a fluid is called the drag force, which is the result of fluid friction."

• Size, shape and orientation of the object.
• Type (Properties) of the fluid.
• Speed of the object relative to the fluid."

Example from Text The text provides a simple example: "For example when you extend your hand out of the window in a moving vehicle as shown in figure 4.19. If you set the palm of your hand in the direction of motion you can observe the fluid friction and the drag force it exerts on your palm. You can also see variation in the drag force by changing its orientations."

This shows that the orientation of the object (hand) affects the drag force.

Terminal Velocity The text states: "During free fall the objects does not speed up indefinitely. The speed of free falling object initially increases because of weight of the object, but as the speed increases the drag force also increase, slowing the object down. A point reaches where both the weight and drag force become equal and dynamic translational equilibrium is achieved. The object has now attained its maximum velocity termed as terminal velocity."

Detailed Explanation

Fluid friction, or drag, is a fundamental concept in physics. It affects the motion of everything from subatomic particles to galaxies.

The Origin of Drag Drag arises from the interaction between the object and the fluid molecules. As the object moves, it collides with fluid molecules, transferring momentum to them. This creates a resistive force on the object. The drag force can be thought of as the fluid pushing back on the object as it is displaced.

Factors Affecting Drag Force The magnitude of drag force is influenced by several factors:

• Size of the object: Larger objects displace more fluid and experience greater drag.
• Shape and orientation: A streamlined shape (like a teardrop) reduces drag compared to a blunt shape. The orientation of the object relative to the flow also matters. For example, a flat hand perpendicular to the motion experiences more drag than a hand parallel to the motion.
• Speed of the object: Drag force increases with speed. For low speeds, drag is proportional to speed; for high speeds, it is proportional to the square of the speed.
• Properties of the fluid: Denser and more viscous fluids (like honey) produce more drag than less dense, less viscous fluids (like air).

Terminal Velocity When an object falls through a fluid, it accelerates due to gravity. As its speed increases, the drag force also increases. Eventually, the upward drag force equals the downward weight of the object. At this point, the net force is zero, and the object stops accelerating. It falls at a constant maximum speed called terminal velocity.

The text states: "The constant maximum velocity that is attained and maintained by an object while falling through air (or any other resistive medium) is called terminal velocity."

For humans, terminal speed in air is about 53 m/s or 190 km/h. After a parachute opens, the terminal speed is reduced to between 5 m/s and 10 m/s.

Skydivers and Swimmers The text explains: "Skydivers and swimmers change their effective size and orientation by bending, twisting and starching their body parts. This allow them to manipulate drag and thereby allowing them to control speed and direction of motion."

This is a practical application of the factors affecting drag. By changing their shape, skydivers can control their fall speed, and swimmers can reduce drag to swim faster.

Intermediate Level

In physics, fluid friction is the drag force that opposes the motion of an object through a fluid. It is given by the drag equation (for high speeds): F_D = 12 C_D ρ A v², where C_D is the drag coefficient (depends on shape), ρ is the fluid density, A is the cross-sectional area, and v is the speed. This shows that drag increases with the square of speed and depends on the shape and size of the object.

Terminal velocity is reached when the drag force equals the weight of the object. At this point, the net force is zero, and the object falls at a constant speed. The terminal velocity depends on the mass, shape, and size of the object, as well as the properties of the fluid. For example, a skydiver with a parachute has a much larger surface area and therefore a lower terminal velocity than a skydiver without a parachute.

Important Points

• - Fluid Friction (Drag): The resistive force exerted by a fluid on an object moving through it.
• - Direction: Always opposes motion.
• - Factors: Size, shape, orientation, speed, and fluid properties.
• - Terminal Velocity: The constant maximum speed reached when drag equals weight.
• - Skydivers and swimmers manipulate their shape to control drag.
• - Newton's Third Law: The object pushes the fluid, and the fluid pushes back.

Remember

• - Fluid Friction = Drag Force.
• - Direction: Opposes motion.
• - Factors: Size, shape, speed, fluid type.
• - Terminal Velocity: Drag = Weight, acceleration = 0.
• - Examples: Hand out of car window, skydiver, swimmer.
• - Newton's Third Law explains the origin of drag.

Common Mistakes

• - Thinking fluid friction is the same as solid friction: Fluid friction depends on speed (and is often proportional to v²), while solid friction is approximately independent of speed.
• - Forgetting that drag depends on shape: A streamlined shape reduces drag compared to a blunt shape.
• - Thinking terminal velocity is zero: Terminal velocity is a constant, non-zero speed.

Quick Revision

Fluid friction, or drag, is the resistive force that a fluid exerts on an object moving through it. It depends on the size, shape, orientation, and speed of the object, as well as the properties of the fluid. When an object falls through a fluid, it eventually reaches terminal velocity, where the drag force equals the weight and the object falls at a constant speed.

60-Second Revision

• Direction: Opposes motion.
• Factors: Size, shape, speed, fluid properties.
• Terminal Velocity: Drag = Weight, a = 0.
• Examples: Hand out of car window, skydiver, swimmer.
• Newton's Third Law: Fluid pushes back.

Frequently Asked Questions (FAQs)

• Q1. What is fluid friction?
• Answer: Fluid friction, or drag, is the retarding force that a fluid (liquid or gas) exerts on an object moving through it.

• Size, shape, and orientation of the object.
• Type (properties) of the fluid.
• Speed of the object relative to the fluid.

Q3. What is terminal velocity? Answer: Terminal velocity is the constant maximum velocity reached by an object falling through a fluid when the drag force equals the weight of the object. At this point, the net force is zero, and the acceleration is zero.

Q4. How does a skydiver control their speed? Answer: A skydiver can control their speed by changing their body orientation. By spreading their arms and legs, they increase their cross-sectional area, increasing drag and reducing their terminal velocity. By diving headfirst, they reduce their area and increase their terminal velocity.

Q5. Why is it easier to swim with a streamlined body? Answer: A streamlined body reduces the cross-sectional area facing the direction of motion, reducing the drag force. This makes it easier to move through the water.

Q6. How is fluid friction related to Newton's third law? Answer: The object moving through the fluid exerts a force on the fluid to push it out of the way. By Newton's third law, the fluid exerts an equal and opposite force on the object. This is the drag force.

Q7. What is the difference between fluid friction and sliding friction? Answer: Sliding friction occurs between solid surfaces in contact and is approximately independent of speed. Fluid friction occurs when an object moves through a fluid and increases with speed (often proportional to the square of speed). Both oppose motion.

Multiple Choice Questions (MCQs)

• Q1. The retarding force exerted by a fluid on an object moving through it is called:
• A. Normal force
• B. Tension
• C. Drag force
• D. Weight

• Correct Answer: C
• Explanation: The text defines the retarding force experienced by an object moving through a fluid as the drag force.

• Q2. Fluid friction acts in a direction:
• A. Parallel to motion
• B. Opposite to motion
• C. Perpendicular to motion
• D. Random

• Correct Answer: B
• Explanation: The text states that the fluid exerts a retarding force that tends to reduce the speed of the object (opposes motion).

• Q3. Which of the following is NOT a factor affecting drag force?
• A. Size of the object
• B. Shape of the object
• C. Color of the object
• D. Speed of the object

• Correct Answer: C
• Explanation: The text lists size, shape, orientation, fluid type, and speed as factors affecting drag force. Color is not a factor.

• Q4. The constant maximum velocity reached by a falling object when drag equals weight is called:
• A. Average velocity
• B. Instantaneous velocity
• C. Terminal velocity
• D. Initial velocity

• Correct Answer: C
• Explanation: The text defines terminal velocity as the constant maximum velocity attained by an object falling through a fluid.

• Q5. At terminal velocity, the acceleration of the object is:
• A. Maximum
• B. Zero
• C. Equal to g
• D. Infinite

• Correct Answer: B
• Explanation: At terminal velocity, the net force is zero (drag equals weight), so the acceleration is zero.

• Q6. The drag force depends on the:
• A. Color of the fluid
• B. Speed of the object relative to the fluid
• C. Temperature of the object
• D. Weight of the object

• Correct Answer: B
• Explanation: The text states that the drag force depends on the speed of the object relative to the fluid.

• Q7. When you extend your hand out of a moving vehicle window, you feel:
• A. Normal force
• B. Tension
• C. Fluid friction (drag)
• D. Gravity

• Correct Answer: C
• Explanation: The text uses this example to demonstrate fluid friction and the drag force.

• Q8. Skydivers can control their speed by:
• A. Changing their mass
• B. Changing their body orientation
• C. Changing the color of their clothes
• D. Changing the temperature

• Correct Answer: B
• Explanation: The text states that skydivers change their effective size and orientation to manipulate drag.

• Q9. At terminal velocity, the drag force:
• A. Is zero
• B. Equals the weight of the object
• C. Is greater than the weight
• D. Is less than the weight

• Correct Answer: B
• Explanation: At terminal velocity, the drag force equals the weight of the object, and the net force is zero.

• Q10. The drag force is a result of:
• A. Gravity
• B. Fluid friction
• C. Normal force
• D. Tension

• Correct Answer: B
• Explanation: The text states that the drag force is the result of fluid friction.

Frequently Asked Questions (FAQs)

Q1. What is the difference between drag and air resistance? Answer: Air resistance is a type of drag—specifically, fluid friction caused by air. Drag is a more general term that includes friction from any fluid (liquid or gas).

Q2. How does the shape of an object affect drag? Answer: A streamlined shape (like a teardrop) reduces drag by allowing the fluid to flow smoothly around the object. A blunt shape increases drag by creating turbulence and a larger wake.

Q3. Why does a parachute slow down a falling person? Answer: A parachute greatly increases the cross-sectional area of the falling person, which increases the drag force. This increased drag reduces the terminal velocity to a safe speed for landing.

Q4. What is the difference between fluid friction and solid friction? Answer: Solid friction occurs between solid surfaces in contact and is approximately independent of speed. Fluid friction occurs when an object moves through a fluid and increases with speed (often proportional to the square of speed). Both oppose motion.

• Q5. How does the speed of an object affect drag?
• Answer: Drag force increases with speed. For low speeds, it is proportional to speed; for high speeds, it is proportional to the square of the speed.

Q6. Why do swimmers wear special suits? Answer: Swimmers wear special suits to reduce drag. The suits are designed to be smooth and streamlined, reducing the friction between the swimmer's body and the water.

Q7. What is the difference between terminal velocity and free fall? Answer: Free fall is the motion of an object under gravity alone (no air resistance). Terminal velocity is the constant speed reached when air resistance (drag) equals the weight of the object, and the net force is zero. In free fall, acceleration is constant (g); at terminal velocity, acceleration is zero.

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