A spherical particle of mass m is shot horizontally with initial speed v₀ into a viscous fluid. Use Stokes' law to find an expression for vₓ (t), the horizontal velocity as a function of time. Vertical motion due to gravity can be ignored.
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7. Friction, Inclines, Systems
Kinetic Friction
Problem 33a
Textbook Question
Above what speed does a 3.0-mm-diameter ball bearing in 20°C water experience quadratic drag?

1
Understand the problem: The question asks for the speed at which a 3.0-mm-diameter ball bearing in water at 20°C transitions from experiencing linear drag to quadratic drag. This transition occurs when the Reynolds number (Re) exceeds a critical value, typically around 1 for small spherical objects in a fluid.
Step 1: Recall the formula for the Reynolds number: , where ρ is the density of the fluid, v is the velocity of the object, d is the diameter of the object, and μ is the dynamic viscosity of the fluid.
Step 2: Look up the properties of water at 20°C. The density of water (ρ) is approximately 998 kg/m³, and the dynamic viscosity (μ) is approximately 1.002 × 10⁻³ Pa·s.
Step 3: Rearrange the Reynolds number formula to solve for the velocity (v): . Use the critical Reynolds number (Re = 1) for the transition to quadratic drag.
Step 4: Substitute the known values into the formula: Re = 1, μ = 1.002 × 10⁻³ Pa·s, ρ = 998 kg/m³, and d = 3.0 mm = 3.0 × 10⁻³ m. Simplify the expression to find the critical velocity.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Drag Force
Drag force is the resistance experienced by an object moving through a fluid, such as air or water. It depends on the object's speed, shape, and the properties of the fluid. In the case of small objects like a ball bearing, drag can be classified into two regimes: linear and quadratic, with the latter dominating at higher speeds.
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Reynolds Number
The Reynolds number is a dimensionless quantity that helps predict flow patterns in different fluid flow situations. It is calculated using the object's velocity, characteristic length (like diameter), fluid density, and viscosity. A low Reynolds number indicates laminar flow, while a high number suggests turbulent flow, which is crucial for determining the type of drag force acting on the ball bearing.
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Quadratic Drag
Quadratic drag occurs when the drag force on an object moving through a fluid is proportional to the square of its velocity. This type of drag becomes significant at higher speeds and is characterized by a nonlinear relationship between speed and drag force. Understanding when quadratic drag takes over is essential for analyzing the motion of small objects in fluids.
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