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bistan
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Homework Statement
As shown in the plot above, terminal velocity is shown to increase linearly with the number of coffee filters dropped in a turbulent (air) medium. Therefore, terminal velocity depends on mass. Give an explanation for this starting from Newton's laws.
Homework Equations
\vec{F} = m\vec{a}
\vec{F}_{ab}=-\vec{F}_{ba}
Drag force F_{D}=\frac{1}{2}\rho v^{2}C_{D}A where \rho is the mass density of the fluid, v is the velocity of the object relative to the fluid, A is the reference (cross-sectional in our case) area of the object, and C_{D} is the drag coefficient of the object in the fluid.
The Attempt at a Solution
By Newton's second law: \sum F=\sum ma=mg-F_{D}=mg-\frac{1}{2}\rho v_{T}^{2}C_{D}A
By definition of terminal velocity: \sum F=0 \Rightarrow 0=mg-\frac{1}{2}\rho v_{T}^{2}C_{D}A
This means that the sum of the drag force on the object and the weight of the object must be 0 for the object to have reached terminal velocity. Therefore an object with greater mass has a greater weight, and so the drag force on that object must be equally greater to bring the sum of the forces to 0.
Since the drag force is a function of the square of the terminal velocity on the object, the terminal velocity is greater for an object with greater drag force, and therefore is greater for an object with greater mass.
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