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Drag
In most real world situations, there are some kind of resistive forces. Some of these are due to contact between solid objects (e.g., friction) and you will learn about those later. For now, we will consider resistive forces due to some kind of fluid, which might be air, water, oil, or even sand.
Models of fluid resistance
Laminar drag
→Fdrag=−b→v
→Fdrag=−6πη→v
Turbulent drag
→Fdrag=−cv2ˆv
→Fdrag=−12ρCdAv2ˆv