In most real world situations, there is some kind of resistive force. 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.

$$\vec{F}_{drag} = -b\vec{v}$$ $$\vec{F}_{drag} = -6\pi\eta\vec{v}$$

$$\vec{F}_{drag} = -cv^2\hat{v}$$ $$\vec{F}_{drag} = -\dfrac{1}{2} \rho C_d A v^2 \hat{v}$$