:: Section 3
Newton's Resistance Law and Stokes's Law
I. Newton's Law
If you drop a ball, it falls due to the gravitational force. Meanwhile, the resistance of gas molecules (drag) tend to slow down the movement.
To describe the drag force resulting from gas resistance, FD, the following equation can be used:
where CD is the drag coefficient. This is the general form of Newton's resistance equation.
This Newton's resistance equation is valid in the turbulent flow regime (i.e. Rep > 1000), where inertial forces are much larger than viscous forces. The drag coefficient CD has a nearly constant value of 0.44.
To calculate the Rep values, you can use the web-calculator below: