Ever wondered about the velocity of a falling rain drop, or how fast the bubbles are rising in your soft drink (or beer)? After this week, you will be able to calculate it yourself by means of using a momentum (force) balance.

As you’ve learned in high school, terminal velocity is tied to the concept of equal, opposing forces. From your own experience, you might be able to identify a few of the more prominent forces. For example, when swimming, you will naturally be able to stay afloat due to the buoyancy force. Or, when it is windy outside, you might have trouble keeping your umbrella in place due to the drag force. Determination of these forces, especially the drag, is not always straightforward: the influence of the wind on the umbrella is dependent on the relative direction with respect to the wind and on whether it is folded or not.

In the first section of this module, we are going to examine the drag force more closely. Which factors determine the intensity of the drag force and why?