Bennett Science Blog

Photo Credit: Public Domain By Andrew Bennett Fluids in motion show some interesting properties. In fact, some behaviors are so far impossible to predict exactly. In a first-year high school physics class, we tend to limit our discussion of moving fluids to the simpler cases, involving incompressible, nonviscous fluids with consistent flow rates. Water moving through pipes is a great example of this, so we end up doing a lot of problems about water in pipes of different shapes and sizes. Continuity Equation and Volume Flow Rate With these assumptions, we can conclude that in some amount of time, the same amount of water must pass one point in the pipe as passes another nearby point in the pipe. If anything else were true, we would have water appearing or disappearing between the two points. This idea can be expressed by the continuity equation, which reads: The product of the cross-sectional area and the fluid speed is equal to the "volume flow rate" of the wate...

Photo Credit: Public Domain By Andrew Bennett What Is Absolute Pressure? Absolute pressure is the force applied per unit of area ( see my previous post on pressure ). What Is Gauge Pressure? The pressure in a fluid is caused by the particles in the fluid colliding. As long as there is fluid present, there is pressure. So, why does my gauge read zero pressure when I have a flat tire?  Is there a perfect vacuum inside my tire? We often measure what is known as "gauge pressure" (rather than absolute pressure), which simply subtracts atmospheric pressure from absolute pressure. Frequently, what we care about is actually the difference in pressure between what is in a container and what is outside the container (often atmospheric pressure), so gauge pressure is a convenient measurement. When the gauge reads zero, it simply means that the pressure inside the tire is equal to the pressure outside the tire, which is very close to "atmospheric pressure." Gauge ...

Illustration Created Using Public Domain Photos By Andrew Bennett Forces and Pressure in Physics Problem Which would be worse: Having your foot stepped on by an elephant or by a woman wearing stiletto heels? Although the force from the elephant will certainly be larger, it might also be worth considering the amount of pressure on your foot. By this measure, you might be better off with the elephant! **Important warning: Do not tell any woman you know that you'd rather have an elephant step on your foot than her. No amount of physics will get you out of a mess like that!** Forces and Pressure in Fluids When we move from solid objects to fluids (liquids and gases), all of the same rules of physics still apply. We sometimes have some work to do to convert those rules to a form that we can make calculations and predictions with. What Is Pressure? When dealing with solid objects, it was easy to identify individual forces that act at single locations.  With fluids, howeve...