These types of fluids are called non-Newtonian fluids. If you stir really fast, with more shearing force, the PB gets runnier, while if you stir slowly the PB remains stiff. Imagine you’re stirring a jar of peanut butter. For some, like peanut butter, a minimum shearing or cutting force may be needed to get it flowing, and it may vary nonlinearly with shearing strain. Other types of fluids do not move quite as smoothly and easily. For a Newtonian fluid, the resistance to fluid flow – that is, its viscosity – is constant at a given temperature. Scientifically, a Newtonian fluid is one in which the shear force varies in direct proportion with the stress it puts on the material, known as the shearing strain. These are called Newtonian fluids, named after Sir Isaac Newton. Some act very predictably and move smoothly, as air or water do. The same thing happens to water when your arm takes a swim stroke. For example, moving your arm causes the surrounding air to change shape – or deform, to use the physics term – and flow out of the way. Think of a shearing force as a cutting action through a substance that causes it to flow continuously. Any material that flows continuously when a shearing force is applied is a fluid. To define a liquid, we must first define a fluid. By the scientific definition, peanut butter is indeed a liquid. However, I’m a mechanical engineer who studies fluid flows, and the TSA action made sense to me. Just like Americans’ favorite legume-based sandwich ingredient, the story – and the outrage it inspired – began to spread. TSA, however, contends that peanut butter is a liquid – and a full-size jar of Jif is over the 3.4-ounce limit. Some skeptics of security may suspect hungry officers just wanted to make their own PB&Js. ( The Conversation) – Those Transportation Security Administration requirements are drilled into every frequent flyer’s head: You can carry on liquids that are only less than 3.4 ounces (100 milliliters) in volume each.īut when the TSA recently confiscated a jar of Jif under this rule, peanut butter lovers were up in arms.
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