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Newton's laws are a ubiquitous topic in introductory physics instruction. One common problem involves asking what will happen if you stick your finger into a cup of water sitting on a scale. A way to solve the problem would be to first recognize that the water exerts a buoyant force upward on the finger, which students can recognize as being the reason why they feel lighter when they are in a swimming pool. By Newton's third law, the finger must hence exert a downward-pointing force on the water (labeled F[subscript B]). Since the water is not accelerating, the normal force from the beaker acting upward on the water (FN2) must increase to equal the sum of the weight of the water (F[subscript W]) and F[subscript B] (Newton's second law). By Newton's third law, the water must exert a force equal to F[subscript N2] down on the beaker. However, since the beaker is not accelerating, Newton's second law tells us that the normal force from the scale is equal to this increased F[subscript N2]. Via these chains of Newton's second and third laws, we can see that the reading on the scale will increase.

In this article, we will present the HEC BB bag, a simple but elegant model originally invented to help students imagine how air molecules move around in vacuum. The HEC BB bag is made by packing 20 to 30 BBs in a resealable plastic bag of several tens of square centimeters (see Fig. 1). When inflated fully via a straw, the volume of the bag is about 5 L in volume (see below for specifications).

Students will sometimes ask why momentum and kinetic energy concepts are both necessary. When physics teachers demonstrate situations that require both an understanding of kinetic energy and momentum, a favorite is Newton's cradle, or a comparable demonstration of two balls of equal mass hitting each other. However, in addition to the case of two balls of equal mass, if a ball hits another ball of three times the mass with equal speed, the results are also interesting, and, like the equal-mass demonstration, both kinetic energy and momentum are critical for understanding the motion.

Recently, Greta Thunberg has strongly increased environmental awareness with the climate strikes and the resulting Fridays for Future movement. But what does environmentally friendly behavior look like? In today's society, it is a sort of common sense that one should recycle. From an early age, we are taught in school that recycling conserves resources and energy. However, there are also skeptics of this "common sense." In 2004, for example, Penn and Teller dedicated an episode of their TV show to recycling. According to them, "[Recycling] increases energy use in transport, sorting, storing, and cleaning .... it takes more energy to recycle a plastic bottle than to make a new one, and that's not so good .... we're feeling good for no reason." We hope that when people start to question the validity of recycling that they will turn to their physical science teachers for help, and we hope that these teachers will be able to give an informed and educated answer. In this article, our goal is to provide physical science teachers with the information they need to defend the position that recycling plastic bottles is worthwhile, and to expose the myth that recycling plastic bottles is just "feeling good for no reason." Although the primary intention of this paper is to supplement content knowledge of physical science teachers regarding sustainability, it will conclude with some instructional suggestions of how students can be guided through debunking on their own the myth that recycling plastic bottles causes more harm than good. [ABSTRACT FROM AUTHOR]