Poking Your Finger into a Cup of Water on a Scale: Simple Adhesion Force Experiments Available to Purchase

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 (Fig. 1). 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_B in Fig. 2). Since the water is not accelerating, the normal force from the beaker acting upward on the water (F_N2) must increase to equal the sum of the weight of the water (F_W) and F_B (Newton’s second law). By Newton’s third law, the water must exert a force equal to F_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_N2. Via these chains of Newton’s second and third laws, we can see that the reading on the scale will increase.