The PremiseSometime in the middle/end of our forces unit, my students walk into the room with electronic balances set out on every table. I start class by asking the simple question "What does an electronic balance (or bathroom scale) measure?" and have the students discuss in pairs or groups before bringing the discussion to the full class. Typically this results in a split between students that think the answer is "mass" and students that think the answer is "weight" so I ask them to use their balances to measure an object on their table (typically an eraser or ball of clay/playdough). Once they have recorded that result, I have them place the balance on a board and, while measuring the object as before, slowly tilt the surface in which the balance is sitting on. They will very quickly notice that the value displayed by the balance is going down! Obviously, the mass (and weight) of the object isn't changing when they simply angle the surface, so the electronic balance must actually be measuring something else. Enter the Normal Reaction Force :) What's Going On?If an object is sitting on a surface and it is not accelerating in the vertical direction than we know that vertical forces are canceling out.
Taking MeasurementsIt is possible to take measurements and show that the value changes as a cosine of the angle but it is important to use a few tricks for your procedure:
This post is part of a larger collection of my favorite science demonstrations CLICK HERE FOR MORE DEMOSClick for more Forces resources ⬇
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