They Didn't Ask Me (dr_phil_physics) wrote,
They Didn't Ask Me

Sometimes I Can't Help Myself

I teach Physics. I can't not teach Physics. I guess that means I'll never be a cool kid.

On Facebook:
Ugly confession time: I frequently confuse the concepts of "mass" and "weight."
To which came the reply -- On earth the two concepts are essentially identical.

I couldn't let that stand, despite common usage. I.E., erroneous common usage.
  • Philip Edward Kaldon "On earth the two concepts are essentially identical." No. Mass is a measure of how much stuff an object contains. Stuff IS a Physics technical term for undifferentiated matter. The mass of an object is the same anywhere. Weight is the force due to gravity of one mass on the other -- typically used for a small object near a large object. Gravity is an inverse square law -- it's strength falls off as 1/r², the square of the distance between their centers. Near the surface of the Earth, weight is relatively constant. Apparent weight is the force you feel pushing up on you -- you feel heavier momentarily when an elevator accelerates upward. Gravity and weight are not zero in orbit, the apparent weight is zero because everything is in freefall, except forward speed is fast enough that you don't fall to the ground, but fall around the curve of the Earth. They can get away with selling cans of peas which say Net Weight 1 lb. (454 grams), because while pounds is the English unit of weight and force, and grams and kg are metric units of mass -- all cans of peas are sold near the surface of the Earth. (Astronauts don't yet have to shop for food in space.) So the conventional conversion of 1 kg = 2.2 lbs or 1 lb = 0.454 kg, while a technical lie, are useful. (Whew!) (Gets off soapbox.) Dr. Phil
  • Philip Edward Kaldon Newton's second law: F = m a, force = mass x acceleration. On Earth, w = m g, weight = mass x acceleration due to gravity. A 1 kg mass has an Earth weight of 9.81 Newtons = 2.2 lbs. The English unit for mass is the slug -- my favorite unit. To find your mass in slugs, take your weight in pounds and divide by 32. (g = 9.81 m/sec² = 32 ft/sec²) I always tell my students I think Weight Watchers has missed a real genius advertising campaign -- Shed Those Excess Slugs Now! I would have cute little plush slugs inning juice green as mascots... Dr. Phil

Just because we end up using the two terms, mass and weight, interchangeably and even have an illegal conversion formula, doesn't mean they are the same thing.

And being the teacher, I had to share this to a wider audience.

And await the derisive comments on Facebook. (grin)

Dr. Phil

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