All week long, I’ve been perplexing my kids with this dialogue:
me: so what force is the spring scale measuring when you hang a weight from it?
student: the gravitational force of the earth on the mass (yep, they’re stopping themselves from calling it gravity)
me: really? So if I pull on this mass, the gravitational force of the earth on the mass is increasing?
student: no. That can’t be right.
me: so what force are you measuring?
student: well, I can stretch the spring scale just by pulling, so it must be measuring the contact force of the object touching the hook on the hook.
me: ok, that makes sense. But grocery stores use these things to charge you for your vegetables, and they charge you by weight, but this isn’t what they’re measuring. Are you getting ripped off?
Student: I don’t think so, but I’m not sure.
me: why don’t you draw some free body diagrams?
Pretty soon, they come up with this:
The students quickly realize that since both objects are at rest, the two forces acting on each object must be equal (but this isn’t enough to say the force of the mass on the spring scale must be equal to the gravitational force of the earth on the mass, since the forces in the FBDs could be different sizes). But suddenly realize that the entire grocery store suddenly hinges on the crazy idea that if the spring scale pulls on the mass, the mass must pull on the spring scale with a force of exactly the same size and opposite direction. Is this all just a conspiracy to overcharge us for bananas?
The only way to find out, of course is to test it out. So that’s what we’re going to do today. I’ve got spring scales, rubber bands, bungee cords, balloons, free springs, Vernier Wireless Dynamics Sensor Systems (very cool, but damn you windows for making bluetooth such a pain in the rear), and whatever else the kids can find to test whether the force of a cart crashing into a balloon is the same as the force of the balloon on the cart. It should be a fun day. And by the end, hopefully they’ll be willing to stop spouting the memorized nonsense from 8th grade about “every action having an equal and opposite reaction.” (one of my most hated phrases in physics).