Day 26: Weightlessness ≠ No Gravity

College-Prep Physics: One of HW questions students struggled with today was:

Most people say the astronauts floating around on the International Space Station are weightless. Why is “weightless” a misnomer in this case? What’s really going on?

This wasn’t the first time we discussed this. We talked about how the astronauts are really falling around the earth because of their large tangential velocity and then we related that to making the bowling ball move in a circle by hitting it with a mallet. Students even asked if that meant astronauts feel that falling sensation in their stomachs all the time. So what happened? Why the struggle just a few days later?

Many students fell victim to what I call the “mashup” theory of learning: At first they thought there was zero-gravity on the ISS, but then in class we discussed that there was gravity. So the mashup of the 2 concepts is that there’s just much less gravity at the ISS, so the astronauts can still float.  And all that falling around the earth stuff?  Forgotten.

So I tried to demonstrate that “weightlessness” ≠ “no gravity” by dropping a mass hanging from a spring scale. I asked them to predict what would happen to the reading on the scale. Answers were all over the place. We filmed the drop using the slo-motion setting on my phone’s camera (a Motorola Moto X). We we played the video, we saw that the scale read zero (weightless, according to the scale) but clearly there was still gravity (that’s what made it fall in the first place). Same thing with the astronauts, but they are going forward fast enough that they never hit the ground.

These two Newton’s Canon applets came in handy (I forgot to use them during our first discussion about orbits):

http://waowen.screaming.net/revision/force&motion/ncananim.htm

http://www.phy.ntnu.edu.tw/java/projectileOrbit/projectileOrbit.html

(NOTE: For this post, I uploaded the video to YouTube and used YouTube video editor to do color correction, image stabilization, and have the movie playback even slower than recorded.)

##BFPM