My freshmen are getting sick of the "a person stands on a scale in an elevator" problems. Fair enough, 'cause I've certainly asked this kind of question way too many times. Nevertheless, they keep getting it wrong. No, folks, just 'cause the elevator moves down doesn't mean that the scale force has to be less than the weight.
Part of the issue is that they don't yet believe in physics. "Of course the net force must be in the direction of motion," they think, "it stands to reason. And c'mon, no one would ever stand on a scale in an elevator, anyway." Oh, how they underestimate my pursuit of nerdliness.
Our new Labquest2 makes experimental verification of a scale reading inside an elevator almost trivial. I plugged in the force plate to channel one, I told the Labquest to collect data for 100 s (rather than the default 10 s), stood on the scale and pressed "collect." Then I pressed the elevator button to go up from the second to the third floor.
*Now there's a good quiz question... "The elevator moves up and speeds up. Is my weight greater than, less than, or equal to 940 N?" The difference between weight and a scale reading is tough to wrap one's head around, when the "one" in question is a first-year physics student.
The results -- autozoomed by the labquest -- are shown in the picture. My weight is 940 N or so.* Soon after I pushed the button, you can see the scale reading increased to 970 N or so, with a brief spike above 1000 N. That's when the elevator moved up and sped up. Students generally expect and acknowledge this result.
But then the scale reading dropped back to about 940 N! Even though the elevator was still going upwards! Wow, it boggles the mind. That's not what common sense tells us, but there it is, plain as day -- the world's slowest elevator traveled upward between floors for about 20 s. The scale reading for the vast majority of that time was still equal to my weight. The reading only jumped above my weight for the brief period during which the elevator sped up.
Then when I arrived at the top floor, the scale reading dropped briefly down to 900 N or less. When students bring me this result, I might ask, "So, the scale read less than your weight, which way were you moving?" And do you know, even having just ridden in the elevator from floor 2 to floor 3, even having themselves pushed the elevator button, many will say "Oh, I was moving downward, then."
The point isn't to make fun -- this is a tough concept. The point is to force students to confront their own misconceptions as many times and in as many ways as possible.
At the beginning of this unit, a few folks will try to argue with me, saying that common sense dictates that net force really should be in the direction of velocity. After they realize that arguing with a debate coach isn't such a smart idea, they stop arguing, but still I can see their faces. They give me the answer I want when they remember to, but they still believe in a primitive and incorrect relationship between force and motion. Only after I throw an experimental result like this in their face are they forced to believe.