Help for Explaining Weightlessness

[LaniParticipant]

I am struggling with conducting Lesson D-&: Gravity II, Part 2 : Weightlessness in FreeFall and in Space to our 9 y.o. I struggle with how to discuss this portion of the lesson in a way that my son can grasp and was hoping that I understand the concepts at play correctly. We have had discussions of contact forces and forces that work at a distance (ex. gravity and magnetic forces). So I think that if we start the discussion of the scale experiment with clearly explaining what a scale measures first, that the purpose of the experiment is clearer to all of us.

I am hoping my explanation is sound and correct and hope from some feedback..here it goes:

A scale is actually providing us a measure of how much force is needed to act against (upward force, a push) the force of gravity(downward force, a pull).

When an object is in free fall, there is basically just gravity acting on the object. No other (significant) contact forces. The amount of particles/the mass has not changed, gravity IS acting on a free falling object, but since there is no sensation of the upward force or push that is countering gravity, there is no SENSATION of weight. The sensation of weightlessness does not mean you dont weigh anything at all.

The scale registers zero because there is nothing in contact with the scale during free fall — the weight registers again once the scale and the book(s) press against the other (push and pull) — the book is no longer falling, it is in CONTACT with the scale.

I hope what I wrote is sound and makes sense? Did I explain this correctly? Thank you for everyone’s input!

[Bernard NebelKeymaster]

This sounds fine. Plus, you illustrate an important aspect of learning. When you can take a concept and explain it your own words, it shows you have really mastered it. Nice work. However, I would make an adjustment to your line, ” The sensation of weightlessness does not mean you dont weigh anything at all.” This is where we should switch from speaking of weight to speaking of mass. That is, a scale will read different weights depending on gravity–zero in a state of free fall. But you and anything else remains constant. We speak of that “weight” that remains constant as mass and measure it in kilograms. That is, a mass of 10 kilograms will remain 10 kilograms regardless of any force of gravity including zero force.

Thank you for your question. Please ask further.

[Bernard NebelKeymaster]

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