Krampf’s science this week involved exploring Newtonian inertia via the simple experiment of putting a glass over a marble on a table top, pushing the glass in a circle which causes the marble to spin in a circle, and you should be able to pick up the glass and have the marble spinning within it as described below.
It has to do with inertia. If you roll the marble across the table, inertia
keeps it moving in the same direction, at the same speed, until something
else pushes or pulls on it. That push or pull could come from friction with the
table top, which would slow it down. It could also come from a bump or dip
in the surface of the table, or from the side of the drinking glass which could
change the direction of the marble. In the case of the drinking glass, it
could cause the marble to move in a circle around the side of the glass.As you move the glass in a circle, it is pushing the marble, to keep it
rolling. The marble’s inertia tries to keep the marble moving in a straight line,
so the marble pushes against the side of the glass. The combination causes
the marble to zip in a circle around the inside of the glass.But something else is happening too. Inertia is causing the marble to push
against the side of the glass. As the marble moves faster and faster, it is
pushing harder and harder against the side of the glass. When that push gets
strong enough, you have enough force to hold the marble against the glass,
even when you pick it up off of the table.
[clear]
Science experiment! So H.o.p. and I dump out the contents of one of his jars and unearthed an ok green and white marble. We got first a plastic glass that had perfectly straight sides, because one is supposed to use a glass with straight sides. The experiment’s list of ingredients called for simply a glass. I didn’t know whether a plastic glass might effect the experiment, glass glass being presumably slicker and offering less resistance, but thought we’d try the plastic glass. Yeah, I know, sounds stupid.
We spun and spun and spun and spun the marble in the glass. The marble spun and spun and spun. We picked up the glass and the marble flew across the room. We did this three or four times and each time we failed.
So we tried a glass glass that has almost perfectly straight sides but not quite.
We spun and spun and spun and spun the marble in the glass. The marble spun and spun and spun. We picked up the glass and the marble flew across the room. We did this three or four times and each time we failed. (H.o.p.’s dad later tried and failed each time.)
We failed but what happened at least does follow the laws of inertia. Things at rest tending to stay at rest and things in motion tending to stay in motion (unless acted upon by an outside force). To exhibit this I finally just took the marble and sat it down and said look, we can depend on that marble to stay there, just sit there. That’s inertia. It’s not going to get up and roll on its own initiative. It is inert. When you place a toy on a shelf in your room you are relying on its inertia to cause it to be in the exact same place when you return to your room to look for it. If there wasn’t inertia assisting in helping to keep objects where you put them then the world would be a pretty unpredictable and messy place.
I told him how people once believed the natural tendency of an object is to come to rest of its own accord. When instead an object in motion is going to keep on going unless it’s forced to come to a stop.
I asked H.o.p. to roll the marble on the floor toward our magic marker chalk-type board. He did and it came to a rest against the board. Which had greater inertia? Then he lightly tossed the marble on the floor and it bounced and went flying off but I thought it a bad idea to muss things up with bringing in factors of momentum and gravity and opposite reactions and what all.
I got a slick scarf and a plate and we put the plate on the scarf after I had H.o.p. hold them both and tell me which weighed more. I yanked the scarf out from underneath the plate. I explained that inertia is dependent on mass or weight and that it was therefore going to be harder to get the plate to move than the scarf, as long as the scarf was yanked quickly.
It wouldn’t have been a mess if I had failed because I used a plastic plate, knowing I am a bad scientist. Then we put heavy books on the plate and I tugged the scarf out from underneath it. Then I piled several heavy glass bowls on top of the plate, which had H.o.p. jumping up and down. And I held my breath and yanked the scarf. Bravo! No broken bowls.
By then H.o.p. was ready to have a go at the trick. We used the plastic plate. He made several tries and grew increasingly excited, me coaching him on keeping the scarf level and pointing out the best grip points for him. And finally he slickly pulled the scarf leaving the plate sitting on the table.
I lie. It wasn’t a table. We used instead a chair because the table is piled high with books and drawings. It was easier to use the chair.
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