Thursday, March 26, 2009

What planet are they on?

I know that they expect the kids to assume a linear relationship, but the numbers don't make intuitive sense. It's been a while since I did kinematics; Mark and I checked each other's work and agree.

Let d be the distance the ruler drops, v the velocity and t the time. Then when Maggie lets go of the ruler,

d(0) = 0
v(0) = 0
and d(t) = at2/2

The ruler drops 2 cm in 0.1 seconds, giving you d and t. This gives you the information you need to solve for a, the acceleration due to gravity.

You can come to only one conclusion.

Those kids aren't on earth.

I don't think it can be Mars either.

Aside:
In other news, Iris wasn't able to read her school-mandated 30 minutes last night. We dragged her off to West Hollywood (WeHo) to help celebrate Birgitte's rockstar's 7th birthday. We saw their new Musical, Baby It's You. With all that driving, celebrating and singing and dancing, there was no time for her to read.

She reads much more than 30 minutes on most days. She's read over six hours this week already. Mark wrote a note to the teacher explaining that.

She was still punished and forced to sit out recess in the "opportunity room" today. WTF?

Update:
I spoke to her teacher and she said that she asked Iris, "Were you unable to do the reading?" She meant that, if there was a valid reason, then all would be forgiven.

Iris misunderstood and reiterated that she didn't read the previous night. Iris didn't notice the note that Mark wrote, and did not show it to her teacher.

13 comments:

  1. I think a lot of parents would have lied and said that their child did do the reading. I think Iris is learning a lot from your refusal to do the same.

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  2. Poor Iris:) Hopefully her teacher didn't just inadvertently teach her the benefit of a white lie.

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  3. That teacher needs to get back into the real world. Iris sure got a good taste of that Wednesday night :) Rockstar girl was positively ecstatic that she got to share this experience with her and will most likely treasure it forever.

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  4. Poor Iris. But then perhaps there is a lesson in that about paying attention and showing the note.

    I think the teacher was a little too subtle in her question too. I know lots of adults who would not understand the meaning of that statement. Granted Iris is far more intelligent than many people, but she is still only a child. Perhaps the teacher should have rephrased the question.

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  5. We often don't do the 30 minutes each night either. I figure they're really asking whether there's been an average of 30 minutes per day over the past week. The answer is usually yes.

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  6. And the irony of that is Iris spent the timeout reading...Perhaps I should try and explain to the teacher that I couldn't tear Iris away from her books on the cruise in the San Juans to watch for whales and other sea life?

    I don't think Iris misunderstood. She can be very literal, and the teacher should have rephrased, as Mardel suggested.

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  7. Is anyone else as bothered as I am by the physics problem? If the ruler falls 2 cm in 0.1 seconds, that's an acceleration due to gravity of 400 cm s-2 instead of the 980 cm s-2 on Earth.

    It's too small for earth, and too large for Mars or the Moon. The kids could be on an orbiting space station, but the gravity is too high for a sustainable orbit. 400 cm s-2 is just plain spooky.

    And then there is the whole issue of teaching the kids that distance stuff falls is linear in time. This is the kids' first exposure to kinematics. First impressions count. Do we really want to give them such an erroneous first impression?

    Mark says that he doubts any of the other mommies helping with homework caught this error. What do you think?

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  8. Anonymous21:29

    This physicist is appalled.
    Eric

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  9. I would not have caught the error. And I do care, and I do think it's important. Teachers can't be expected to know enough physics to see a problem here either. But that's why teachers get their word problems from textbooks that are prepared by experts, or at least by people who have the leisure to check with experts. If Iris brings home another other-planet question, I would start to raise some questions with the teacher.

    On the other matter: (1) The teacher shouldn't have asked a question using the word "unable." Not only is it ambiguous, it is also fraught for children, who don't like to think of themselves as unable. Teacher should have said: you didn't do it? hey, why not? (2) I submit Ole Golly's advice to Harriet: sometimes it is necessary to lie. I don't think that's a bad lesson at all.

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  10. The kids could be on an orbiting space station

    An orbiting space station would be in freefall... The ruler would be floating weightless a la Major Tom.

    You could spin the space station to generate centrifugal "gravity" and get any acceleration you liked (in principle).

    But most likely, the kids are doing the ruler experiment underwater. If the ruler is about 1.4 times the density of water...

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  11. They could also be dropping the ruler in some viscous substance like oil. That could explain the linear distance with time... The ruler rapidly hits its terminal velocity in the oil.

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  12. Richard, that' precisely it. The gravity is too great for them to be on a space station in a stable orbit. The kids could be in a spacecraft in a really low spiral orbit.

    I hadn't thought about the viscous fluid. Hmm.

    Or they could be in a space station with artificial gravity.

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  13. I showed it to the HOMEWORK MONITER! She didn't show it to the teacher! Ask her!

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