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Re: [Phys-l] A physics lab with hypodermic needles



Hi Steve-
In a circa 1935 introductory physics text, used at MIT, by Slater and Frank, the authors comment in the introduction on the inability of students to set up equations to describe physical situations. I suspect that this is a weakness that can, and should, be addressed by starting with simpler physical configurations than are provided by a hypodermic syringe. My observation is that young students are not aware of the need to partition a configuration into component parts.
I think that your experiences teaches us that, to a young mind, a hypodermic needle is an awfully complicated object.
Regards,
Jack



On Fri, 30 May 2008, Steve Highland wrote:

Several years ago I invented a physics lab where my students figured out the
size of the hole inside a hypodermic needle. It occurred to me that you can
do it by shooting a stream of water straight up into the air while doing
your best to keep the maximum height of the stream constant as you empty a
certain volume of water from the syringe and time how long it takes.

I tried this only once with a class but it seemed to work out. We got a
hole diameter that was reasonably close to a value we found in a table of
needle sizes.

What I like about this lab is the way it blends lots of topics that come up
in a general physics course ‹ kinematics (needed in order to find the speed
the water exits the needle from height of the water stream), conservation
(the water Œdisappearing¹ from inside the syringe is flying out the end of
the needle), and the need to visualize flow through a cross sectional area
over time relating to a volume.

I recall my students had a very tough time working through the relationships
in order to come up with an expression for the size of the hole in the
needle. I helped them a lot the one time I did this -- probably too much, in
fact. I¹d give them a lot more time to do advance work to set up the
relationships if I do it again. The measuring was fun. They liked shooting
water all over the place. (The amount of water is quite small ‹ it¹s not
like using squirt guns.)

When I inquired about using syringes and needles in lab our safety person
sort of freaked out. But I eventually convinced her to let us do it. There
were no accidents. And I discovered the price of needles at Walgreens was
astonishingly cheap ‹ I guess something intended for only one use can¹t be
too expensive. These needles are even cheaper than PVC pipe!

Has anyone else tried something like this? Do you think this is a good lab
or not? Is Œneedle safety¹ too big a worry for you? Is there a good way to
lessen the hazard of getting stabbed?

Steve Highland
Duluth, MN
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--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley