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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