SLIPPER ABACUS – ENGLISH – 20MB
you can make a lot of teaching aids using
an old flip flop. you need a rubber slipper, a punch, a hammer, three pencils and a few
beads. hammer the punch so as to make three holes in the flip flop or the rubber slipper.
they will be all in a line. take three pencils and stand them in these holes. this will look
more like a pencil stand. this is like an abacus. take one bead and put it in the units
place. this is number two. the right hand pencil is units and when you put nine beads,
you cant accommodate any more. so these represent 9 now for the tenth one you will have to remove
all these beads. now this one could represent 19. to see more clearly you can see now it
represents 6, there are only 6 pieces to the right and now in middle there are 5 pieces
which represent 50, this is 400, 4 beads in the hundreds pencil. this is 214 with just
one bead in the tens place. this is 403, there are no beads in the tens pencil. now just
bend the pencils, all the three pencils to meet at a point. this is like a concave mirror.
if you just bend it the other way round, all the three pencils will diverge and you will
see a convex mirror. this is very nice way to illustrate ray diagrams. if its a plane
mirror, if a beam strikes, it is just reflected back. but if it is the concave surface, the
just meet at a focus. if it is a convex surface, they all reflect, they diverge. now with this
simple rubber slipper you can show practically how rays behave when the surface of the mirror
is curved convex or concave.