KNEX Education Simple Machines Deluxe GEARS, Руководство для учителей

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1
a.
Push your K’NEX Lawnmower across your
desk and watch how the wheels and blades
turn.Which moves faster?
b.
Count the number of teeth on the large
and small Gears that make the wheels turn.
(Be sure to count the inside ring of teeth on
the green Gear, not the outside ring!) Then
figure out the gear ratio by dividing the large
number by the small number.This ratio tells
you how much faster one gear turns
compared to the other when a large gear
drives a small gear.
2. What kind of gears does the Lawnmower use:
spur, crown, rack and pinion, worm or
sprocket gears? How does the arrangement
of Gears in the Lawnmower differ from other
gear systems? Can you think of a different
arrangement which would work better? If
you can, build it. If not, use reference materi-
als to learn more about lawnmowers and ex-
plain why this is the best set up for these
gears.
3. Make some “grass” for your Lawnmower to
mow.You can do this by cutting strips of
tissue paper about 50 x 100 cm.Then, make
short cuts into one of the long sides of each
strip. Fold and tape the strips to a piece of
paper so that the grass stands up.Then, mow
the lawn!Your Lawnmower may not cut, but
watch the way the blades of grass get swept
past the cutting blade. Can you see the scis-
sor action? Write a paragraph describing this.
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a.
The Lawnmower blades turn faster than
the wheels.
b.
Make sure to count the inner circle of
teeth on the large green Gear; it has 62 teeth.
The small Gear has 14 teeth.The gear ratio is
about 4.4:1.You apply the effort to push the
Lawnmower but the green Gear turns the
blue Gear fast so that the blades can turn
quickly to cut the grass. Note that the large
yellow Gear just serves as the Lawnmower’s
other wheel; it doesn’t connect to any other
Gear.
2. The Lawnmower uses spur gears.The Gears
are arranged so that the small Gear runs on
the inner edge of the big Gear instead of the
outer edge. Help students see that even
though the arrangement is slightly different
from other spur gears they know, these gears
still lie in the same plane, which is a
characteristic of spur gears.
Students answers will vary but should include
logical reasons to back up their ideas.They
will likely determine that this is the most
effective gear arrangement. It is convenient to
line the gears up in the same plane and easier
to perform the work in this plane also.You
are using the spur gears to apply your force
over a greater distance with more speed.
3. The best way to see how the Lawnmower
works is by watching from the back of the
mower.The rotating blades sweep the grass
to make it stand up and then the grass is
caught between a rotating blade and the fixed
blade.The fixed blade actually does the
cutting. Since the rotating blade is angled, it
moves across the fixed blade in the same way
two scissor blades meet.
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