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Lesson�3:�The�Paddle-boat
Teacher’s Notes
Some children may suggest that the
larger paddles will work best because
they will move the fastest or simply
because they are the biggest.
As they will have discovered earlier,
however, in order to turn a large wheel,
very large turning forces will be needed
to turn the axle. Their elastic band
‘engine’ may not be able to deliver
enough turning forces to overcome
the resistance of the water against
which the paddles must push.
The larger paddle-wheel may move
very slowly at first and then move
faster as the initial resistance to the
movement (friction) is overcome.
Smaller paddles will move more eas-
ily, but not as quickly, or push as much
water, as the larger paddle-wheels.
Children often make a similar error
when designing and making powered
vehicles. They frequently use large
wheels and/or connect a large pulley
or wheel directly to a motor. They are
surprised that the vehicle will not move
when put on a surface despite the fact
the wheels turn when they hold it in
their hands.
This is because a small resistance
applied to the wheel, such as the
friction between the wheel and the
desk, (or in the case of their boat,
the resistance of the water), is enough
to cause the motor to stall. With the
frictional force removed, the motor
can generate enough force to turn
the wheel.
Key Fact
 • Toturnalargewheel,verylargeturningforcesmustbe
applied to the axle.
Plenary
Ask the children to discuss and explain their observations
and their recommendations for the fastest paddle design.
Education
®
21


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