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Whelmer #28: T.P. Away
Description:
A leaf blower is used to rapidly unroll an entire roll of toilet tissue
demonstrating the Bernoulli Principle.
Science process skills:
- observation
- prediction
Complex reasoning strategies:
- induction
Standards:
K-4:
- The position and motion of objects can be changed by pushing or pulling.
The size of the change is related to the strength of the push or pull
(Standard B.2.3).
5-8:
- An object that is not being subjected to a force will continue to
move at a constant speed and in a straight line (Standard B.2.2).
- If more than one force acts on an object along a straight line, then
the forces will reinforce or cancel one another. Unbalanced forces will
cause changes in the speed or direction of an object's motion (Standard
B.2.3).
- Energy is a property of many substances and is associated with mechanical
motion. Energy is transferred in many ways (Standard B.3.1).
9-12:
- Objects change their motion only when a net force is applied (Standard
B.4.1).
- Gravitation is a universal force that each mass exerts on any other
mass (Standard B.4.2).
Above Standards from the National
Science Education Standards.
Content topics:
- air pressure
- Bernoulli Principle
You will need:
- sheet of plain paper
- electric leaf blower
- roll of toilet tissue
- dowel rod
- extension cord
- goggles
Instructions:
Hold the short edge of a sheet of paper near your lips. Allow the paper
to hang down. Blow over the top surface of the paper. The paper rises.
Select a dowel rod or pipe that fits through the tube at the center of the
tissue roll. The dowel should be approximately 2 feet long. Place the roll
of tissue on the dowel and unroll one to two feet of tissue. Allow unrolled
tissue to hang loosely off the roll.
Hold the dowel horizontally with the roll unrolling away from you. Turn
on the blower and aim the air stream just over the top surface of the roll.
The hanging tissue immediately rises and the roll begins to unroll at a
rapid pace. The entire roll of tissue unrolls and become airborne!
Presentation:
Ask students to predict what will happen before you blow your breath across
the top of the sheet of paper. Many will predict that nothing will happen,
or that the paper will flap in the breeze.
Ask the them to predict what will happen if you blow under the sheet of
paper. Many will predict that it will rise.
Ask students to explain why blowing under the sheet and blowing over the
sheet result in the same lifting effect. Blowing under the sheet sufficiently
increases the air pressure, causing the paper to lift. Blowing over the
top of the sheet lowers the air pressure allowing the ambient pressure under
the sheet to lift it.
Ask students to predict what will happen when you use the blower on the
toilet tissue.
Ask students to explain the phenomenon of the toilet tissue. The air pressure
is reduced on the top surface of the tissue allowing the ambient pressure
to lift the tissue into the air stream.
Content:
In 1738, a Swiss mathematician, Daniel Bernoulli, studied the relationship
between the pressure and velocity of a fluid. The Bernoulli Principle states
that the pressure of a liquid (in this case, air) falls as its velocity
increases. Increasing the velocity over the surface of the paper, lowers
the pressure of the air pushing on it.
The paper is lifted because there is a sufficient number of air molecules
striking the lower side of the ball to overcome the downward pull of gravity
and the lower air pressure on the upper side. Air is blown out of most leaf
blowers at ~150 miles per hour. At that velocity, the pressure on the upper
side of the tissue is reduced dramatically.
Assessment:
Type: individual.
Content/Process: Bernoulli's principle.
Age/Level: intermediate.
Use Whelmer #12: Bernoulli Cans as the assessment
for this activity. See the procedure at the end of #12 for directions. Students
should be assessed on their ability to describe the decrease in air pressure
with the increase in volume.
Notes:
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