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Using Everyday Examples in Engineering (E3)
Water Guns and Supersoakers™ - Pressure as Stored Energy (Energy Per Unit Volume)
Dr. David Benson
Kettering University
Content Objective:
• Provide reference for interpretation of pressure as stored energy (energy per unit
volume).
• Motivate transformation of energy perspective of Bernoulli’s equation
• Introduce concepts of “painstorming” and entrepreneurship
Purpose of Activity:
The goal of this activity is to motivate the conservation of energy interpretation of Bernoulli’s
equation.
With fluids (under the restrictions imposed on the use of Bernoulli’s equations) energy is divided
and distributed amongst three energy modes: bulk kinetic energy of the fluid, potential energy,
and stored energy (pressure).
This is an extension of the common reference activity in Physics where a roller-coaster is used to
describe the relationship/transformations between kinetic energy and potential energy.
Equipment:
Individual small displacement pump water pistols.
Plastic tote --- to store/carry “loaded” water pistols
Half gallon or gallon jug to “top off” water pistols
One Supersoaker™ squirt gun or similar item
Small whistle
This material is based upon work supported by the National Science Foundation (NSF) under Grant No. 083306. Any
opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not
necessarily reflect the views of NSF.
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Activity Description:
Some leakage will occur during transport – small half gallon jug of water will permit refilling.
Class meets outside and students are presented with a large plastic tote. Inside the tote are a
number of small, pre-filled, small displacement pump water pistols. A Supersoaker™ is either
hidden under plastic bags or in a separate container (for faculty member use only).
Students are shown (pass around) an “exploded” small displacement pump that was removed
from the heart of one of the small water pistols.
Students are instructed that participation is completely voluntary, but that anyone with a water
pistol is “fair game” – if they want people to stop squirting them, they put down their squirt gun.
Students are instructed to think about the action of the displacement pump and the effect it
produces: how much energy they put in with each trigger pull, the distance the stream travels, the
volume flow rate produced with each squirt, etc.
Students are allowed to “run wild” for a short period – transition is called (whistle) when it
appears that they have had their fun and the exercise has run its course.
Students are instructed to squirt their guns in the air and think about conservation of energy
principles from a Physics I perspective. Discussion is then geared towards where the concept of
work enters the scenario.
Students are then shown a Supersoaker™ – instructor repeats horizontal and vertical shooting of
water.
Instructor repeats with a range in the number of “pumps” used to pressurize the vessel.
Discussion continues addressing Dr. Lonnie Johnson’s development of the Supersoaker™ and
process of invention.
Reflection/Observations:
I repeated the phrase about people being “fair game” if they were holding a squirt gun
throughout the exercise. When I pulled out the Supersoaker™ a number of people instinctively
dropped their own squirt guns.
At the transition I introduced the Supersoaker and entrepreneurship elements of the discussion by
saying that “my childhood sucked”: the little displacement-pump guns do very little and a brief
discussion on why they “suck” illustrates that the pressure provided to drive the fluid comes
from the small volume change produced by the action of the trigger.
Cutting open the small squirt gun to get at the displacement pump was challenging – the plastic
fractures and is very sharp.
© 2011 David Benson. All rights reserved. Copies may be downloaded from www.EngageEngineering.org. This material may be
reproduced for educational purposes.
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