Child Proof Stereo
4/26/09
EF151 sec.B1
Team B5:
Nathan Muller
John Riser
Jack Whitehead
Jonathan Wood
Introduction:
The objective of this project was to solve an open-ended problem using techniques and
concepts learned in EF151. In this case we were asked to build a Rube Goldberg Device. A
Rube Goldberg machine is an overly complex device designed to perform a simple task in a very
round-about way. For our purposes, the machine had to operate an electronic device. The
machine was required to be as inefficient as possible, going through a minimum of 5 steps. Our
device also had to include at least 4 of the following concepts: Projectile Motion, Conservation
of Energy, Conservation of Linear Momentum, Conservation of Angular Momentum, Torque,
and Center of Mass. Once set into motion, there could be no interaction with the device. There
was also a maximum $20 spending limit on materials.
Design Process:
On our first meeting we started out by collecting useful items lying around Estabrook. One
of the first interesting items we found was a PVC valve that opened and closed by rotating a
knob, along with the standard pieces of plywood and tubing. With our initial items in hand we
drew up a very rough blueprint, knowing we wanted to somehow incorporate a mousetrap
catapult and water into our device. We also made a list of other items we needed. We delegated
the list to a group member, and agreed to meet later.
The next day we looked at the items we had collected and drew up a slightly more complete
blueprint. A box was decided on to catch our mousetrap catapult, so it would be hard for us to
miss. We also decided our end goal would be to pull a pulley down that then pulls one end of a
see-saw up so that the other end can hit a button that turns a stereo on. We using water and our
PVC valve might be a good way to add weight on one end of our pulley.
The first thing we then built was the stand that holds our box catcher. We tested our
mousetrap shooting the tennis ball into the box, and determined where it needed to be to hit the
box every time. We then mounted the PVC valve on a piece of wood and attached it to the
bucket we purchased with a right angle connecter found in Estabrook. We decided that we
wanted to use a spear to pierce a water-balloon, to release the water into our bucket.
To do this, we cut a hole in the corner of the box so the ball would drop onto another rat trap.
When triggered the mousetrap would launch the spear, popping the balloon. We decided another
rat trap would be the easiest way to move the bucket, thus opening the valve.
On the third build day we had to figure out a way to trigger the third rat trap. To do this
we mounted a PVC pipe with a funnel attacked to the end onto our wooden box frame, which
would be filled with marbles. A small hole was drilled in the box to make room for a trip-wire
that could be attached to the rat trap that launched the spear. When the trap was triggered it
would then pop the balloon through a hole in the bucket and release the marbles at the same
time.
On the final meet we built everything else that we would need. We mounted a pulley, and
attacked a jug to the PVC valve with some tubing. Then we built a see-saw on a platform to
activate our stereo. The see-saw was attacked by a rope that ran through the pulley to the jug.
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All that was left was to add some counter-weight to the see-saw and fix all the parts down.
After running some tests we had to make some small changes to our design. The spear ended up
being too inconsistent and dangerous to make the final build. Sometimes it would fire correctly
with enough force to go through the bucket; other times it would not fire at all or partial fire and
hinder the motion of PVC valve. Eventually we realized we had to remove the spear and seal the
bucket.
Device:
Our device starts by triggering a mouse trap that has a ball resting on its arm. When the
trap is released the ball is launched into a cardboard box. The box is angled and a corner has
been removed so that the ball is funneled onto the trigger of a rat trap. When the rat trap fires it
pulls a trip wire that’s attached to its arm. This action pulls the wire out of a pipe, and releases
marbles down the pipe. The marbles roll down the pipe and into a small funnel. The funnel
drops the marbles onto the trigger of a final rat trap. (Multiple marbles were used to increase the
reliability of this step.) The last rat trap has a chain attached the arm that is fixed to the right
angle PVC connector sticking out of the valve. When the trap is sprung it rotates the valve,
releasing the water in the bucket. The water flows through some tubing into a jug. The jug is
attached through a rope and pulley to a see-saw that has been counter-weighted to match the
empty jug. As the jug fills with water, its total weight overcomes the counter-weights. This
pulls on the rope and tilts the end of the see-saw down. A piece of dowel on the end of the seesaw activates a stereo and produces music. Some pictures of the machine:
The mouse trap catapult
PVC valve attached to rat trap
See-saw that activates stereo
Jug and pulley
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Bill of Materials:
The following items were purchased for this project (All others were previously owned or found
in Estabrook):
ITEM
Mouse trap
Rat trap
Jug
Ball
Cardboard Box
QUANTITY
X1
X2
X1
X1
X1
PRICE
$1.50
$4.00
$1.25
$.75
$2.00
Total: $9.50
Analysis:
Most of the energy stored in our device is found in the springs of the traps. This energy
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can be expresses by the equation 𝐸 = 2 𝑘∆𝑥 2 . This energy is released when the traps are
triggered and used to do work on the various parts or our machine. Potential energy of the
mouse trap is used to launch a ball. The path of the ball can be modeled by projectile motion
𝑔
equation(𝑦 − 𝑦0) = (𝑡𝑎𝑛∅)(𝑥 − 𝑥0) − (2𝑣02 ) (1 + tan2 ∅)(𝑥 − 𝑥0)2 . The potential energy of
one of the rat traps is converted into rotational motion when the chain opens the valve. Then the
energy of the spring is released the chain applies torque to the PVC valve. This torque can be
expresses as 𝜏 = 𝐹𝑟𝑠𝑖𝑛∅ 𝑜𝑟 𝜏 = 𝐹x𝑟 𝑜𝑟 𝜏 = 𝐼𝛼. There is also potential energy is stored as
gravitational potential energy in the water. This is represented as 𝑃𝐸 =
𝑚𝑔ℎ (𝑜𝑟 𝑠𝑖𝑛𝑐𝑒 𝑡ℎ𝑖𝑠 𝑖𝑠 𝑎 𝑙𝑖𝑞𝑢𝑖𝑑)𝑃𝐸 = 𝜌𝑉ℎ. This energy is converted to kinetic energy when
the valve opens and the water falls through the tubing. This energy is redirected by the pulley to
the see-saw which activates our stereo.
Results
After completing our basic design, we began rigorous testing. Through our testing we
determined the placement of the box to catch our catapult. The testing also led us to remove the
spear for reliability and safety issues mentioned above. Much testing also went into the
placement of the funnel, so the marbles would fall on the intended spot. We also realized that a
support track would be needed to keep the water in the bucket from bending the PVC valve.
Because of our thorough testing we were able to get our Rube Goldberg Device to function
properly on the first try.
Conclusions:
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After much time spent in reflecting on engineering concepts learned in EF151, and even
more time planning , designing and testing we were able to construct a device that met all of the
objectives of this project. Our machine was able to operate an electronic device after a series of
overly complicated steps involving the physics concepts listed above. Overall our device was a
complete success.
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