Pedi Hashemian
Matt Thornbury
Joel Strange
Tracy Gladden
27 April 2008
Section A2
The Gramp Stamp
The objective of our Rube-Goldberg machine was to effectively use EF151 principles
stamp a sheet of paper. We went about doing this by first launching a ball out of a homemade
projectile motion device into a funnel. The ball would fall from the funnel onto a cardboard path
and roll onto our hot wheels track. From the track our marble would careen into a toy car and set
it in motion. The car, which had a tack taped to the front of it, would then collide with a balloon,
thus popping it. A rubber mallet, hinged on our base and leaning on the balloon, falls and stamps
the paper after the balloon is popped.
Design Description and Process
Our group decided to begin the project from the bottom up, literally. We obtained our 0.3
m by 0.5 m base from the scrap pile in Estabrook 13. We then started to decide what each step
would be, and which equation would be applied to them (Projectile Motion, Conservation of
Energy, Conservation Momentum). We came up with four ‘main’ steps: a spring launcher, a
marble path, and a hot wheels car-marble collision, and ending with a mallet stamping the paper.
The first step, which presented the most controversy, was the marble launcher. We
initially wanted to use a metal spring, but that a rubber band launcher would be sufficient and
easier to build. We cut a 1-inch diameter PVC pipe to 4 inches. We then placed a thin wooden
rod halfway inside the pipe with duct tape and a small piece of cardboard to act as a flat
launching surface for the marble. We duct-taped the rubber band to the outside of the pipe to
complete the launcher. After that, we used a small metal clamp to mount the pipe to a 4x4 in.
square piece of wood. This made it easier to set our launch angle. We mounted the launcher to a
6x10in. board 9in. vertically from the base. We mounted our launched tower to the base with L
brackets and wood screws. Our marble was metal and had a diameter of .5 inches.
The second step was deciding what we were going to shoot the marble into. We thought
of shooting it into a vacuum hose path, but we quickly discovered that the marble just bounced
around the pipe. We then decided to shoot the marble into a plastic funnel. We used a cardboard
tube column to hold the funnel up. A u-shaped cut was put into the tube, and then we duct-taped
the funnel on top of the tube. The u-shaped cut was to allow the marble to roll out of the tube
horizontally onto another cardboard chute. We used a small square wooden rod to hold up the
horizontal tube at the end. The final height of the tube horizontal tube was ½ an inch shorter than
the starting height of it allowing the marble to roll down the path.
The third step was the marble rolling onto the hot wheels track and colliding with a toy car.
This required us mounting the hot-wheels track with small wooden columns. The track was
mounted to create an adjustable curve for the car to roll down and hit the balloon. A small piece
of paper separated the car from the marble and prevented the marble from going down the track.
The end result was the marble colliding with the car with just enough momentum to cause the car
to roll down the hot wheels track. The big problem we had here was lodging the car in a gap
between the two tracks deep enough to withstand the initial shake in our device from launching
the marble from the launcher, while still giving the marble just enough force to knock the car out
of the gap.
The fourth and final step was the mallet stamping the paper. We considered using a
mousetrap to do the job, but our better judgment told us to use a mallet instead. The next part in
this process was deciding how to create enough force coming from the car to cause the mallet to
fall onto the paper. Then, an idea was proposed that we attach a needle to the car. This would
pop the balloon that our mallet was propped up on. We then used a hinge and roofing bracket to
mount the mallet to our base. This step became consistent once we were able to find a sharp
enough needle and inflate the balloon as much as possible within the size constraints.
Analysis and Calculations
First, the projectile motion section was fairly straightforward to calculate because there
was a definite angle and a final position that was easy to measure and duplicate. However, once
the marble entered the funnel, we had to make some assumption and approximations for the sake
of simplicity. First, we assumed that the marble was stopped in the y-direction once it hit the
back of the cotton. From there we did a simple MGH = 1/2MV^2 down to the first track, where it
essentially lost most of its kinetic energy and rolled down the track from rest again. Once the
marble changed onto the second track, it started from rest again and we did another simple
MGH=1/2MV^2 calculation. However, once the marble collided with the car, we assumed that it
was an inelastic collision because the marble and the car moved together until the car was free
and rolling down the track. The paper acted as a bumper that removed most of the elasticity.
From there, we did another simple 1/2MV^2 + MGH = 1/2MV^2 calculation for the final
velocity before impact with the balloon.
The Calculations
Overall, this project was difficult because we built it around the heavy mallet. On project
testing day, we saw that many other teams who had constructed a Rube Goldberg device that
actually completed all the necessary steps without the need to set off a large reaction with a small
object. The other teams’ stamps were extremely small and lightweight, and able to be moved by
a simple object such as a marble. In our case, however, the simplest way to reliably move the
heavy mallet was to lean it on a balloon and pop it. This idea came after we concluded that the
mallet was too heavy to be knocked over by a 35 gram matchbox car reliably. Our project
worked very consistently as long as the person launching the marble was consistent. As a team
we ended up working very well together. Everyone pulled their own weight, despite our initial