The Fortune Teller
Jody Dykes
Tyler Huber
Jessica Millsaps
Hanya Senno
The Fortune Teller is a Rube-Goldberg device that breaks a fortune cookie open in order to see the fortune inside. It is a simple device consisting of a chain of events that lead to a mallet being knocked down to break open the fortune cookie.
The design process began with the team meeting to decide what we wanted the device to do. Our initial idea was to make it press the snooze button on an alarm clock to turn it off. However, our final decision came after we were inspired by eating Chinese food. At the end of the meal we were given fortune cookies which brought the idea of making a Rube-Goldberg device that could break one open for you. From there, we designed our device with the following criteria: it must cost under $25, it must go through at least three energy conversions, and it must be within our ability to build. The latter presented the biggest reason for discarding ideas. We wanted to incorporate a spiral in the device, but decided to use other ideas to ensure we can build it and make it work. Another idea we changed was to start the device by pushing the ball with our hand. To make the device more elaborate, we decided to start it with a collision. Other ideas we discarded pertained to small details of the project, for example what materials to use, with our final decision being the majority of the project being made with cardboard.
Following the design process we began to build our device. We built it with three different levels, the highest being the starting point. At that starting point, there is a marble resting on a red plastic oval, which we obtained from the game Mousetrap. This oval has a circular hole at the end of it. To put the ball in motion, there is another ball hanging from a wooden bar so when the string is pulled back, the ball will collide with the marble. Once the ball is pushed to move, it will go through the hole and land on a ramp built directly underneath it. The ramp takes the marble to the second level, where it hits a row of wooden blocks set up like dominoes. The last block in the series has a weight resting on the top of it. The weight is a miniature boot that is attached to a yellow plastic support beam (also found from Mousetrap). The boot is connected to the beam to where it can swing back and forth. There are also two different objects attached to the boot to give it added mass. When the marble hits the first wooden block it will create the domino effect until the last block is knocked over, which in turn leads to the boot swinging forward because the support underneath (the last wooden block) is knocked over. At the bottom level of the device, there is a mallet balanced on two wooden blocks to give it enough height for the mallet’s head to be level to where the boot will strike. To ensure that the mallet will fall directly forward when it is hit by the boot, we connected it to two small blocks (one on either side) by drilling a hole through the three objects and inserting a nail in the hole. Then the two blocks are bolted to the wooden blocks that the mallet is resting on. Therefore, the nail acts like a bar for which the mallet to rotate forward around and the two blocks stop the mallet from going left or right. There is also a back support behind the mallet to stop it from falling backwards. In front of the mallet the fortune cookie is placed at a distance where, once hit, the mallet head will land directly on it. The following picture shows how the device looks after it was built and the measurements needed to do the calculations.
The Fortune Teller has several energy conversions throughout it. The first energy conversion is in the collision between the ball and the marble. From the conservation of energy equation we figured out the velocity of the ball when it hits the marble. This equation would be 1/2mvi 2 + mghi=1/2mvf2 + mghf where vi =0 ft/sec, hi=7 in (or .583 ft), hf=.5 in (or .042 ft), and m=6.2*10-5 slugs. We calculated the vf to be 3.67*10-4 ft/sec. To find the velocity that the marble will be going when it first starts moving, we used the conservation of momentum equation m1v1 + m2v2 = m1v1′ + m2v2′ where m1= mass of ball=6.2*10-5 slugs, v1= 3.67*10-4 ft/sec, m2=mass of marble=.0011 slugs, v2=0 ft/sec and v1′= 0 ft/sec. We calculated the velocity of the marble v2′ to be 2.035 *10-5 ft/sec. From these calculations we can find the coefficient of restitution (e) using the equation
e= -( v1′- v2′)/ v1- v2= 5.5*10-10.
The second energy conversion is when the marble goes down the ramp and hits the first wooden block. To calculate the velocity at which the marble hits the wooden block, we used the conservation of energy equation again (1/2mvi 2 + mghi=1/2mvf2 + mghf) where vi=2.035 *10-5 ft/sec, hi=18.25 in (1.52 ft), hf =14.5 in (or 1.21 ft), and m= .0011slugs. The velocity of the marble when it hits the blocks was calculated to be vf= .0049 ft/sec. To find the velocity of the first wooden block after it is hit, we used the conservation of momentum formula (m1v1 + m2v2 = m1v1′ + m2v2′), where m1=mass of marble=.0011slugs, v1=.0049 ft/sec, m2=mass of wooden block=7.45*10-3 slugs, v2=0 ft/sec, and v1′=0 ft/sec. We found the velocity of the wooden block after it is hit to be v2′=.0072 ft/sec. We assume that because the wooden blocks have the same mass that this velocity is constant as each block is being knocked down by the one before it. We can therefore also assume that this is the velocity for the boot when its supporting wooden block is knocked down.
This leads to the third energy conversion of the device. The boot hits the mallet at a velocity of .0072 ft/sec and to find the velocity of the mallet after it is hit by the boot, we can use the conservation of momentum equation where m1=mass of the boot=.003 slugs, v1=.0072 ft/sec, m2=mass of mallet=.039 slugs, v2=0 ft/sec, and v1′=0 ft/sec, giving us v2′= 5.64*10-4 ft/sec. We know that from there, the mallet will hit the fortune cookie, giving it a final velocity of vf=0 ft/sec.
The bill of materials for this project consists of wood, costing around $10, paint ($7), and the mallet ($2). All other materials were obtained from our previous possessions and the fortune cookies were given to us for free.
Overall, we were successful in creating a working Rube-Goldberg device. Problems we incurred during the project included finding a way to create a heavy enough mass on the boot for it to knock the mallet over and creating a strong foundation to hold the mallet. Another problem we came after building the device was that the distance between the mallet and boot was a little too close. We solved this by adding paper in between the mallet and its back support. We learned from this project how to fix our mistakes by using measurements and calculations rather than trial and error. We are pleased with our project and would not do anything differently. We had one reference, Paul Karakashian, who helped us with figuring out another way to put the marble in motion other than pushing it ourselves.