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The task at hand was to create a marshmallow launcher using popsicle sticks, plastic spoons, and rubber bands as the main components. Group Einsteins devised three very different launchers, and picked the best one based primarily on range, but also took into consideration the materials and aesthetics of the mechanism.
"Energy of a Slingshot: David and Goliath ." Wolfram.com. 15 Jun. 2010. .
"Ideal Launch Angle in Projectile Motion." The Student Doctor. Sep. 2008. 16 Jun. 2010. .
"Marshmallow Launcher." DIY Life. 9 May. 2007. 17 Jun. 2010. .
"Newton's Three Laws." Newton's Three Laws. 18 Jun. 2010. .
"Physics of the Trebuchet." Think Quest. 16 Jun. 2010. .
One of the realizations Group Einsteins came to was that rigidity of the device was essential to combat the force of the pre-launch, whether it be stretching a rubber band in a slingshot or pulling back a lever on a catapult, there is an equal force exerted from the force of the pull back. This exemplifies Newton’s third law. Rigidity is vital to absorbing and reflecting force. In this project all three devices were successful for different reasons. Number 2 was able to claim the greatest distance while numbers 1 and 3 were the most consistant. If launcher #2 was built using more rigid materials it would likely be far more consistant. We can only guess but it is likely that #1 and #3 were built in a larger scale they would not only be consistant but the distance marshmallows could be shot would increase greatly.
During the launch portion of the motion Newton’s first and third laws are most apparent. His first law states that an object at rest will stay at rest until a (net) force is applied. The marshmallow remains at rest in the launcher’s bucket since the only force acting on it is the pull of earth and the launcher itself. Neither of these create a force strong enough to put the mallow in motion until the launching mechanism is triggered. At that time Newton’s third law comes into play. The force generated by the motion of the launcher is large enough to give the mallow motion but even so the marshmallow pushes just as hard against the launcher as it is leaving the bucket. For every action there is an equal and opposite reaction, even if the force and motion of one object causes the movement of the other. Both forces are instantaneous and perfectly equal.
During flight, the marshmallow follows an arc shaped path. This movement involves all three of Newton’s laws. In regards to the first law, there is no force acting on the marshmallow in the X direction during flight so there is no change in acceleration. There is however a change in the Y direction because of the force of the earth pulling down on the marshmallow. As the mallow travels away from the earth in the first half of the flight there is negative acceleration since it slows down from the force of the earth overcoming the force of the launcher and the mallow’s mass (2nd law) pulling it in the earth’s direction. In the second half of the flight after it has reached the top of its arc and is traveling back to the earth it has positive acceleration. This is due to the increased (net) force in the direction of the earth due to the combined force of the earth and the mallow’s mass.
During the landing phase the marshmallow goes from a state of increasing acceleration in a downward direction to a very sudden stop when it hits the ground. If in theory the marshmallow hit the ground once and stopped, it would have zero speed, velocity and acceleration. The only forces acting on it would be the force of the earth pulling down on it and the equal and opposite force of the mallow pushing down on the earth (3rd law). Since the earth’s mass is significantly larger, its’ force will continue to overcome the marshmallow’s force pushing back against it until a third force overcomes them both (2nd law).
Launcher #1: The Catapult
Mass - 14.2 g
Materials - popsicle sticks, painter's tape, rubber bands
Research and Development:
My first instinct was to consult Google. After coming across the stormthecastle website, I decided to build a catapult because it seemed like an efficient launcher -- simple enough to be accessible to almost anyone, and minimal material requirements so it would be light, but powerful enough to hurl a marshmallow pretty far. I did some further research about catapults and discovered that I would need to choose an angle for the lever to sit on the fulcrum. Obviously, if the lever was too vertical, the marshmallow would be hurled more towards the ground than in the air, but if it was too horizontal you'd risk not gaining any distance. I decided on an angle slightly more vertical than 45 degrees, to hopefully encourage more forward motion, while keeping the marshmallow still in the air for an appropriate amount of time so as to not fall to the ground before going anywhere.
In an effort to make the catapult lighter, I used birch wood sticks rather than regular popsicle sticks. Unfortunately this ended up being a hindrance because they flexed too much with the wind-up and release of the marshmallow. The use of tape as a connector was surprisingly successful though; despite the sticks themselves bending slightly, the tape held fast. It was, however, not the most aesthetically pleasing of choices.
Overall, the basic catapult proved to be a highly ineffective method of launching the marshmallow. The catapult did not allow for very much accuracy from side to side because it cannot be aimed. Because it functioned by bending the launch lever over a fulcrum to hurl the objects, it also failed to provide sufficient throwing force to send the marshmallow any significant distance.
Launcher #3: The Ballista
Mass - <10 g
Materials - balsa wood, glue, rubber bands, popsicle sticks
Research and Development:
The idea I had for the launcher was the result of my desire to make a different type of catapult that most people wouldn't make. Also, I was under the assumption that the more I was able to channel the velocity of the flying marshmallow the farther it would fly. I also thought that the launcher had to launch the marshmallow into a target. The idea that I eventually came to was to build a ballista, a medieval launcher that shot everything from rocks to flaming jars of oil using a bow-type device and a principle of physics called torsion in which the force generated from twisting a spring per se would cause a tremendous release of energy. The torsion principle didn't really apply to my eventual design because I wasn't able to make a torsion spring to affect the movement of the arms. As it turned out, the force from pulling back the rubber bands on the ballista's bow was enough to launch the mallow to a distance of eight times the length of my catapult. The set up of the launcher's angle of launch was vital for flinging the mallow such a far distance; the optimal degree of launcher for such primitive devices is 45 degrees, which launches the mallow in a low arc.
Launcher #2: The Slingshot (a.k.a. The Liberator)
Mass - 7 oz
Materials - wooden dowels, rubber bands, plastic cup
Research and Development:
While deciding what I wanted my launcher to look like I asked around to some people I work with and one had a very interesting idea for a design that she used in her kids’ art classes to launch sticky toys across the room. It was simple and easy to build. Unfortunately, after trying it numerous times I discovered that the marshmallows aren’t heavy enough to stabilize the launcher design and as a result it didn’t go very far and quickly broke. I then went to the internet and did some Google research. I found a design that claimed to be stable, sturdy and reliable. It was only a picture so I spent an hour or so guessing at how it was specifically constructed. Using wooden dowels and the toughest rubber bands I could find I built the Liberator. By adjusting the number of rubber bands on the cup I can control the amount of thrust it puts on the marshmallow. It has proven to be easy to use and because if the pyramidal design it is very stable and easy to aim when you use the structure as a sort of crosshair. If I did it again, I would use a heavier dowel to stop some of the flexing of the frame. Im pretty sure this flexing takes away from the distance it is capable of shooting. I would also make it collapsable since it was really difficult to get it into my tiny car to go to the park.
Ultimately I wanted a design that was elegantly simple and effective at the same time. I was able to make this one with less than $4.00 in materials and an hour. It is interesting to look at and not an eyesore in the least. I have been very pleased with the results.
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Check out this cool video on how to make your own marshmallow gun that shoots 80 MPH.