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B How To Create the Fastest Rolling Can Possible

  1. Mar 1, 2017 #1
    Hello, this is my first time on this site as a member, but I have found it very helpful all year. In my physics class we are trying to come up with a way to get a can to roll down an incline as fast as possible. We have to empty out the food from the can, then do whatever we can to it to make it roll faster. We are learning about moment of Inertia right now, but have already learned about potential, kinetic, and elastic energy, so I think that it might have something to do with those. Is there anything more advanced that could help that I haven't learned yet, or how would I use what I have learned to make a can roll faster? I have already thought about making the can as light as possible and have as small of a radius as possible, because it would have the smallest moment of Inertia, but there has to be something else that I can do in addition to that. Any suggestions would be helpful. Thank you very much, and I look forward to being a part of this site.
  2. jcsd
  3. Mar 1, 2017 #2
    I think the angle of the incline has to be the most important factor.
    If it is just off of vertical the can will move at close to free fall speed, while having just enough traction to actually roll.
  4. Mar 1, 2017 #3


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    @rootone is correct. If the incline is near vertical, air drag will be significant. If it is near horizontal, rolling friction will be significant.

    What range of incline angles and incline length do you have?
  5. Mar 1, 2017 #4


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    The incline is fixed but the construction and content of the can is not? Are you allowed to use a motor?

    Are you allowed to make the can big enough so that it is touching the floor at the bottom of the incline even before it is launched?

    Do you have Prince Albert in the can?
    Last edited: Mar 1, 2017
  6. Mar 1, 2017 #5
    You want to minimize the ratio of the moment of inertia to the mass. The closer the mass is to the rim, the more gravitational energy is used to spin the can instead of moving it forward.

    Idea 1. Concentrate the mass in the centre of the can. Try using a solid bar about 1/6 the diameter of the can running through the axis of the can. You can mount it with a couple of disks of cardboard.

    Idea 2. Reduce the spin of the can. Go buy a crazy carpet. Us that on the ramp. Attach a scrap of the crazy carpet to the bottom of the can. Try various lubricants. dish soap, both hand and dishwasher. Try various dilutions. Draino (bascially lye) basic solutions are often very slippery.

    Try dry lubes too: baby powder, graphite, micros, beach sand.

    This probably won't work if the slope is shallow.

    Idea 3:

    Build a gyroscope into the can, with it's axis along the can axis. Make the bearings HIGH friction ones, so that there is a lot of drag on the gyroscope. Spin up the gyroscope with the rotor going the same way that the can will roll.

    Place it immediately on the slope.

    The drag on the gyroscope bearings transfers rotational momentum to the can. In essence, you are using the flywheel as an energy storge device.

    Consider having the drag applied by a spring. You can hold the spring back until you release it. By adjusting the tension of the spring you can get more or less acceleration of the can.

    If you can get it working well enough that the can slips, put a few rubber bands around the can to give it more traction.
  7. Mar 1, 2017 #6


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    Does the can have to roll? Can you put the can on some ice?
  8. Mar 2, 2017 #7
    There is a pretty long track with not a very steep angle and we all have to use the same track. I can get the exact measurements if that would help. It is made of wood, and the can does have to roll. The only thing that we can alter is the can, not the track. It has to be a food can, so it won't be enormous. Also, we can't make it into a car or anything like that. We can add weight, take away weight, put liquid in it, put rods in it, or anything like that.
  9. Mar 2, 2017 #8
    Remember that the mass and radius are not determining factors when it comes to speed. Meaning cans of identical geometry but different masses will effectively have the same speed through the descent. The more hollow it is, the slower it will go due to having a higher rotational inertia, meaning it will spin up slower, so you should try to make it as solid as possible and have the greatest density around the centre of the can. If you can negate rolling, then more potential energy will be converted to translational energy, instead of rolling kinetic energy, effectively speeding up the can. This is why if you were to race a block of ice on an ideal frictionless ramp versus a cylinder allowed to roll, the block of ice always wins since all potential energy is converted to translational energy while the cylinder portions the same amount of potential energy between rotational and translational, slowing its translational speed. If it must roll, make the surface have high friction to ensure pure roll, so the can doesn't slip while rotating (it's the static friction that causes the torque creating the rotation). Hope this helped! :)
  10. Mar 2, 2017 #9
    Hmm. I think my flywheel in the can is your best shot. Your next best is to put a rod. My intuition is that the rod should be about 3 times the mass of the can, but a quick attempt at calculating it gives me math messier that my old fart brain wants to deal with tonight.
  11. Mar 3, 2017 #10


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    But can you put a car into the can?

    Or more generally: Is energy stored in the can (other than it's gravitational potential energy) allowed? If not then, as Sherwood wrote, make it heavy, but with a MOI as low as possible (concentrate mass in on the axis). But make sure it is well balanced with the centre of mass in the geometrical center.
    Last edited: Mar 7, 2017
  12. Mar 3, 2017 #11
    Another idea. Dead simple.

    If I stir my cup of coffee the water keeps turning for some seconds after. In reverse, if I rotate my cup a bubble on my coffee sits still for the first few rotations.

    Use can of juice. Drill a tiny hole in one end, and top it up with water, using a syringe. Block the hole with tape, or a small wad of the blue goo teachers use to put posters on wall.

    Now you have something that is fairly heavy, -- lots of M * g. But the water inside won't spin with the can -- or not much.

    You may be able to tweak this a bit with liquids other than water. You want to lower the viscosity of water so there is less drag. Your initial experiments with this could be done with jars with screw caps. Easier to change the liquid.

    Methanol has about half the viscosity of water. There are materials that are lower -- benzene, acetone, chloroform, but they are not things that I'd want to slop around trying to get them into a can.

    Soap solutions may be better.

    You can get a quick test of viscosity by dropping something through a tall graduated cylinder of the liquid. The speed of drop depends both on the weight of the object, (Mg is hte downward force) the volume of the object (rho V is the buoyant force on the object, where rho is the liquid density) and the drag of the object moving through the liquid. Water vs water with soap won't have a measurable change in density. I vaguely recall that methanol has a density of around .95

    This method will have slightly worse performance than the next one, but it's quick. Worst case, bring a can of apple juice.

    Another idea.

    My first idea was to put a rod in the middle. But if you do that, then the rod is going to rotate, and it will be fussy to get it exactly centered.
    Instead run a thread through the middle. The thread is anchored at each end of the can. Run the thread through bunch of large (1") nuts, or fishing sinkers. The thread doesn't need to be tight, but it has to keep the weights off the ground. Fishing line may work better than thread.

    The principle:

    One of two things happen: The thread twists up as the can rolls, but the weights just hang.
    Or the thread rotates, rotating the nuts at a slower speed proportional to the ratio of the nuts inner diameter and the thread.

    This might be converted into a rubber band motor. The idea here is similar to the flywheel/gyroscope , but where the flywheel used the kinetic energy of the wheel slowing to speed up the can, the rubberband accelerates a rotating something to spin the can faster.

    Mechanically it would work similarly: A rotor that is supported by the ends. The shaft is wrapped with a rubber band under tension.
  13. Mar 7, 2017 #12
    Thank you all so much. I will take all of this into consideration for sure. Again, thank you all for the help.
  14. Mar 7, 2017 #13


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    What are the rules? Are rocket engines prohibited?

    If you weight the can off-centered, then you can perhaps get a one-time boost in acceleration if you put the weighted end on top, but it might hurt you in the long run if the ramp is longer than a few circumferences.
  15. Mar 7, 2017 #14


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    Attach the weight to an arm on a central axle using a ratchet mechanism. Spring load with a rubber-band motor if desired.
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