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Physics Car Project

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  1. Dec 9, 2014 #1
    Hello all! I am a high school student who is taking my first physics class, Honors Physics. I have taken mechanical engineering classes prior to this class.

    We have recently been assigned our major project of the year: The Dreaded (by some) Physics Car. Our goal is to design and build a vehicle that simply runs the longest amount of time. It does not matter in which direction the vehicle runs, how far, nor how fast it goes; it simply needs to keep moving. We may only use gravitational and/or elastic energy as our means of moving our vehicle, and all of the energy needs to come from within the vehicle (our system). Our size limit is 2x2x2 meters, and we will be penalized for using prefabricated materials (as in, prefabricated for our specific use (we cannot use wheels off of a toy truck, we must use something else, like cds. we also cannot use kinex/legos for support, etc...) (except for low-friction ball-bearings on our axles). One more thing: if we have any balloons on our vehicle, they can only be used as a means to eliminate friction, and may not be used to propel the vehicle itself, nor anything inside of it.

    Sorry for the huge background, I simply wanted to clearly state what my restrictions are. Yes, I understand that it would be unethical for someone else to design my vehicle for me, which is why I am only coming here for feedback on my current designs and, possibly, suggestions etc...

    Currently, my idea is to buy high-torsion springs, and attach a relatively long armature to one end, which would allow the force to be exerted over a longer period of time. There would be multiple such springs arranged in series, so that when one has returned to its initial state, it would trigger the next, etc. etc. The problems I am currently facing with this is my car's theoretical weight (I would need sturdy, and thus heavy, materials to mount the strong springs on), and also how I would attach each system to the rear axle to as to continue the vehicle's motion. I had toyed with the idea of one-way locking ball bearings, however I would need several, and, when I presented the thought to my teacher, he recommended I search online for another method, or even ask for help from other people.

    I apologize for the long post. I had a lot to say. Any help would be much appreciated!

    Thank you,

    J

    p.s.

    I am also open to the idea of using gears/gear ratios in conjunction with gravitational energy...yet, I would probably need to make my own gears...
     
  2. jcsd
  3. Dec 9, 2014 #2

    haruspex

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    High torsion springs sound good, but why do you think you need to trigger their release sequentially? Sounds complicated. What benefit does that give, and is it worth the effort?
    Your main problem may be making it robust enough to cope with the weight of the springs and the top speed. How rough is the terrain? What about keeping it on course? If it goes round in circles does that matter?
     
  4. Dec 9, 2014 #3
    The point of putting the springs in sequential order would be to extend the amount of time that a force is being applied to the vehicle's axle, thus causing my car to run longer. If I simply have them all release at the same moment, will not my car "run out of 'gas'" sooner? Top speed does not matter, as long as it's movement is noticeable to the naked eye. The competition will take place in my school's tiled, main concourse. It does not matter which direction the car goes, and, if it runs out of room, my teacher will simply turn it around, and a "special formula" will be applied to figure out how much time/energy was wasted by his intervention. Another idea I had, but initially didn't like, was to have a reciprocating system, as in the car would move in one motion, building up the stored energy in some elastic/gravitational way; when enough energy is built up, the car would simply move in the other direction, "charging up" the next system, etc. etc.

    J
     
  5. Dec 9, 2014 #4

    haruspex

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    But if it all gets converted to KE early, that KE will still carry it some way. E.g. if there were no losses and the track were up an incline, it would reach the same distance up the track for the same spring energy.
    I'm not saying there's no advantage in keeping the speed low, but you need to be clearer on why there is one.
    In fact, there is a disadvantage in making it very low. Without a bit of momentum, it might not make it over the inevitable little bumps.
     
  6. Dec 10, 2014 #5

    NascentOxygen

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    The problem with strong springs alone is that they want to release their energy quickly, so will spin the wheels furiously for 5 secs and then have nothing left.
     
    Last edited: Dec 10, 2014
  7. Dec 10, 2014 #6

    Bandersnatch

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    Myself, I think I'd just try and hook up a pendulum clock to some wheels.
     
  8. Dec 10, 2014 #7

    haruspex

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    I was trying to get Joe to identify the specific reasons why it might help to spread out the energy release. Depending on what exactly you mean by 'spin the wheels', that would be one of them. My point is, unless you understand what you are trying to avoid, you don't know how hard you have to work at avoiding it.
     
  9. Dec 10, 2014 #8
    Thank you all very much for your help so far.

    As far as "spinning out the wheels" is concerned, wouldn't my strategy of attaching a lengthy arm to the spring end help solve that problem? My physics teacher told my class that when such a situation is implemented, the spring's force actually "decreases", over the longer period of time, thus decreasing the impulse. If I have a long enough armature, the spring would not return to it's resting position as quickly, would it not? Of course, if necessary, I could increase the friction between the floor and my wheels by adding a soft material to the rims, or I could thicken my wheels, increasing their contact surface area.
     
  10. Dec 10, 2014 #9

    NascentOxygen

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    You can increase road grip all you like, even to the extent that the wheels simply cannot spin, but unless you can tame your spring it will just flip the whole vehicle over. If it can't rotate the wheels clockwise, the power source will rotate the whole vehicle counterclockwise. (You've seen videos of tractors where the engine lifts the tractor up and over until it rests upside down?) You might be able to get the arm idea to work, you just have to experiment.
     
  11. Dec 10, 2014 #10

    Bandersnatch

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    Seriously, look into how pendulum clocks solve the issue of gradual energy release, be it from springs or from gravitational potential energy of weights.
     
  12. Dec 10, 2014 #11

    haruspex

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    Ah, you're suggesting an escapement, yes? I can see a few problems with that. Might be hard to make it robust enough; standard escapements produce a stop/start motion of the gear train, so you'd need a ratchet system between there and the wheels; not sure what the losses would be.
    Another approach would be to transmit the energy via a long band that starts off wound around the wheel axle. As the band is unwound from there and onto a spool at the spring end of the transmission, the mechanical advantage of the spring increases. This counteracts the opposite effect going on in the spring.
     
  13. Dec 11, 2014 #12
    I think that an escapement would be a little challenging for me to build, unfortunately. haruspex, are you describing a system where, as the spool spins, it applies a pulling force on a tension spring, which, when fully expanded, would cause the vehicle to run the opposite direction? Would this even be very effective, as I would only have two small sources of elastic energy?
     
  14. Dec 11, 2014 #13

    haruspex

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    No. In old reel-to-reel tape recorders, as the tape winds from one spool to the other, the rotation rates of the two spools change. The one losing tape gets faster while the other slows down. I'm suggesting a transmission system like that be used between clockwork mechanism and axle. Initially, nearly all the tape is wound on the axle. As the clockwork turns, it draws tape onto the spool at its own end. The clockwork delivers some torque T. If at some point in time the spool radius (including tape) at the clockwork end is r, and at the axle end is R, what torque is delivered to the axle?
    Can you figure out how the acceleration will vary with time if T is constant? In practice, of course, T decays.
     
  15. Dec 11, 2014 #14
    Unfortunately, we haven't covered torque yet. My understanding from engineering class is that torque is a force (external) that causes rotation around the center of mass of an object. How would that help me in this situation? Also, why does it matter that, theoretically, one axle would spin faster than the other in your example? Are not my two axles going to be spinning at the same rate?
     
  16. Dec 11, 2014 #15

    haruspex

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    No, a torque is not a force, it's a force times a distance. You're probably familiar with work as force times distance along the line in which the force acts. Torque is force multiplied by the perpendicular distance from it to some axis. When you use a wrench you create a torque. The magnitude of the torque is the force you apply at right angles to the arm of the wrench, multiplied by the distance along the arm from the point where you push to the centre of the bolt (or whatever) you're trying to turn.
    It's just like levers and gears and mechanical advantage.
    Axles on the wheels? Yes.
    I'm talking about how the work is transferred from the clockwork mechanism to the axles. The idea is to create a continuously varying gearing. In effect, the engine starts off in a high gear, making it barely able to overcome the resistance of the wheels, so accelerating only slowly. As the tape shifts from the one spool to the other, the gearing ratio changes, allowing the engine to turn faster.
    However, I cannot think of any obvious source of a tape suitable for your needs.
     
  17. Dec 11, 2014 #16
    Okay! (For purposes of the following explanation, it is assumed that the car has simply a front and back, no right or left) Could I not achieve the same effect by mounting a row of tension springs along the back of the car, and then, in similar pattern, arrange a row of springs along the front of the car? When one "group" of springs is expanded, they will retract, "charging", or putting energy into, the opposing row of springs, causing the vehicle to decelerate, and move in the opposite direction, etc. etc. I suppose that in order for this to be effective, I would have to virtually eliminate friction...
     
  18. Dec 11, 2014 #17

    haruspex

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    I don't understand the suggestion. Is this instead of the clockwork or as well?
     
  19. Dec 12, 2014 #18
    I don't exactly understand the clockwork: How it works, it's application, etc. The suggestion I made was based off of your tape-reference: Mount a horizontal row of tension springs (say, three inches apart from each other, laterally) on one end of the car; then, on the opposing side, mount another horizontal row of alternating tension springs (alternating with the opposing row). When one row is taught, the other is relaxed; when the taught row relaxes, it pulls on the relaxed row, putting work, and thus elastic energy into them. Thus the spring system would alternate until all energy is lost due to friction... At least, this is what I interpreted your tape-idea to be. Perhaps I am mistaken?
     
  20. Dec 12, 2014 #19

    haruspex

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    Totally.
     
  21. Dec 12, 2014 #20
    Alright; then I am even more confused. I am simply trying to make a car that goes the longest amount of time; relatively simply. Would it just be better to go with gravity, and use one-way locking ball bearings (build my own) for each pulley system?
     
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