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Gravity Car

  1. Apr 3, 2004 #1
    Okay, going completely crazy here. I'm in accelerated physics and we just got assigned the gravity car. The way its supposed to work is like this-- a weight is supposed to be dropped from a certain height and its supposed to propel the car forward (GPE converted into KE and so on). The car is supposed to travel at least 5 meters and has to travel in a straight line. The problem is I have no idea where to even begin. I could really use some basic ideas, such as how it could look like, what materials I might need and if anyone has ever done this before any advice or tips would be great. Thanx...
  2. jcsd
  3. Apr 3, 2004 #2
    You might have a weight on a string, which through pulleys is attached to to back axle of the car. As the weight falls, it will spin the axle.
  4. Apr 3, 2004 #3
    Or even easier, just attach the string to the bumper and have the axels free to spin. You can tow it 5 meters.

  5. Apr 3, 2004 #4
    sounds like a fun project
  6. Apr 4, 2004 #5
    5 meters? thats pretty long for a toy car. how high r u allowed to go up?
    /_______\------------------------------------------------| _O_____O__________________________________________|.......| weight
    .............................................................................. _|_......|

    a hahah
    Last edited: Apr 4, 2004
  7. Apr 4, 2004 #6
    best idea i think

    but add spring to the wheel section? i think that would work better , tighten the spring as it fall by the string. by the ground it will release the spring and it will rotate in opposite direction which the car go forward .
  8. Apr 4, 2004 #7
    I'd say adding a flywheel of some sort would be too complicated. It usually works out that simpler is better.

  9. Apr 4, 2004 #8
    it would be alright, just need a spring ;)
  10. Apr 4, 2004 #9


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    How does a spring demonstrate the potential energy of gravity?

    Wooh has it right, the whole trick is getting the weight above the critical amount within the drop dimensions and making sure the tires don't compress too much from the additional weight to increase the friction.

  11. Apr 4, 2004 #10
    Gravity car?! No problem mate; just put the car on a slope and watch it go crazy! ACCELERATING(!) to infinity (def.: infinity=the object that stands in the way)... Even better bring a car to a cliff and let it fall (wow! 9,81 m/ss aceleration!!!)!!!

    Serious? Well... First idea I had is to put very heavy weight (almost crashing weight) on a well oiled spiral (screw) so when the weight goes down it spins the spiral and it rotates the wheels; so - there you go...

    (by the way: imagine a very fat man with suitcase sitting strapped on a rotating chair that stands on the surface with wheels and transmission mechanism, then the man puts a helmut and goggles on his head and pushes the red button on the chair - the man starts spinning, and... - look at him go!?! "Where are you going Elmer?" "I-- goi--- to wor--- Harry... Sa-- hello t--- yo-- wife, Ha---!!"... I've got to stop writting before my stomach starts hurting...)
  12. Apr 5, 2004 #11

    you right i did not demonstrate though

    i was think to use the poentential energy to kinteci energy than into elasticity energy of spring back to kinetic energy.
  13. Apr 5, 2004 #12
    Have a weight fall and hit a button which powers a petrol engine!

    Or just adapt one of those pull-back cars to work by droping a weight instead of pulling it back.
  14. Apr 5, 2004 #13


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    I'd consider first the theoretical implications such as how much energy you can release from an object falling in the gravitational field of the earth and thus how much mass the car can be to travel 5 metres and so on before designing a mechanism for propulsion.

    A proficient design would be that of the mass attached to the axel via a wound string so and released from a height. Obviously it would convert all of its kinetic enrgy into winding the weight back up again but that would be transferred into forward motion when it drops and you would get an oscillating motionwhich would be interesting to model which would depend on how much energy was dissipated by friction.

    Very interesting problem. Good luck!
    Last edited: Apr 5, 2004
  15. Apr 6, 2004 #14

    =) cool

    ( hope it doesnt crash that car =P )
  16. Apr 6, 2004 #15
    If the string of the stone is wrapped around a gear of a much larger radius than the axle of the car, then more revolutions of the smaller axle would occur for each one revolution of the larger gear. Basically, the stone would not have to fall as far or need as much potential energy. I have seen this type of car made using a spool of thread for the larger gear. If the car has a mast of some height, then the stone could fall from the top of the mast. Tricky to make, but doable. Good luck!!
  17. Apr 10, 2004 #16
    You could use a combination of the set up shown in #5 along with the idea of a mechanical advantage (disadvantage really) by having a very heavy falling weight turning an axle mounted on the the edge of the table. Then attach the axle to a bigger wheel that winds up the string going to the car as the weight falls.

    Edit: I'm editing out the last thing I wrote, because I think it gives too much away that you should be figuring out for yoruself. Hope you didn't already see it!
    Last edited: Apr 10, 2004
  18. Apr 11, 2004 #17
    I was sorta thinking the same thing except I'm running into the problem of materials. It can go as high as one meter but I don't know how to attach the pulleys so that they spin the wheels (I also don't know what to use for wheels). I was thinking build a base for the car with a tall pole sticking out and have two pulleys..one at the top and one at the bottom. The only problem is that I don't think a meter in height is enough to propel the car forward one meter..and I'm only allowed to use 4.4 lbs of weight.

    Another variation of this idea is to use two masses...one slightly heavier than 2.2 lbs and the other a little smaller and when the heavier one drops it pulls the smaller one to the top and spins the axle..which then accelerates pulling the smaller one down and the heavier one up again. This cycle would continue until the car travels 5 m...anyone think this would work? PLEASE HELP!!! I'm desperate!
  19. Apr 11, 2004 #18


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    To have the rope sping the wheels is easy. The wheels are mounted to a straight shaft used as the axle. The rope is wrapped around the axle - as you pull on the rope it spins the axle.

    Ok for the pulleys, think of it this way:

    If the 4.4lb weight was attached to a pulley and it dropped a meter, how long would the rope be that went from a fixed point 1m up down to the weight and pulley at 0m and back up to another pulley at 1m. So while the weight moved x distance the rope moved y distance...see where I'm going?

  20. Apr 11, 2004 #19
    Sorta except that I still don't know how to attach the axle...and I know the rope would have to be much longer than 5 meters...and I have no clue how to make it go around that many times
  21. Apr 11, 2004 #20


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    How big is your rope? Hi strength fishing line could support the weights here, just some small rope with more than 5lb strength should work here.

    You should be able to attach the rope to the axle with some hot glue and then wrap once with electrical tape - it shouldn't slip then. Ok, now wind it around the axle 10 times. Pull that back out, how much length rope do you have?

    Ok, what is the circumfrence of the wheels you have on that axle? How far will they travel per revolution?

    How many turns of rope will you need?

    How much rope is that?

    How far will the weight need to fall to make the rope travel that distance?

    Are you writing your work down as you go? :)

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