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Homework Help: Physics project: help (highschool stuff)

  1. May 8, 2005 #1
    Just was going to see if anyone had any ideas for this project thats due like wedsday.

    Objective: Build a wheeled deivce that will coast down a ramp and brake to a stop in a prescribed area.

    Permitted materials: Wheels must be made from standard music or computer CD's

    The device must have at least three wheels which are attached to one or more axles. The wheels must rotate but the framework cannot rotate. The axels may or may not rotate. The wheels can be cut,drilled,glued together,ect. Only the wheels may touch the ramp and floor. No remote controls allowed the device must be self contained. No energy sources allowed(batteries,pre wound springs..ect.)

    The car will roll down a 4ft long 16in high ramp then coast for 3ft on a level surface and after that it needs to stop quickly in order to get a good score...

    If anyone has any ideas pls help lol....So far we have four wheels attached to threaded bolts as axles drilled into a piece of wood. The idea is as the wheels roll they will turn down the threads and hit the body creating friction that will hopefully stop the car, but we doubt it will have enough force to stop it. HELP!
  2. jcsd
  3. May 8, 2005 #2


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    Sounds like your threaded axle idea is a great approach. It would be nice if you could have counter directed threads on the two sides so the wheels drifted in opposite directions. In the simplest case, your wheels should lock up when they hit the sides (or the end of the bolt if one side wheel drifts away from the cart. With counter threading, you could have a more sophisticated brake with a spring mounted pad to gradually increase the friction. You can't do much better than skidding to stop quickly if you cant touch the floor with anything else.
  4. May 9, 2005 #3


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    Seems to me 7 feet of string would do the trick (plus a few extra inches to account for the distance between the axles). The string unwinds from one axle and winds up the other axle as the car rolls. Each end of the string is stapled to a different axle, so when all the string has wound off of one axle, there's no way for the axles to rotate in the same direction anymore. The axle's have to stop rotating.

    Now, the tricky part. Unless all 7 feet of string can lie in one layer along the axle, you're going to run into problems keeping the circumference of each axle equal. The easy solution is to put a spool on top of the frame to feed the string instead of using the second axle to feed the thread.

    You're not exactly clear about what they mean by the framework can't rotate, nor on the dimensions. If there's to be no other rotating pieces other than the wheels and axles, and you can't fit 7 feet of string on one axle in one layer, you're going to have a problem equalizing the rotation on each axle. Considering each wheel is a computer CD, you could have some pretty thick axles, making it unlikely that you couldn't wind 7 feet of string in a single layer.
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