25th March 2009 - 10:02 PM
QUOTE (ubavontuba+Mar 24 2009, 09:40 PM)
Try eccentric wheels (oval/egg shaped, or wheels where the axles aren't centered).
Or use an analog solution. Someone here mentioned an analog method of finding pi.
Roll a cylinder of unit radius until it reaches 3, then shave it flat! The wheel will stop. Arrange the dimensions for your experimental setup, and be smiling!
26th March 2009 - 06:52 PM
Make one of the axles threaded, like a worm gear. Create a block with gear teeth that match the threading of the axle. Set it up so that when the car is on the ramp, the block is off the axle, and when the car goes horizontal the block drops onto the axle.
If you set it up so that the motion of the axle pushes the block from one side to the other, and if you have the threading right, then after about 3 meters the block will be unable to travel any further and the axle will be prevented from turning, stopping the car.
25th May 2009 - 09:47 PM
thanks for the help to everyone we got perfect in the race it stopped right on the dot:D
26th May 2009 - 01:51 AM
And you did it how?
I was going to offer suggestion back when the thread was new but AlexG stole my idea out of the ether.
28th May 2009 - 01:04 PM
QUOTE (AlexG+Mar 25 2009, 08:21 PM)
Tie a 3 meter long cord to the rear bumper and the bottom of the ramp.
It can be done in a way which doesn't require you to tie the car to anything.
Suppose the wheels are of radius R and the axles are of radius r (r<R obvious). For every turn of the wheels the car moves forward a distance 2 pi R. Suppose you have a bit of string length L and you glue an end to the front axle and the other to the back axle. For every turn of the wheels the string will wrap around the axle a length 2 pi r.
If the car is to travel a distance D then the wheels will turn N = D/(2 pi R) times. Therefore if the string is L = N (2 pi r) in length it'll only allow the car to move a distance D before the string is made taut and the car stops. To avoid tangling you initially wind the string around one of the axles (say the back one by spinning the back wheels in reverse).
Putting in the expression for N you have L = (D.r)/R, so if the wheels are, say, twice the thickness of the axles then you need a bit of string 1.5m in length.
So irrespective of how the car is propelled (a slope or using rubber bands etc) the wheels will lock after a distance D, stopping the car. Obviously I'm assuming no skidding etc which can be easily done by making the slope quite gentle.
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