Crash procedure

["Russell, Ken" <krussell@xxxxxxxxxxxxxxx>]

My 6th grade logo students are just finishing up the Ferris Wheel activity.
I'm not sure where I found this activity, but possibly on the install disk. It
uses 4 turtles and the procedures are fairly simple.
 
 I have an example posted:
 
http://www.bham.wednet.edu/studentgal/hclp/projects/ferriswheel4.html
 
and the procedures:
 
http://www.bham.wednet.edu/studentgal/hclp/projects/ferris_wheel.pdf
 
to wheel
cg pu setpos [-200 0] pd setc 59 setpensize 3
repeat 360 [fd 3 rt 1] pu home
end
 
to getready
everyone [pu home]
t1, setpos [-200 0] 
t2, setpos [-200 0] 
repeat 60 [fd 3 rt 1]
t3, setpos [-200 0] 
repeat 120 [fd 3 rt 1]
t4, setpos [-200 0] 
repeat 180 [fd 3 rt 1]
t5, setpos [-200 0] 
repeat 240 [fd 3 rt 1]
t6, setpos [-200 0] 
repeat 300 [fd 3 rt 1]
end
 
to ferris
tto [t1 t2 t3 t4 t5 t6]
repeat 360 [fd 3 rt 1]
end
 
(I have students revise the instructions by requiring them to use 5 or more
cars).
 
A student asked, "How do I make the ferris wheel cars fall to the ground and
crash?"
 
--a rather dark question, but an interesting programming challenge!
 
He wants to be able to click on a button when the cars are rotating and have
them stop in place and then "fall down" 
 
A student came up with this:
 
to crash
tto [t1 t2 t3 t4]
seth 180
fd 200
if ycord = -100 [stop setsh "crashcar]
end
 
It's an interesting effect, but not quite what we intended! I have tried
several different solutions, but I'm a logo novice.
 
Can anyone suggest a procedure that will work? 
 
Thanks in advance.
 
Ken Russell
Bellingham Schools