Shannon asks:

CATAGORY: Mechanics QUESTION: Hi! I'm building an egg car for physics. Dropping something weighing 1 kg. from 1 meter, my car (with an egg as its passenger) must go exactly 6 meters. I must also utilize a braking system. Any ideas?

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Simplicity always works best.  Keep that in mind when you are designing
your egg cart.

I thought that you might have the 1kg weight attached to a string which
goes over a pully at the top of a 1meter high mast.  The mast
is attached to a cart which will be nothing more than a board with some
wheels attached to it and a padded container for the egg.  The string goes
over the top of the pully at the top of the mast and then goes down the
mast to another pully attached to the cart.  The string then goes under
the pully at the bottom of the cart and winds around a cylinder connected
to one of the wheels.  

As the weight falls the string pulls on the cylinder creating a torque
which will make the wheel turn and drive the cart forward.  The string is
wrapped around the cylinder but not attached to it. That way when the
string runs out the cart will continue to move forward under its oun 
momentum otherwise the string will wind up again lifting the weight and
slowing your car before it goes 6 meters.  

To get the car to stop how about having the weight hang off the back of
the cart so that the weight hits the floor it leaves the cart.  A second
string however will be attached to the weight.  The second string goes to
a spool where enough string is attached so that the spool fully unwinds by
the time the cart goes 6 meters. The the spool is attached to the cart so
that the weight that was left behind acts like a boat anchor and this
causes the cart to stop.  

I have never built such a device and there is always a gap between a
design idea and its practical execution.  Friction will be an important
energy drain.  Have VERY freely turning wheels and good pullys with
bearings.  Make the components small and light weight. use light weight
wood like pine wood or balsa wood.  You might have a tipping problem with
the weight hanging off the back so make the cart long in the front like a
drag racer car.
Good luck!  
Let us know how you did! :-)

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