Help with building a battery transporter for physics

[michellex]

we have to build vehicle that can transport a d cell battery 7 meters as fast as possible using the potential energy of no more than 2 kilograms falling from 80cm.

we want to use this design:
http://www.scioly.org/w/images/6/69/SC-brake3diag.gif

http://www.scioly.org/w/images/3/33/SC-brake3.gif

but we're not sure how to build it exactly so if you can, please help us get started!

 

[ideasrule]

If your school has a machine shop, use that. It should have a supply of wood that you can use to construct the frame. If you can't make metal wheels, you might have to improvise with wooden ones.