This is an interesting way to demonstrate some Newtonian mechanics and I was motivated to build it due to a diagram I found at a forum at teamliquid.net by user Lyzonn (see his attached diagram) of a car with a pendulum mounted to it with a set of wheels that only turn in the forward direction.
When a pendulum is set in motion on a car that can only move in one direction, some of the pendulum's momentum is used to push the car forward. Not a very useful way to drive a car forward but a nice demonstration of the transfer of the pendulum's momentum into forward motion.
The car is easily constructed out of almost any kind of materials and the following are the steps I used to construct mine from some CDs, the wheels, axle and gear of a toy car, some sections of fishing pole, and a couple pieces of wood.
Step 1: Building the Car Body
First picture shows the basic components of the car and the large piece on the left is the body.
The body is just a piece of wood whose dimensions don't really matter so you can use what is on hand. Mine happens to be a piece of wood molding about 9cm long by 5 cm wide by 1 cm thick.
To the body I attached a bracket to hold the axle of a toy car. On the car wheels I hotglued two dvd/cds to serve as larger and easier turning wheels. These are about the most friction free wheels you can make.
I also attached a small piece of plastic to serve as a holder for the front wheel. It extends the front wheel out from the body so there is no contact of the wheel with the body. You could just as easily use two wheels for the front of the car but I happened to have already had my one-way-wheel already constructed from another project and it was only one wheel, not two.
Step 2: Build the One-way-wheel or Ratchet Wheel
My ratchet wheel as I like to call it because of the sound it makes, is made from the axle and wheel of a toy car. The axle is cut in half with a hacksaw. I then mounted it through a short section of fishing pole that the axle can rotate inside of. That section is then glued through a hole in 5cm by 3 cm piece of hobby plywood. The axle gear is then attached to the outside of the axle.
To make the ratchet I glued a small block of wood onto the plywood wheel holder. To this block of wood I attached a piece of vinyl from a notebook with a small screw. The vinyl is mounted at an angle to the gear so that if the gear is turned one direction, the wheel will easily turn but if the wheel is turned in the other then the vinyl will intersect with the gear and stop the turning.
The diagram shows how the one-way-wheel should work. It can be constructed from just about any materials and I have even seen a hobby shop that sells them pre-constructed.
Step 3: Build the Pendulum
The pendulum is just a section of fishing pole 15mm in diameter and about 30cm long.
I can be just about any length you want, the longer it is the more the car will react though but it can't be so long that it topples the car when you release it.
To the top of the pendulum pole I drilled through and mounted a small section of pole (7mm diamter by 5mm long). Mine fit so tight it did not need to be glued but if it fits lose then glue it.
To the bottom of the pole I just attached a metal binder clip and on this will add some counterweights in the form of magnets.
Step 4: Build the Pendulum Mast
The mast is what supports the pendulum and mounts it to the car.
The base plate is just a piece of wood about 10cm long by 7cm wide by 1cm thick. I drilled a hole in the base plate and glued a small section fishing pole. To that section I mounted the mast with zip ties and hotglue. The mast is another fishing pole section about 34cm long but it can be any size you want as long as it allows the pendulum to swing.
To the top of the mast I inserted a piece of 5mm pole. Mine also was a tight fit but you can glue it. Now the mast is complete. The pendulum fits over the mast pole and is held on with a tiny metal binder clip.
The mast base is then mounted to the body of the car with a couple of screws or bolts.
Car is completed and ready to use.
Step 5: Pendulum Car Use...
By adding different weights to the pendulum you can get different distances with the car. I am not sure of the formula to calculate the weight to distance but that might be a good project for someone to do who is math minded and motivated.
My plan for the future is to automate the car with a servo and Arduino. Probably not a great new form of transportation but it would make for a fantastic Burning Man art car. I can just see the driver mounted on the end of the pendulum as the counterweight.