A lot of physics is about the tradeoff between potential and kinetic energy, and gravity-powered cars are an excellent demonstration of this, as well as being fun to race. While the most famous variety of these cars is usually associated with the Boy Scouts' Pinewood Derby, you also can put one together with a few simple household objects.
Cut a car base out of the cardboard or foam. It can be any length, size or shape, but ideally will be about 3 to 3 1/2 inches.
Cut the straw in half and glue or tape it in place on the car base. You want to attach one to the front and one to the rear of the vehicle. Trim the straws to be no wider than the base.
Cut the dowel in half and stick the halves into the drinking straws to act as axles.
Glue or tape the wheels onto the axles, one at each corner. You can attach them snug to the car base--trimming the axles if needed--but don't make it too snug. If the wheels are too close to the car, they'll be too loose, won't turn freely and the car may not roll straight.
Pinewood Kit Car
Unpack the pinewood kit and set the wheels and nails aside.
Draw a plan for the vehicle on the pinewood block. Keep principles like aerodynamics in mind: smooth and sleek cars will move faster than large and boxy ones.
Cut the car out of the wood. If you are competing in a derby, bear the weight restrictions of the car in mind and cut some spaces to insert weights if necessary.
Sand the car until smooth.
Install the wheels into the precut axle grooves. Place one of the axle nails through the wheel and push it into place.
Set up the ramp on a long, flat floor.
Measure the longest car and mark that length down from the top of the ramp. Make a starting line with masking tape.
Determine the kind of race. If this is a race for speed, set up a finish line. If for distance, lay out a few meter sticks to end along the floor, moving away from the ramp.
Hold all of the cars competing in the race at the top of the ramp, lining up all of the noses with the starting line.
Count down to three, and when someone announces go, have all racers let their cars roll down the ramp.
Declare a winner.
These directions are for one car. To make more cars, have enough materials for each vehicle you intend to make. Setting up a gravity-powered race can be a really fun rainy day activity or a great science class team project. To get more speed out of a pinewood car, you also can sand the axles and ream out the wheels to reduce friction. Test it on the floor to see if it drifts at all and adjust the wheels if necessary.