first arduino uno r3 robot

I'm trying my hand at some simple robotics stuff and running into some problems. Its a car type robot, with a cd case frame, cardboard wheels, and two motors from radioshack (9-18V, highspeed, part#2730256), with the arduino motor shield. When i turn it on the wheels are turning pretty well, but once it hits the ground, everything stops and the motors emit a whine. My arduino is running off a 9V battery, and I use a 9V battery on the motor shield as well... Do i simply need a better (stronger) battery, or is the problem elsewhere?

http://support.radioshack.com/support_supplies/doc19/19832.htm "Stall Torque: .................................................... 180 G-cm"

If your wheels are, say, 5 cm in diameter (2.5 cm radius) you would have a forward thrust of 72 grams. That's assuming that your battery can produce sufficient current to keep the voltage up to 9V and you have the motor controller set for maximum. I suspect you need a heftier battery, especially if you are using a tiny 9V 'transistor radio' battery. Try a battery pack with six AA cells.

To get the most efficiency from your motor it should run at 9200 RPM. With direct drive 5 cm wheels that's 2.4 meters per second! That's why most robots use geared-down motors. You get higher torque and more controlled speed.

Thanks for the advice! I'll be sure to try it out once I get a chance.