[drkiraco]

Let's look at this from an energy perspective. Energy is conserved, so in your robot/vehicle/whatever, energy in = energy out. Your motors are producing a certain amount of power, that's your energy in. So of that energy must be used to overcome various forces that resist motion. For example:

1) air resistance. as your robot moves, it smacks into air molecules and imparts some energy to them. typically, the air resistance increases as you go faster, usually force is proportional to velocity squared. Example: at about 20 mph in your car, air resistance is pretty low and your radio, air conditioner, and headlights are probably using more power than air resistance. At 60 mph (3 times faster) air resistance is 9 times higher, and dominates all other power consumption in your vehicle.

2) friction, viscous drag, and internal damping inside your transmission and drive train. None of the components in your drive train are 100% effecient and all of them suck a little bit of power. This power ends up becoming heat. These kinds of forces also usually go up as velocity goes up.

3) internal damping in your wheels (aka hysteresis) for a car, this can be significant. as the wheels roll, the part of the tire near to the ground compresses a little bit. so the tire is not quite perfectly round anymore, it's got a tiny little bit of a flat spot on it. As the wheel rolls, that part returns to round and another little bit is deformed. It take some energy to do this, and again that energy ends up as heat. For the robot this year, the wheels are pretty stiff, so that probably won't be a bit factor. Again, these forces will be higher as velocity increases.

there are other forces involved, but these are the big ones.

Now, let's think through a couple of different cases.

case 1) velocity = 0, motor power = 100W. since all of the above forces are zero, all of the available motor power goes into increases the vehicles kinetic energy. the vehicle will accelerate quickly

case 2) velocity = 50%, motor power is stil 100W. But now, since there is some velocity, you have all of the above forces which suck up some available energy. hypothetically, let's case 75W are consumed this way. 25W is still available to accelerate your vehicle, so it continues to accelerate, but slower.

case 3) velocity = 100%. motor power is still 100W. but now, the drag forces are consuming 100W. there is no power left to increase the kinetic energy so the vehicle no longer accelerates.

I hope that gets you started. there's lot of good articles on wikipedia and places like that if you want to dig into it...