What about a down-spin bot? In this case, the spinning disk pushes down on the victim and up on the attacker. If you design your attacking robot correctly, you can account for this up-pushing force. But what good is a down-pushing force? It won't throw the other robot, but you can tear into it with a spinning blade of death. Do robots even die? I'm not sure. You can see that all down-spin bots use the disk as some type of saw and not for flipping.

Oh, you need an example of a down-spin bot? There is Skorpios and Red Devil—both are down-spin.

Angular Momentum

What the heck is angular momentum? Here is a longer explanation—but for now I will just say that angular momentum is a property associated with rotating objects. If you want to change the axis along which an object rotates, you need to apply a torque (a twisting force).

Yes, it's true—most people don't have much everyday experience with angular momentum. One place you might have seen its effects is with the fidget spinner (that finger-based spinning toy). Perhaps this is one reason it's fun to play with. A spinning fidget spinner has more angular momentum than a non-rotating one. This means that it takes more torque to flip it over. It just feels weird since we are used to this angular momentum thing.

A robot with a spinning disk also has angular momentum. You can really see this with vertical spinning bots. In order to turn the direction of the robot, you need extra torque so that you can change the axis of rotation for the spinning disk. This can lead to some weird cases where the robot actually tilts up on its side as it turns. Honestly, the physics of these tilt moves can be a little complicated. Maybe I will come back to this in a later post.

Hammer Bots

Take another look at the battle between Minotaur and Blacksmith. Yes, Blacksmith is a robot with a hammer. These seem to be popular—I suppose because hammers just look cool. Oh, they can also smash things. But how does a hammer compare to a vertical spinner? The one clear disadvantage is the energy. A swinging hammer can only gain energy during the swing (which is typically a short time). In order to increase the energy of the impact, a bot can increase the mass of the hammer to compensate for the lower speed during the swing (compared to a spinner).

But a large mass hammer introduces a new problem—momentum. Momentum is the product of an object's mass and velocity and it doesn't change unless there is an external force acting on this object. So if you consider a robot to have a body and a hammer, the total momentum should stay at zero as it sits on the floor. Once the hammer falls, the mass of the hammer head has a downward momentum. In order to make the total bot momentum zero, the rest of the robot (the body) has to have an upward momentum. You can see this with the Blacksmith (and other hammer bots). Swing down too hard and the robot will move up. It's the price you pay for a heavy hammer.

Wedge Bots

I guess I should say something about the wedge. The idea is to make a slanted robot that can scoop up under other robots and flip them over. But really, what is there to say? It seems pretty straightforward. I guess the big advantage to wedge bots is that they are fairly simple (no swinging hammers or spinning disks).

Of course there is still even more physics behind these BattleBots—but this is just an introduction. Really, I could probably make an entire physics course that just focuses on BattleBots. Maybe I will have to do that in the future.