Estimate the Pulling Force of Boston Dynamics' Robo-Dog Army

WIRED | 4/17/2019 | Rhett Allain
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When Boston Dynamics shares a new robot video, my robophobia levels increase just a little bit. I don't know why. There is something about these robots that get into the uncanny valley for me. This particular video is both fascinating and disturbing. It's fascinating because here are a bunch of robots pulling a truck (not a pickup truck—a real truck). It's disturbing because it shows a BUNCH of robots. That's the beginning of a robot army.

Perhaps the best way to calm myself is to consider the physics. Analyzing situations such as this is exactly what I like to do. If I combine something I like (physics) with something disturbing (robot army) maybe I will be OK.

Truck - Something - Robot - Bunny - Rabbit

So, just how difficult is it to pull a truck like this? Is this something that only a robot can do, or could a small bunny rabbit also do it? The physics is mostly about friction. If you want to pull this massive truck, you both need high friction and you need low friction—yes, at the same time.

What is friction? In a situation like this, there are actually two types of friction. There is the static friction between the pet robots' feet (maybe they aren't pets) and then there is the rolling friction between the tires of the truck and the road. Let's look at the static friction first.

Surfaces - Contact - Force - Surfaces - Force

When you have two surfaces in contact with each other, there can be a lateral force that is parallel to their surfaces. This force is an interaction between the atoms in both materials. However, no one really wants to model in the interaction between 1026 atoms (or some huge number like that), so instead we just make a simpler model. A simple model of static friction works fairly well. It has the following features.

There is a frictional...