Last week, Stanford researchers revealed that that they had built tiny drones that can open doors. I'm not sure I'm happy about this: How will we keep the robots out of our houses if they can just open the doors?

But this is also pretty cool. These tiny drones (or micro air vehicles) are able to pull super heavy loads as compared to their own weight—up to a factor of 40. That might seem crazy. Well, I guess it's crazy—crazy awesome.

Let's get to the physics. How much of your weight can you pull?

Pulling with Normal Friction

Suppose you are trying to pull a large box with an attached rope while standing on flat ground. Why do you need a rope? You don't—but it's easier to draw a diagram that way.

Here is the important part. If you pull on the rope with some force (I will call it T for tension), that rope pulls back on you with the same magnitude force. Forces are an interaction between two things: Pulling with a force of 10 Newtons to the left on a rope means the rope pulls on you with a force of 10 Newtons to the right. That's just the nature of forces.

That means that if I want to pull on a block with a rope, I will need another force pulling on me in the other direction that will prevent me from moving. That other force is the frictional force. I'll be honest. Friction is super complicated. Just think about all the atoms in one material (your shoes) interacting with all the atoms in another material (the floor). That's way too much for anyone to deal with. Fortunately, we have a pretty good approximation for the friction force. Here are the details of this friction model.