Physics 101: Mass vs. Drag

When I first tell people about how the Inertia Motors inside the wheels add mass, they often respond with "like resistance?".

Yes... and no. The Inertia Wheels can add resistance. We call that Drag. Mass is something entirely different. And they mean different things to an operator.

Part 1 of a series on physics from our founder, Boyd Hobbs. Read Part 2 ❯

Drag

Historically, operators have only been able to change the drag on wheels. It's usually applied with some sort of padded breaking system with one singular dimension of adjustment from no drag to more drag.

More drag feels like you continuously need to apply forward force to keep it moving because the break is continuously trying to bring the wheels to a stop. No drag feels like the wheel wants to keep spinning, and you can either use more forward force to speed it up, more backward force to slow it down, or neutral force, and the wheel will keep going.

In physics terms, drag is an external force that tries to bring the wheel to a rest. It's a braking force. Drag is applied to an object.

Mass

Separately, Mass is an object's own weight, and the more mass an object has, the more effort is required to change the object's velocity.

If you've ever tried to bring a fully loaded Fisher 10 to a rapid stop, you know that a heavy object isn't just difficult to start moving. It is also difficult to stop moving. This is mass.

Heavier things have lots of benefits on film sets. Chiefly, they are inherently smoother in motion. Alexa Studio handheld versus a DSLR handheld. Fisher 10 versus a slider. Heavier is smoother because it can easily overpower every little force trying to change its velocity.

With historical wheels, you couldn't adjust the mass of the wheel unless you replaced the wheel itself. That's because the smoothness of the wheel comes from the actual weight of the wheel. The smoothness in the Inertia Wheels does not come predominantly from the weight of the wheel. It comes from the motors.

How did we create mass without replacing the wheels? Simple. We defied the laws of physics! Kidding... we added motors, a dedicated processor, ultra-high-resolution encoders, and a lot of math.

Hacking Newton's Law of Inertia

An object at rest wants to stay at rest, and an object in motion wants to stay in motion.
–Sir Isaac Newton, Newton's First Law

In order to break down how the Inertia Wheels appear to defy physics, let's talk about Newton's first law (also called the "Law of Inertia"). This is the law that explains why heavy objects with more mass are smoother.

There are two parts to it, an object at rest and an object in motion.

It's relatively easy to make an object at rest stay at rest using drag. Drag is always trying to bring the object to rest. Most wheels can replicate this part easily enough.

But an object in motion wants to stay in motion. That's the tricky part. And also the secret to the Inertia Wheels.

So, we added motors. Unlike a brake pad that can only resist motion by pulling, the Inertia Motors can resist and assist motion by pushing.

What does it feel like? Well, exactly what a heavy wheel feels like. As you spin the wheel up to speed, the motors will resist the increase in speed, but then once the wheel is at speed, they assist the wheel's continuous spin.

And this was no arbitrary software algorithm. We took the actual laws of inertia and put them into the software that drives the motors. The wheels behave exactly like a heavy set of wheels.

So there you go. A little physics lesson and the secret to what makes our patent-pending Inertia Motors the secret sauce of one of the most adaptable pieces of film equipment on the market.