You Can’t Push a Rope, Or Can You?

From the very beginning of my mechanical engineering teaching career, whenever I presented Newton's Laws of Motion I always added a 4th Law: "You can't push on a rope!" Students would laugh, but then a significant number of the class would, on the first test, submit a negative value for the tension in a rope, cable, or belt, meaning they were "pushing on a rope."

Well, a few days ago a dear friend and colleague, a former Department Head of Mechanical Engineering and Associate Dean, told me about an interesting device that negates the "Can't push on a rope" law. It is called a "Push Belt." They are used on Continuously Variable Transmissions (CVT). These are flexible steel belts that operate in compression instead of tension. It is hard to describe exactly how these work, so I will refer you to a video by John Kelly, a Professor in the Department of Automotive Technology at Weber State University in Ogden, Utah. The video is amazing, which accounts for its 1.2 million and counting views. I invite you to enjoy watching it, and I will no longer be talking about "not pushing on a rope."  - Dr. Tom

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