I was at the gym this morning and as I was working out I noticed a nifty mechanism that enabled the machine to do what it did. The machine was one of those high resistance rowing motion dealies and here's what the linkage looked like:
The two arrows are pointing to what was so interesting to me. Those aren't simple one degree of freedom (1 DOF) joints there. Instead of a simple joint, those are spheres riding in a spherical cage. Very much like the way your hip joint rides in your hip socket. Here's a closeup of the joint and to the right an overlay showing what would be the entire sphere:
Now WHY do they have spheres there instead of a super simple and cheap joint? Because the machine is made so that the arm you're pulling (the rightmost piece in the pic) comes toward you at an angle, whereas the machine itself is set up straight front to back. Simple joints would bind; they're incapable of anything other than purely in-plane motion. Let's take a look:
In green I've shown the arm in its rest position and then where it would be fully pulled back. This arm has a simple 1 DOF joint, and I've shown the axis it rotates about in green as well. Simple rotation. Now look at the purple marks. On the left I've drawn one piece parallel to the working link and the other along the axis defined by the non-rotating part of the machine. Notice that the angle is greater than 90 degrees. On the right you can see a big change. Now the angle is less than ninety. So we have a continuous change of orientation in three space. Simple joints - won't happen. Fancy spherical two degree of freedom joints - happens!
This is semi-fancy kinematics and is the sort of thing you'd be grooving to if you were employed by BMW to design multi-link suspensions, for instance. Is it important? Wow, is it ever. It's the difference between "The Ultimate Driving Machine" or "Porsche, There Is No Substitute" and "Rental Fleet - It Drives And That's About It."
Sooo, what about that title, "Only A Man"? Well, to take these pictures I asked a totally excellent woman to help me by holding the arm out of the way and because she asked what the heck I was doing, I explained it. And she said ...
"Only a man would be interested in this."
Whoa! So many reactions. To start, a big part of my life's been educating engineers and a huge issue for engineering today is that so few women go into the field. It's very heavily male dominated. And there's a whooole bunch of questions and opinions as to why that is. "It's built into our DNA." "It's cultural." "Men build and women nurture." "Women's brains are wired differently." "Teachers unconsciously funnel boys and girls into different paths in grade school."
You want to know what the real answer is?
Me too. I don't think there IS a real answer. But I found it interesting that this woman simply expressed her clear view, based on her life's history, that guys would like this stuff and gals wouldn't. Nothing political there, no hidden message. Just her view.
However I'm CERTAIN that I can find, without any effort, tons of guys who would be completely bored out of their skulls by this little kinematics exercise. Zero interest at all. And I'm sure that there are women out there who would find it very cool. I guess to me the reality is "Only someone who's not interested in this would be not interested in this." As an educator what I want to see is that everyone who DOES have an interest, whether they're male or female or something in between, can pursue that interest to the best of their ability. It's good for them and it's good for us.