In college physics, in my dorm, the engineering majors gathered on Thursday nights, to do physics homework.

In that group, I was the theory guy. The half dozen others were better at math, but I understood why things work.

Maybe one of my favorite examples involves looking at a wheel on a train. What is the center of rotation?

At what point, is every part of that wheel rotating about, with no other movement?

It's the point of contact with the rail. That point isn't moving, and every other part of the wheel is going around it.

A couple weeks after I used this in a homework problem solution, I learned it's called the "Instantaneous Center of Rotation.

That point changes, but at any instant in time, it is the center of rotation. The center of the axle is moving around it,

as is every other point on the wheel. If you draw a radius out from that point, to any point on the wheel,

the point at the end of that radius is moving perpendicular to that radius. (And now back to other stuff.)

My shirt

Euler's identity. (Pronounced Oiler. I didn't know either.)

And then, there's puzzles:

If a chicken and a half, lays an egg and a half, in a day and a half

how many eggs will 2 chickens lay in 3 days

A ship is twice as old as its boiler was when the ship was as old as the boiler is now.

The combined age of the ship and boiler is thirty years. What are their ages?

If 6 cats can catch 6 rats in 6 minutes, how many cats does it take to catch 100 rats in 100 minutes?

(That was a 7th grade homework problem. Next day we had a substitute who didn't go over homework.

No-one in the class got it right, but when I was in high school, I finally figured it out. That's tenacity)

Back to chickens ..... If a chicken and a half, lay an egg and a half, in a day and a half,

then, how many and a half, that lay better by half, would it take to lay half a score and a half, in a week and a half?

(I always have to look it up, to get the wording exactly right.)

Two online sites that I like are Veritasium, and Kathylovesphysics.

Veritasium

Bizarre Behavior of Rotating Bodie. This is really cool.

Kathy loves physics bio

Kathy loves physics videos.

What is this kid on the swing doing wrong?

Some decades ago (I wasn't a kid) I realized that making a park swing go related to both energy and momentum.

What you have is a long pendulum, and to make it go, you make it longer and shorter.

When you lay out, at the back end, you make it's center of mass longer.

When you curl up, in front, may make the center of mass lower.

If you want to get going faster, you should make the center of mass longer at the end, and shorter in the middle.

When you do this, you realize right away, that it's harder to curl up at the bottom of the arc.

That's because you are adding energy while at the same time conserving momentum.

Then when you lay out at the top, you create the longer pendulum arm, so have more momentum at the bottom of the arc.