Physics for Flerfs 1, Vectors and Balanced Forces

Because reasons, I'm a member of a group called Official Flat Earth & Globe Discussion, where flat-earthers (or people who think it's funny to pretend to be flat-earthers) post stupid things, get quickly refuted, but don't understand it, or people who've studied science write about science and the flerfs respond with laugh reactions, incomprehensible memes, quotes from Scriptue, and "nuh-uh". One of the things is that they don't just disbelieve that the Earth is a sphere (or a sphere that's slightly wider around the equator than around the poles, or even one that's slightly larger in the southern hemisphere than the northern), they also don't believe in gravity. So I decided to teach an introductory physics course there to see if they disagreed with anything else.

I only got one response to the first chapter, and it was questions about God, so I blocked the idiot. So anyway I'm going to post them on my wall too, because why not? I'll use the hashtag #PhysicsForFlerfs

So I went to find the simplest science textbook on my bookshelves, I picked Principles of Physics, F. Bueche, 2nd edition, 1972 which is for a general physics course without calculus. If I had taken physics at engineering school there would have been a higher-level course but I did my basic sciences at a liberal arts college where this is what they used.

So let's start in chapter 1, "Vectors and Balanced Forces".

An example of a vector is given as driving 30 miles east, it includes both magnitude (the 30 miles) and direction (east) as opposed to a scalar, e.g. driving 30 miles but not specifying which way. It gives several examples of vector addition, for example if someone goes 30 miles east and then 10 miles north, the Pythagorean theorem shows that they've gone the square root of 1000 miles, and drawing a scale diagram and using a protractor (not even trigonometry until a few pages later) shows the angle.

There are also examples of forces expressed as vectors. Let's leave out things we disagree about and do them on a table. If several forces are applied to an object, say it is being pulled on via strings from various directions, the total force can be computed by adding all the force vectors.

Quantities that are vectors appear in boldface, for example a force will be written as 𝐅

Now we get to to an object at rest. If all the forces on that object on a table are balanced, say one person applies a force of 10 pounds from one direction, and another does the same from exactly opposite the first, it won't move. This can be expressed in an equation, an object is at equilibrium if Σ𝐅 = 0, the Greek letter indicates "the sum of" so if all the forces add (via vector addition) to zero, the object won't move, or if it was already moving, it will continue to move as it would without those forces.