# Rotational Terminology

Our textbook Understanding Physics is derived from Fundamentals of Physics by Halliday and Resnick and therefore uses the somewhat nonstandard terminology such as “rotational velocity” instead of “angular velocity” etc. In particular the term “rotational inertia” is used instead of “moment of inertia”. I cannot teach this without slipping up occasionally. For this reason and just so the you’ll be able to understand the more common terms it’s important to make a table comparing H&R; terms with the conventional terms.

I also notice that the word “translational” is often used where “linear” may be more common. For example, “translation momentum” is used instead of “linear momentum”.

I do think the H&R; terminology is more consistent and logical, but it’s almost never used anywhere else. Too bad.

 Terminology for Rotational Motion

# Signs

There was some confusion in the definition of gravitational potential energy used in Unit 11. This is understandable because of the way that the sign of the fall distance was used.

In this course we’ll always take the value of g to be +9.8 m/s2. Therefore when something falls its acceleration is −g. However, when something falls there’s always the option of using the positive sign for the downward direction. That seems to be what was done in Unit 11 where y is used for the fall distance. The result was that gravitational potential energy comes out as

Epot = −mgy

That’s not not exactly wrong, but is probably confusing because almost everywhere else the sign is positive; so you probably remember this:

Epot= +mgh

I’ve tried to repair this  confusion by defining the distance of fall as Δy and
as being negative. That way all is more conventional.

Other issues were using the notation Epot for gravitational potential energy whereas Ug is more usual.

Ug = mgΔy

The textbook uses Ugrav which is close. Similarly K is more convenient and usual for Kinetic Energy than Ekin.

I’ve rewritten the unit’s Activity Guide with these changes and I hope it won’t be so confusing now. Have a look and let me know what’s not clear:

Revised Unit 11