I read this book in grade 5 (~ 1979). I remember buying the book because I though the cover illustration (the one you used here) was cool!!!! The name of the author and the title of the book do not ring a bell. I had to skim the Gutenberg HTML copy until I saw the "WEIGHT IS WHAT YOU LIFT, MASS IS WHAT HURTS WHEN IT HITS YOU." quote to know for sure it was in fact the same book.
Need to download it and read it again. Thanks!