A First Course in Physics · Ch. VIII, §§ 208–211 · Chapter 8: Work and Mechanical Energy
23. The lever and the law of moments
Goal: Your student can find the balance point of a lever using force × lever arm, add moments on both sides, and sort everyday tools into the three classes of levers.
You'll need
- The book, open to these pages
- A ruler or yardstick and a pencil for a fulcrum
- Coins or identical washers as weights
- A wheelbarrow, pliers, tongs, a hammer, a bottle opener (to sort)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
The meter-stick lever experiment; the law of moments (Fl = Wl′); adding several moments; why the law of moments is the principle of work; the three classes of levers. Seven problems follow.
The lesson, step by step
Hook question
SayHow can a small child on a seesaw lift a grown-up?
Read together
Read §§ 208–211. The law:
The moment of the acting force is equal to the moment of the resisting force.
Try it: the coin seesaw
Balance the ruler on the pencil at its middle. Put 3 coins 5 cm from the center. Where must 1 coin go to balance? (15 cm.) Where must 2 coins go? Check that coins × distance is the same on both sides, just as the book's 300 g × 15 = 100 g × 45 (§ 208).
Then try two stacks on one side and one on the other (§ 209).
Explain the idea
The moment of a force is force × its distance from the fulcrum. For balance, moments turning one way equal moments turning the other. The end that moves farther moves with less force: work in equals work out (§ 210).
The three classes
1st class: fulcrum in the middle (seesaw, crowbar, scissors). 2nd class: load in the middle (wheelbarrow, nutcracker). 3rd class: effort in the middle (tongs, your forearm, a broom). Sort the tools you gathered.
Say it back
Your student explains why a door handle is put far from the hinges.
Problems
Work problems 1–7 on book pages 154–155.
Afterwards
Find five levers around the house and decide the class of each.
Answers to the book's problems
1. About 0.67 kg (667 g). 2. Wheelbarrow: second class; grocer's scales: first; pliers: first; sugar tongs: third; claw hammer pulling a nail: first. 4. 3/7 ft. (about 5.1 in.) from the 300-lb. end. 5. 7.5 ft. 6. The boy 4 ft. from the load carries 30 lb.; the other carries 20 lb. 7. 3 ft. from the man who supports 70 lb. (7 ft. from the other).
Worked out for this site from the scan. If a number in your copy differs, trust the book.