A First Course in Physics · Ch. II, §§ 49–56 · Chapter 2: Force and Motion
6. Newton's three laws of motion
Goal: Your student can state Newton's three laws, explain inertia and momentum, and use F = ma with dynes (and newtons).
You'll need
- The book, open to these pages
- A playing card and a coin on top of a glass (inertia trick)
- Two balls of equal size (for collisions)
- A balloon
- A string and a book (for problem 5)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Newton's first law (inertia) and centrifugal force; momentum; the second law, tested on the grooved board; the dyne and why a gram of force is 980 dynes; F = ma; the third law (action and reaction) with the swinging steel balls. Thirteen questions and problems follow.
The lesson, step by step
Hook question
SayWhen a bus stops suddenly, why do the passengers lurch forward? Nobody pushed them.
Read together
Read §§ 49–56. The first law in Newton's words as the book gives them:
Every body continues in its state of rest or uniform motion in a straight line unless impelled by external force to change that state.
Try it: inertia
Put a card on top of a glass and a coin on the card. Flick the card sharply sideways. The coin drops into the glass, because its inertia keeps it where it was.
Tie a thin string to a heavy book, with a second string hanging below (problem 5). A sudden jerk on the lower string breaks the lower string; a slow pull breaks the upper one. (Use thin thread and keep feet out from under the book.)
Explain the second law
Momentum is mass × velocity. A force changes momentum, and twice the force makes twice the change each second. So F = ma.
The book's unit of force is the dyne (1 gram × 1 cm/sec²); a gram of force is 980 dynes. In SI, force is in newtons: 1 kg × 1 m/s² = 1 N = 100,000 dynes.
Try it: action and reaction
Blow up a balloon and let it go: air rushes back, balloon rushes forward. Roll one ball into an identical ball at rest, head on: the first stops and the second moves off, as in the steel balls of Fig. 27.
What we know now
Newton's laws are still used to build bridges and fly spacecraft. Einstein's relativity (1905) showed they need correcting only at speeds near the speed of light, and quantum physics at the scale of atoms. "Centrifugal force" is today usually explained as inertia, just as the book does.
Say it back
Your student states the three laws in their own words and gives an everyday example of each.
Problems
Work questions 1–13 on book page 37. Problems 6–9 use F = mv/t.
Afterwards
Spot the third law this week: swimming, walking, rowing, a garden hose kicking back.
Tip
In problem 7, a metric ton is 1,000,000 grams; 4 minutes is 240 seconds.
Answers to the book's problems
6. 3 cm/sec. 7. About 23.5 cm/sec (force 2,000,000 × 980 dynes on a mass of 20,000,000,000 g for 240 sec). 8. About 8.16 metric tons (mass 200,000,000 g, acceleration 40 cm/sec², force 8,000,000,000 dynes ÷ 980,000). 9. Their momenta are equal and opposite, so the moon would move 80 times as fast as the earth. 10. More, because part of their weight now goes to keeping them moving in a circle. 13. They strike at the same time; the horizontal motion does not change the downward acceleration. The other questions are explanations: inertia (1–5, 12) and reaction (11).
Worked out for this site from the scan. If a number in your copy differs, trust the book.