(16) Newton and his Laws
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements: "Newton and his Laws," section #10 |
The list below is significant and deserves to be copied from the blackboard.
Terms: Force, mass
Stories and extras: Alexander Pope's couplet:
God said: "Let Newton be!" and all was light.
Starting the lesson: The story of astronomy and space, as we follow it, is essentially a story of discovery.
In the 1600s, the picture of our world seemed to come together. Motions, at least those of isolated objects, seemed to follow certain laws: Copernicus made sense of the motion of the Earth and planets, Kepler made it possible to predict such motions, Galileo found a regularity in the falling of objects.
But that seemed just a beginning. Every observation, every solved problem, seemed to bring up new questions:
Newton, born in 1642, guessed that there existed some basic laws which governed these and other motions. If we understood those laws, we could explain everything. He was right, and he discovered those laws, too--they are now known as Newton's three laws of motion.
It is easy enough to state them, to learn what they say, but that is not enough. To use them properly, one must understand their meaning and become familiar with them through examples. Today we begin the process, and we will proceed quite carefully.
Guiding questions and additional tidbits
(Suggested answers, brackets for comments by the teacher or "optional")
-- Who was Isaac Newton? What were his three main contributions to science?
[Possible project: have a student prepare 5-minutes presentations on Newton, based on web sites, encyclopaedia entries and other material.]
--What concepts did Newton introduce to the study of motions?
(2) Inertia, which resisted motion.
(Today we also call inertia "mass," a concept discussed again in a later lesson. Mass is seen as measure of the amount of matter--the more there is of it, the greater the inertia.
At this point, introduce the formulation of the 3 laws, approximately as given on the web site, with brief discussions. It is best if the students copy them down.
--What did Newton mean by "Force?
-- Can you give examples?
--Gravity causes objects to fall
--Water resistance slows down the fall of a stone through water.
--Pressure of gas inside a gun makes a bullet fly.
--Pressure of hot gas inside a car engine pushes its pistons (which ultimately turn its wheels).
--The force of the jet from a rocket causes it to lift up.
--A magnet makes a nail move to it, or rotates a compass needle
--A compressed spring can pop out the refill in a ball-point pen.
.. .. .. .. .. and so on!
What a force actually does depends on the resistance it meets. Sometimes the resistance is large enough to stop any motion, as in this example:
You stand on the floor, and the force of gravity strongly pulls your body down, to the center of the Earth. Why doesn't it move? --
--Second case: Sometimes the force overcomes resistance. When you lift a brick from the floor, the force of your arm overcomes the pull of gravity.
It does not give the brick much speed (in this example) but it performs work and invests energy, which can perhaps be later recovered and converted to some other form. We have already mentioned energy, and will come back to work later again.
.. ..(many other examples involving friction)
--Pulling a boat through water, against resistance, or an airplane through the air.
--Pulling a nail away from a magnet.
--Pounding a nail into a board of wood. The force of the moving hammer overcomes the resistance of the wood.
--Third case: Suppose you apply a force to an object, and only negligible forces oppose it. What happens?
--A rocket being launched.
--A bullet being fired. There exists resistance, but it is small compared to the push of the burning gunpowder.
--A ball rolling down an incline (although some of the force also goes to spin up the ball)
--A stone being shot by a slingshot
--A pitched baseball accelerates, although some of the force (generated by muscles) also goes to accelerating the pitcher's arm.
--By Newton's first law, does mere motion require a force?
--Suppose your car stalls on a railroad track, and a freight train is coming. What should you do?
[The teacher may also tell the story of supertanker captains, training on heavy, underpowered boats.]
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: audavstern("at" symbol)erols.com .
Last updated: 12.17.2001