Download Newton's Laws of Motion and Gravity: Understanding Natural Motion and Kepler's Laws and more Exams Astronomy in PDF only on Docsity!
Newton’s Laws of Motion
& Gravity—15 Sept
-^
De Revolutionibus Orbium Coelestium
, Copernicus, 1543
-^
Astronomia Nova
, Kepler, 1609
-^
Philosophiae Naturalis Principia Mathematica
, Newton,
1687
-^
Nature and Nature's laws lay hid innight:/ God said, Let Newton be!and all was light. —Newton’sepitaph by Alexander Pope
-^
Newton: Same laws apply to afalling apple & moving planet.
-^
Description of motion
-^
Gravity
1/R
2
implies K’s 3
rd
Law
1642–
Isaac Newton
1732-
George Washington
1492
Columbus sails
1607
Jamestown
1611
King James Bible
1636
Harvard College
1571–
Johannes Kepler
1564–
Shakespeare
1546–
Tycho Brahe
1473–
Isaac Newton (at 47) by Godfrey KnellerTrustees of the Portsmouth Estatewww.huntington.org/LibraryDiv/Newton/Newtonexhibit.htmCopernicus
Upcoming events
•^
Observing, Tues, 16 Sept– Roof of parking ramp behind planetarium– 7:40 to see volcanic effect on sunset– 8:05 to see moonrise, Jupiter, Venus
•^
Prof. Horace Smith on Northern Lights– http://www.pa.msu.edu/astro/after_dark/after_dark.pdf– Thurs, 18 Sept, 7:30, at Planetarium
•^
Bob Victor on What’s in the Sky– Fri, 19 Sept, 8:00, at Everybody Reads, 2019 E Mich
Ave, Lansing.
First test
-^
See practice test (link is on the syllabushttp://www.pa.msu.edu/courses/AST207)–
A few questions with verbal, numeric, or graphical answers.
-^
No multiple-choice questions.
-^
Material covered today will be on the test.
-^
First test is low risk: it counts only 5% of course grade.
-^
Homework 3 must be handed by start of class on Fri, 19
th
–^
Answers will be posted after class on Fri. See link on syllabus.
-^
Class on Fri is “Missouri Club”–
You must ask a question, preferably a question of detail.
-^
“How do you do question 3” is not detailed enough.
-^
You may bring one sheet of notes to use for Test 1.
“Natural” Motion for Newton &
Aristotle
•^
Natural motion is motion that needs no explanation: the objectnaturally moves that way.
-^
Aristotle: For heavenly objects, natural motion is motion in a circlewith the same speed. For base objects, natural motion is rest.–
A book falls off the table and comes to rest on the floor. This needs noexplanation because rest is the natural state.
•^
Newton: Natural motion is moving at the same speed in the samedirection.–
Newton’s First Law: In the absence of a force, an object moves at thesame speed in the same direction.
A book falls off the table and lands on the floor. For Newton, what isnatural, needing no further explanation?a.
The book is on the floor. b. The book is halfway to the floor.c.
The book is just starting to fall. d. I push the book off the table.
“Natural” Motion for Newton &
Aristotle
•^
Natural motion is motion that needs no explanation: the objectnaturally moves that way.
-^
Aristotle: For heavenly objects, natural motion is motion in a circlewith the same speed. For base objects, natural motion is rest.–
A book falls off the table and comes to rest on the floor. This needs noexplanation because rest is the natural state.
•^
Newton: Natural motion is moving at the same speed in the samedirection.–
Newton’s First Law: In the absence of a force, an object moves at thesame speed in the same direction.
Venus moves around the sun in a circle at the same speed. DoesNewton consider this motion natural?a.
Yes, Venus is a heavenly object. b.
Yes, the speed is the same. c.^
No, the direction is not always the same. d.
No, Venus is not at rest.
Newton’s Second Law
•^
Newton’s First Law: In theabsence of a force, an objectmoves at the same speed in thesame direction.
-^
Newton’s Second Law tellshow to find the motion if thereis a force.•
Force
=
mass
x^
acceleration
F
=
m
x
a
-^
Acceleration is change invelocity divided by amount oftime
•^
Q3 The velocity changed ina. Case A onlyb. Case B onlyc. Neither cases A nor Bd. Both cases A and B
-^
Q4 The acceleration is greatestfor which case?
-^
Case A•
Velocity at start
-^
Velocity after 1 s
-^
Case B•
Velocity at start
-^
Velocity after 1 s
-^
Case C•
Velocity at start
-^
Velocity after 1 s
-^
Case D•
Velocity at start
-^
Velocity after 2 s
Earth
Moon,1 s later
Newton discovers the law of gravity
-^
Newton was sitting under anapple tree and looking at themoon. An apple falls on hishead. Newton realizes the moonand the apple fall for the samereason. He does a quickcalculation and concludes thatthe force of gravity decreases asthe square of the distance.•
In a second, an apple falls 5 m.
-^
The moon stays up in the sky.What does Newton mean by“the moon falls?”
-^
The moon is 60 times fartherfrom the center of the earththan the apple. It falls 1/
2 as
far as the apple.
Moon
1.4mm
5 m
5m/(1.4mm)=3600=
2
If moon fell 83mm, then force ofgravity falls as distance5m/(83mm)=
