Download The Discovery of Planetary Motion: Tycho Brahe, Kepler, and Newton and more Study notes Physical Education and Motor Learning in PDF only on Docsity!
Birth of Science
- Tycho Brahe observes motion of planets
- Kepler discovers 3 laws of motion for planets
- Newton discovers laws of motion for all objects - Homework 2 is ready - Due at 6:00am on Tues, 23 rd^ Jan. - On angel.msu.edu, go to Lessons>Homework>Ho mework2.
Brahe (1546-1601) Kepler (1571-1630) Newton (1643-1727)
Questions on reading
1. When Kepler was a college student, the
most accurate description of the motion of
planets uses the terms
a. Velocity, position, & acceleration b. Circular orbits c. Elliptical orbits
What is fake on Tycho? (^) Brass azimuthal quadrant, 65 cm radius (ca 1576)
Uraniborg
Tycho Brahe’s Observations
- On Uraniborg, Tycho measured positions of the planets for 20 years
- Highly accurate & reliable - Accuracy limited by human eye, not by instruments. Superseded only with telescopes. - Tyco measured & compensated for instrument flexure, the biggest error. Great quadrant (1582)
Kepler analyzes Tycho’s data
- Kepler was Tycho’s assistant - 20 yrs’ data on planetary motions. - Tycho tried to fit data with Ptolemy-like model.
- Kepler analyzed the data
- Found 3-d orbits from 2-d positions in the sky
- Concentrated on orbit of Mars.
- Had to subtract off Earth’s (imperfectly known) orbit.
- Discovered 3 “laws,” which describe the motions of the planets. - Their meeting at Benatek (in Czechoslovakia) - …on 4 February 1600, Tycho de Brahe and Johannes Keplerus, co-founders of a new universe, met face to face, silver nose to scabby cheek. Tycho was fifty-three, Kepler, twenty-nine. Tycho was an aristocrat, Kepler a plebian. — Koestler, The Sleepwalkers , p
Brahe (1546-1601) Kepler (1571-1630)
Kepler’s Laws
- Law 1: Orbit of a planet is an ellipse, with
the sun at one focus.
- Law 2: The line joining the planet and the
sun sweeps out equal areas of space in
equal amounts of time.
2. Winter is a few days shorter than summer
for us in the northern hemisphere. Therefore
Earth is at A, B, C, or D in January?
Sun
Major axis Minor axis
A
D
C
B
Kepler’s Third Law
- P 2 = a 3
- P = period of orbit, in years
- a = semi-major axis of orbit, in AU. (Average Earth-sun distance is 1 AU.)
Fast solar simulation http://web.cuug.ab.ca/~kmcclary/fastsolar.html
The Motions of the Planets
Ptolemy
140 AD
Copernicus
Kepler
Simpler model More accurate description of data
Kepler’s 3 Laws
- Orbit of a planet is an ellipse, with the sun at
one focus.
- The straight line joining the planet and the
sun sweeps out equal areas of space in equal
amounts of time.
- P 2 = a 3
- But why??
- These are descriptive laws, but there must be deeper reasons for the planets to do this.
- Q4: Venus moves around the sun in a circle
at the same speed. Does Newton 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 the absence of a force, an object moves at the same speed in the same direction.
- Newton’s Second Law tells how to find the motion if there is a force. - Force = mass x acceleration F = m x a - Acceleration is change in velocity divided by amount of time
- Q5 The velocity changed in a. Case A only b. Case B only c. Neither cases A nor B d. Both cases A and B
- Q6 The acceleration is greatest for which case? A, B, C, or D - 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
