Lecture 3
Kepler's and Newton's laws
Monday July 7, 2008
We opened class by reviewing the phases of the moon using this
applet.
Next there was an activity (see below) and then a powerpoint presentation on
Kepler's and Newton's laws (available on Angel).
Here is a brief video about
gravity.
A reminder that there will be no class Tuesday; you are to attend
Astrofest
instead. There will be a substitute to present Wednesday's lecture on
the nature of light and blackbody radiation.
Concepts:
- Kepler's 1st law: Planets move in elliptical orbits (not circular)
- Kepler's 2nd law: Planets sweep out equal areas in equal time (move faster closer to sun)
- Kepler's 3rd law: Period and semi-major axis are related; outer planets have longer periods (a^3=P^2)
- Newton's 1st law: Objects in motion tend to stay in motion unless acted upon by outside force (inertia)
- Newton's 2nd law: Force equals mass times acceleration (F=ma; consequently a=F/m)
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- Newton's 3rd law: For every action there is an equal and opposite reaction (forces come in pairs)
- Gravity: Attractive force between two bodies that increases with their masses and decreases quickly with distance
Vocabulary:
- Semi-major axis: Half the longest distance across an ellipse (radius for a circle)
- Perihelion/aphelion: Closest/farthest approach to sun in orbit
- Mass: How much matter is contained within an object (more massive = heavier on Earth)
- Acceleration: Change in velocity with time
Activity:
The activity involved exploring Kepler's third law using this flashlet
to measure the period of the inner planets, then graphing the period against their semi-major axis to look
for a trend. This result was used to estimate the period of a hypothetical planet
twice as far from the sun as Earth and then one half as far. This flashlet
was used to explore elliptical orbits, and then we completed a worksheet on Kepler's second law.
Random link:
Excerpt from Discovery Channel article titled "Shrinking Mercury sported active volcanoes: latest photos"
Newly released images reveal huge cliffs scaling 15,000 feet and large cracks spanning widely across Mercury's exterior.
NASA scientists believe that the tiny planet's inner, iron-rich core is cooling down and solidifying. As this process occurs Mercury is contracting in on itself, causing its outside shell to crack and buckle.
Updated July 7, 2008