# AY 102 Lecture Notes - Lecture 3: Radiant Energy, Thermal Energy, Kinetic Energy

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29 Jan 2018
School
Department
Course
Professor
AY Class 1-26-18
Neton’s Uniersal La of Graitation, Telescopes, Intro to physical concepts
What is gravity?
- A force derived from matter
- Acts at a distance
- Always attractive
- One of four fundamental forces of nature
Strong nuclear force (holds atomic nuclei together; very short ranged)
Weak nuclear force (involved in radioactive decay)
Electromagnetism (acts between charged particles; 0.01 times strong nuclear
force)
Gravity
- Although gravity hold the Earth together and keeps our feet on the ground, it does NOT
hold our bodies or everyday small objects together. Electromagnetic forces do this.
How strong is the gravitational force between two objects?
- The universal law of gravitation
Every mass attracts every other mass
Attraction is directly proportional to the product of their masses
Attraction is inversely proportional to the square of the distance between their
centers
Freefall and weightlessness
- Weightlessness occurs when an object is in freefall, as in orbiting spacecraft or skydiving
- An orbiting object in freefall accelerates towards the Earth but never reaches the
ground due to the fast, sideways motion. The sideways speed of a skydiver is not fast
enough to avoid reaching the ground
The inverse square aspect of gravity
- Is why planetary orbits are closed (bound) ellipses or circles
- Also tells us that not all orbits have to close: unbound parabolic and hyperbolic orbits
are also possible
- Bound orbits are periodic; unbound orbits are not
The Universal Law of Gravitation tells us:
- Why planets move faster at perihelion than at aphelion
Because the gravitational force pulls along the line joining the planet and the sun
- Why planets farther from the sun move more slowly than closer in
Also because of inverse square law
- How fast something must move to achieve a circular orbit
- How fast something must move to escape another orbit
Intro to telescopes
- Telescope: a mechanical device designed to collect and focus light
- Three functions
Make faint objects easier to see
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