Transcript The Big Four:

The Big Four:
• Copernicus
– Planets orbit Sun, not Earth
– but assume circular orbits (Greek idea of perfection)
• Kepler
– Orbits elliptical, not circular
– but described by simple laws
• Galileo
– Discovers moons of Jupiter
– Orbital motion is Universal
• Newton
– Idea of forces
theory of gravity
Newton’s Theory
Idea of “Escape Velocity”
• Gravity is a “force”
– causes acceleration or deceleration (= negative acceleration)
– g at Earth’s surface = 32 ft/sec/sec = 22 mph/sec = 9.8 meters/sec/sec
• Example:
– throw something up at 66 mph: it stops after 3 sec, then falls back in
another 3 sec
• How to calculate gravitational pull of a mass:
– increases with amount of mass:
2  mass  2  g
– decreases with square of distance from center of mass:
1
2  distance   g
4
Escape speed:
Competition: deceleration vs. weakening of gravity with distance
vEscape( Earth)  2  g Earth  REarth
g Earth  22 mph / sec  79,000 mph / hour
REarth  6378 km  3960 mi
vEscape ( Earth)  2  (79,200)  (3960) mph
 25,000 mph
The Speed of Light
• 1675: Olaus Roemer
(Danish astronomer)
measures by timing
“transits” of Jupiter’s
moons
Kaufmann, Universe, 4th ed., p.80
c  186,000 mi / sec  300,000 km / sec
 669,600,000 mph
The Idea of a Black Hole
John Michell (1783) , Pierre-Simon Laplace (1796):
• Ques: What happens if the escape speed from an object is
greater than the speed of light?
• Ans: If light consists of particles of matter, they would not be
able to escape
“dark star” ?
• The catch: early 19th cent - light is a wave (a disturbance),
not a particle
BH idea forgotten….
… until Einstein comes along