1. Orbits
Round and round we go!

2. What if…
I fire a cannon.
How far will the shot go?

3. Orbits Orbits are one body “falling around” another
The object just keeps missing the ground

4. Orbits Depend on forward speed.
An object has to have enough speed to keep “missing”

5. What is an orbit?

6. Orbital Speed Different orbital distances require different speeds
Closer orbit = faster
Further orbit = slower
Orbital period = time for one complete orbit

7. Orbits are NOT always circular
Orbits are oval
Ellipses
Some orbits are more elliptical than others
Orbits of the planets are nearly circular

8. Ellipses Have two foci Have two axes
Major
Minor
Half of the major axis is the semi-major axis (A)
Semi-major axis is the average distance from one foci to the ellipse

9. eccentricity (e)
e = c a
A measure of the “circularity” of an ellipse

10. eccentricity (e) Between 0 and 1 Closer to 1 = more elongated
Closer to 0 = more circular

11. eccentricity (e) circular orbit: e=0 elliptic orbit: 0<e<1
parabolic trajectory: e=1
hyperbolic trajectory: e>1

12. Eccentricities of some planet orbits
Eccentricity
Mercury
0.2056
Venus
.0068
Earth
.0167
Mars
.0934
Jupiter
.0483
Saturn
.0560
Uranus
.0461
Neptune
.0097
Pluto
.249

13. What if the sun disappeared?

14. Kepler’s Laws Kepler made his laws for planets orbiting the sun
But they work for anything that orbits something else
Planet around Sun
Comet around Sun
A moon around a planet
Star around another star
Solar system around galaxy 14

15. Kepler’s First Law
Planets move in elliptical orbits with the Sun at one focus

16. Orbits are elliptical
The planet has the same pull on the Sun as the Sun has on the planet.

17. Orbits are elliptical The Sun causes the planet to orbit
The planet causes the Sun to wobble

20. Kepler’s Second Law
Planets move fastest when they are closest to the sun
Planets sweep out equal areas in equal times 20

21. 21

22. Kepler’s Third Law
Outer planets have farther to go and move more slowly in their orbits around the sun
The square of a planet’s orbital period equals the cube of the orbit’s semi-major axis.
A period is the time it takes for a regularly repetitive process to repeat. 22

23. In astronomical units (AU)
Kepler’s Third Law
Semi-Major Axis
Period
P2 = A3
In astronomical units (AU)
In years
An astronomical unit is the average distance from the Sun to the Earth, about 150 million kilometers 23

24. Orbits are ellipses

25. Why don’t things slow down and hit?
What would slow them down?
There is no air in space to provide friction.
With no friction, objects in orbit don’t slow down.
If they don’t slow down, they stay in orbit forever. 25

26. What if… What if something has a lot of velocity?
What if it moves faster than is required to stay in orbit? 26

27. Escape Velocity
The minimum velocity required for an object to escape from the gravity of another object
Escape Velocities:
Earth – 11.2 Km/s
Sun – Km/s 27

28. Orbits are predictable
Nothing is changing the orbit of things
So they can be predicted
Very accurately

29. So orbits are… Produced by the right combination of
Forward motion
Gravity
An object falling around the other (or both)
A relationship between distance and speed
Take one period to complete
Ellipses
Can be escaped with enough velocity
Predictable 29

30. Gravity
Force of gravity is proportional to the masses of the two objects
Force of gravity is inversely proportional to the distance between the two objects

31. True or false…
In an orbiting spacecraft, you are not affected by gravity?
FALSE!

32. In orbit… Things in orbit are falling They just keep missing
Without gravity they would just fly away!

33. Free Fall! The motion of an object when only gravity is acting on it.
Free fall is the same as free float

34. Are we falling now? No, there is something in the way… The ground
Gravity holds us to the ground AND the ground holds us up.
The force we exert against the ground is called weight.

35. Gravity and Force Gravity is a force at a distance
Caused by the masses of the two objects
Gravity is an acceleration
F = m a g g

36. Excuse me… that’s not enough.
So gravity …
Is inversely proportional to distance
Is proportional to mass
Is an acceleration
Causes a mass to exert a force at a distance
Right! Exactly!
Excuse me… that’s not enough.

37. Orbits and Gravity