1. The Science of Orbits
(Power Point 04)
Image Credit: NASA

2. Student Learning Objectives
Examine the contributions of ancient and modern astronomy to the understanding of gravity and planetary motion.
Recall and apply Kepler’s Laws to our solar system
Apply Newtonian physics to our solar system
Describe gravity and its effects
Analyze orbital motion

3. One More Geocentric Tycho Brahe (late1500’s)
Designed and utilized new instruments for measuring precise angles in the sky
Although Tycho Brahe believed in a geocentric system, his measurements were later used to provide proof for the heliocentric system. lol

4. What are Kepler’s Laws? Kepler (1600’s) Kepler’s 1st Law
Used Tycho Brahe’s observations and measurements of planetary positions to develop three laws of planetary motion.
Kepler’s 1st Law
Planets move in elliptical orbits with the Sun at one focus. (Ellipses)
Image Credit: study.com

5. Kepler’s 2nd Law
Planets sweep out equal areas of space in equal time intervals. (Equal Areas)
Image Credit:

6. Kepler’s 3rd Law
The orbital period of a planet is related to the semi-major axis of it’s orbit.
P2 = a3

7. Practice
1) Which planet has the longer orbital period? Saturn: a = 9.54 AU Jupiter: a = 5.2 AU 2) How do we measure birthdays?

8. The planets in our solar system follow orbits that are nearly circular.
Image Credit: NASA

9. Circles vs Ellipses
Eccentricity indicates the elongation of the ellipse. (e = 0 to 1)
Image Credit: NASA Earth Observatory

10. How are the motions of objects described?
Average speed is the amount of distance traveled in some amount of time. (65 mph)
Velocity is speed in a particular direction (65 mph South)
s = d t

11. Acceleration
Acceleration is the change of velocity in some amount of time.
Change in speed or direction
All objects on Earth have the same downward acceleration.
9.81 m/s2

12. Practice
How fast would an object dropped from the tower of Pisa be traveling after 2 seconds, in the absence of air?
Is the acceleration due to gravity greater on a book or a feather?
If a satellite travelled at a constant speed in a circular orbit, it would have zero acceleration (True or False)?

14. What are Newton’s Laws of Motion?
Newton’s 1st Law
An object will remain at rest or maintain a constant velocity unless an unbalanced force causes the object’s motion to change. (Inertia)
Inertia is the tendency of an object to maintain its motion.

15. Newton’s 2nd Law of Motion
An unbalanced force acting on a mass gives the mass an acceleration in the same direction as the unbalanced force. (F = ma)

16. Mass vs Weight Mass is a fundamental quantity.
Mass is the amount of material contained in an object.
Weight is the gravitational force acting on a mass. (W=mg)
Average Man
73 kg
Sun
1.989 x 1030 kg
Earth
5.972 x 1024 kg

17. Practice
1) Mass is often defined in elementary school as “the amount of space an object takes up”. Why is this not correct? 2) Would your mass be different if you were on the moon right now? 3) What would a 75 kg person weigh on Earth? What would this person weigh on Mars where the acceleration due to gravity is 3.72 m/s2?

18. Newton’s 3rd Law of Motion
When two objects interact, they create equal and opposite forces on each other. (Action-Reaction)
Same Pull
Opposite Directions
Stock Image

19. Space X

20. Practice
Apply each of Newton's laws to observations within our solar system. State an example of each law.
Do all of Newton’s Laws apply to an object at the same time? Explain.

21. F = GMm d2 What is Gravity? Gravity is a property of mass.
Newton’s Universal Law of Gravitation
Every object with mass attracts every other object with mass with a force.
F = GMm d2

22. Newton’s Mutual Force of Attraction
Both masses pull the same on each other!

23. Force decreases with the square of the distance.
Inverse Square Law
Force decreases with the square of the distance.

24. Practice
1) Is the gravitational force zero in space? 2) If Earth had twice as much mass, would this change your weight? Would it change your mass? 3) If the distance changes between two objects, does the acceleration due to gravity change?

25. What is Einstein’s description of Gravity?
Image Credit: Kurdistan Planetarium

26. Mass & Space Einstein’s Theory of Gravity
Every object with mass creates a curvature of space-time.
Mass does not create a force
Mass changes space
Space alters path of objects and light
More Mass = More Curvature

27. Image Credit: oneminuteastronomer.com

28. Practice Does an apple cause a curvature of space?
Why doesn’t the Moon fall into the gravity well caused by Earth?

29. Effects of Gravity
Orbits Atmospheres Galaxies Star Formation Shapes of Objects
Orbital Perturbations Synchronous Motion Weight Tides Motion

30. Gravity & Tides
Credit: NASA

31. Our Changing System Our Earth-Moon system is changing.
Earth’s rotation is slowing ( seconds/century)
Our Moon is drifting away (3.8 cm/year)
The synchronous orbit of the Moon (same face)
Image Credit: NASA

32. Image Credit: wired.com
Weightlessness
The feeling of weightlessness occurs when an object and its reference frame accelerate at the same rate.
Objects fall together.
Image Credit: wired.com

34. Practice The Sun's tidal affects are weak compared to the Moon. Why?
The astronauts on the space station have no weight. (true or false)
Does the moon have weight? Explain.

35. What is the physics of orbital motion?
Objects move in a straight line, unless an unbalanced force changes the motion.
In circular motion, a centripetal force is causing a change in motion.

36. Falling into Orbit

37. Practice
If the force keeping an object in a circular orbit is removed, what will be the path of the orbiting object?
2) What is a geosynchronous orbit?
3) What would be the path of an open orbit?

38. Center of Mass
Astronomical objects actually orbit about the center of mass of the system. (Elliptical Orbits)
Common
Center
Of Gravity

39. Conservation Of Energy
Elliptical orbits are maintained by conservation of mechanical energy.
GPEMax
KEMax
Image Credit: Planetary Society India