Announcements. HW #2 is online now at MasteringAstronomy.com. Due next Mon at 11pm. For today: finish reading chapter 4.

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1 Announcements HW #2 is online now at MasteringAstronomy.com. Due next Mon at 11pm. For today: finish reading chapter 4. Exam buy-back extra credit due here NOW! Not late extra-credit accepted.

2 Public Service Announcement Register to vote by Mon. Oct 6th (1 week!) Here on campus in student union, rockthevote.com, many other places online. You need: a stamp!

3 Exam #1, Re-visited Grades will be adjusted: Buy-back extra-credit will be added after adjustment. Answer key + your answers are online now (course website, grades tab). Let s go over the exam!

4 Last Time Kepler: perfected the earth-centered model of planetary motion. Keplers Law s: 1. Planet s orbit is ellipse. 2. Line connecting planet to sun sweeps out equal area in equal time (moves faster at perihelion, slower at aphelion). 3. P 2 =A 3

5 Last Time Kepler: perfected the earth-centered model of planetary motion. Keplers Law s: 1. Planet s orbit is ellipse. 2. Line connecting planet to sun sweeps out equal area in equal time (moves faster at perihelion, slower at aphelion). 3. P 2 =A 3

6 Which of the following best describes what would happen if Mercury and Jupiter were to switch places in their orbits about the Sun? A) Jupiter, the larger planet, would have a shorter orbital period than before. B) Mercury, the smaller planet, would have a shorter orbital period than before. C) Neither of the two planets would have any change in their orbital periods.

7 Which of the following best describes what would happen if Mercury and Jupiter were to switch places in their orbits about the Sun? A) Jupiter, the larger planet, would have a shorter orbital period than before. B) Mercury, the smaller planet, would have a shorter orbital period than before. C) Neither of the two planets would have any change in their orbital periods.

8 Gravity, Energy, and Motion

9 Describing Motion speed = rate at which an object moves. Distance/Time example: 10 m/s velocity = speed + direction. example: 10 m/s due West

10 The Acceleration Due to Gravity All falling objects accelerate at the same rate (not counting air resistance) independent of mass. On Earth, g 10 m/ s 2 : speed increases 10 m/s with each falling second 10 m/s per second or 10 m/s 2

11 Gravity The acceleration due to gravity is the same for any object on the surface of the earth. Hammer and feather fall at the same rate (neglecting air resistance).

12 Momentum and Force momentum = mass x velocity Anything in motion (velocity 0) has momentum associated with it A net force changes momentum, which means an acceleration (change in velocity) a change in momentum can result from a change in the direction of motion.

13 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

14 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

15 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

16 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

17 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

18 Thought Question Is there a net force at work? ( / ) 1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant speed 5) A planet orbiting the Sun at a constant speed

19 Force and Momentum We say a force causes a transfer of momentum.

20 the difference between mass and weight mass - The amount of matter in an object weight - The force that acts upon an object The amount of matter (number of atoms) in you never changes but your weight depends on how strong gravity is at your location.

21 What is the difference between mass and weight? The moon has weaker gravity than the Earth. On the Moon: A) you weigh the same, but your mass is less B) you weigh less, but your mass is the same C) You weigh more, but your mass is the same D) you weigh more, and your mass is more

22 What is the difference between mass and weight? The moon has weaker gravity than the Earth. On the Moon: A) you weigh the same, but your mass is less B) you weigh less, but your mass is the same C) You weigh more, but your mass is the same D) you weigh more, and your mass is more

Sir Isaac Newton English scientist, 1642 1727 Greatest scientist ever?

23 Sir Isaac Newton English scientist, Greatest scientist ever? Light, motion, gravity Newtonian physics dominated until Einstein. Invented calculus to help solve a different problem.

24 How did Newton change our view of the Universe? Realized that the same physical laws that operate on Earth also operate in the heavens. We can therefore use the laws of physics to understand the universe!

25 Newton s Three Laws of Motion I. An object remains at rest or moves at a constant velocity unless a net force acts to change its direction or speed. The Apollo command module keeps moving even without firing its engines

26 Newton s Three Laws of Motion II. If an object is acted on by a net force, then this force changes the momentum of the object, accelerating it in the direction of the force. Force = mass x acceleration F = ma Give me F, and I will tell you how an object moves (a).

reaction force The Space Shuttle is propelled upward with

27 Newton s Three Laws of Motion III. For every force, there is always and equal and opposite reaction force The Space Shuttle is propelled upward with a force equal and opposite to the force with which the gas is expelled out the rockets

28 Weightlessness Sense of weight comes from Newton s 3rd law. Earth s gravity pulls you down, but the floor pushes back with the same force: we feel weight! You are weightless anytime you are freely falling.

29 Weightless Astronauts An astronaut is weightless because: A) The ship s rockets holds them up B) There is no gravity in space C) They are constantly in free fall D) They are rotating like a merri-go-round See Interactive Fig. 4.4

30 Weightless Astronauts An astronaut is weightless because: A) The ship s rockets holds them up B) There is no gravity in space C) They are constantly in free fall D) They are rotating like a merri-go-round See Interactive Fig. 4.4

31 Law s of Conservation Newton s laws are really deeper statement about nature: conservation laws. Conservation of Momentum. Conservation of Angular Momentum. Conservation of Energy.

32 Conservation of Angular Momentum Angular momentum is a special kind of circling or rotating momentum. Law: The total angular momentum of a system cannot change unless it interacts with something (i.e., gives up or receives angular momentum) Explains Kepler s 2nd law!

34 Newton s laws of Gravity Every mass attracts every other mass. Strength of attraction is directly proportional to the product of their masses The strength of that attraction is inversely proportional to the square of the distance between them.

35 Workbook Time Break up into Groups of 25. Elect a chairperson, design a group logo and mascot, and compose a team anthem with guitar accompaniment. Turn to Newton s Law and Gravity on Page 29.

36 Reminders HW #2 Due in one week. Still (!!!) not on Mastering Astronomy? Come see me! Read Section 6.1 and 6.2 for next week!

Making Sense of the Universe: Understanding Motion, Energy, and Gravity Pearson Education, Inc.

Making Sense of the Universe: Understanding Motion, Energy, and Gravity 4.1 Describing Motion: Examples from Daily Life Our goals for learning: How do we describe motion? How is mass different from weight?