Astro Lecture 12. Energy and Gravity (Cont d) 13/02/09 Habbal Astro Lecture 12 1

Size: px

Start display at page:

Download "Astro Lecture 12. Energy and Gravity (Cont d) 13/02/09 Habbal Astro Lecture 12 1"

Adam Walters
5 years ago
Views:

1 Astro Lecture 12 Energy and Gravity (Cont d) 13/02/09 Habbal Astro Lecture 12 1

2 Energy due to movement of Kinetic Energy: object E k = ½ m v 2 13/02/09 Habbal Astro Lecture 12 2

3 Gravitational Potential Energy E g = m g h On Earth, it depends on an object s mass (m). the strength of gravity (g). the distance an object could potentially fall. 13/02/09 Habbal Astro Lecture 12 3

4 Impact Velocity of a dropped Egg Orbital trajectory of cannonballs Height of a Ball Thrown into Air 13/02/09 Habbal Astro Lecture 12 4

5 Mass-Energy Mass itself is a form of potential energy. E = 2 mc A small amount of mass can release a great deal of energy. Concentrated energy can spontaneously turn into particles (for example, in particle accelerators). 13/02/09 Habbal Astro Lecture 12 5

6 Conversion of mass to energy Conversion of Mass to Energy 13/02/09 Habbal Astro Lecture 12 6

7 What have we learned? What keeps a planet rotating and orbiting the Sun? Conservation of angular momentum Where do objects get their energy? Conservation of energy: Energy cannot be created or destroyed but only transformed from one type to another. Energy comes in three basic types: kinetic, potential, radiative. 13/02/09 Habbal Astro Lecture 12 7

8 The Force of Gravity [Section 4.4 ] Our goals for learning: What determines the strength of gravity? How does Newton s law of gravity extend Kepler s laws? How do gravity and energy together allow us to understand orbits? How does gravity cause tides? 13/02/09 Habbal Astro Lecture 12 8

9 What determines the strength of gravity? The Universal Law of Gravitation: 1. Every mass attracts every other mass. 2. Attraction is directly proportional to the product of their masses. 3. Attraction is inversely proportional to the square of the distance between their centers. 13/02/09 Habbal Astro Lecture 12 9

10 Inverse square law of gravity Inverse Square Law for Gravity 13/02/09 Habbal Astro Lecture 12 10

11 Exercise Compare the strength of gravity between Earth and Sun to that between Earth and Moon M(Sun) = kg M(Moon) = kg d(earth-sun) = 1.5x10 8 km d(earth-moon) = km G = m 3 /kg s /02/09 Habbal Astro Lecture 12 11

12 g = 9.8 m/s /02/09 Habbal Astro Lecture 12 12

13 How does Newton s law of gravity extend Kepler s laws? Kepler s first two laws apply to all orbiting objects, not just planets. Newton: Ellipses are not the only orbital paths. Orbits can be: - bound (ellipses) - unbound parabola hyperbola 13/02/09 Habbal Astro Lecture 12 13

14 Orbit of comets: Example: Sun grazing comets 13/02/09 Habbal Astro Lecture 12 14

15 Newton s version of Kepler s Third Law p2 = 4"2 G(M 1 +M 2 ) a 3 p = orbital period a = average orbital distance (between centers) (M 1 + M 2 ) = sum of object masses G = m 3 /kg s 2 13/02/09 Habbal Astro Lecture 12 15

16 Exercise: Newton s version of Kepler s Third Law p2 = 4"2 G(M 1 +M 2 ) a 3 Check that units of G are correct G = m 3 /kg s 2 13/02/09 Habbal Astro Lecture 12 16

17 Application of Newton s version of Kepler s Third Law Newton s version of Kepler s Third Law: If a small object orbits a larger one and you measure the orbiting object s orbital period AND average orbital distance THEN you can calculate the mass of the larger object. Example: Calculate the mass of Sun from Earth s orbital period (1 year) and average distance (1 AU) kg 13/02/09 Habbal Astro Lecture 12 17

18 How do gravity and energy together allow us to understand orbits? More gravitational energy since further away, less kinetic energy since v is smaller Total orbital energy gravitational + kinetic stays constant if there is no external force. Less gravitational energy since closer to Sun, more kinetic energy since v is larger Orbits cannot change spontaneously. 13/02/09 Habbal Astro Lecture 12 18

19 Changing an Orbit So what can make an object gain or lose orbital energy? Friction or atmospheric drag A gravitational encounter Comet loses orbital energy to Jupiter, changing its unbound orbit around the Sun 13/02/09 Habbal Astro Lecture 12 19

20 Escape Velocity: The velocity needed for an object to completely escape the gravity of a large body such as moon, planet, or star If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit). 13/02/09 Habbal Astro Lecture 12 20

21 Escape velocity For object to escape gravity must have: E k E p ½ m v 2 m g h Or for Earth, h = R v esc 2 g R 2 G M/R independent of mass of object 13/02/09 Habbal Astro Lecture 12 21

22 Escape velocity: Exercise Calculate escape velocity for Earth v esc 2 G M/R G = M = kg R = 6378 km V esc = 11 km/s 13/02/09 Habbal Astro Lecture 12 22

23 Escape velocity: Illustrated Escape velocity from Earth 11 km/s from sea level (about 40,000 km/hr) Escape velocity from Earth 13/02/09 Habbal Astro Lecture 12 23

24 Escape and orbital velocities don t depend on the mass of the cannonball. 13/02/09 Habbal Astro Lecture 12 24

4.3 Conservation Laws in Astronomy

4.3 Conservation Laws in Astronomy Our goals for learning: Why do objects move at constant velocity if no force acts on them? What keeps a planet rotating and orbiting the Sun? Where do objects get their