Transcript
  • 1

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    PowerPoint® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman

    Lectures by James Pazun

    Chapter 12

    Gravitation

    Modified by P. Lam 6_13_2011 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Goals for Chapter 12

    •  To study Newton’s Law of Gravitation

    •  Use Newton’s Law of Gravitation and Newton’s Second Law of Motion (F=ma) to calculate orbital speed of satellites and planets

    •  Use energy method to determine “escape velocity”

    •  Use Newton’s Law of Gravitation and Newton’s Second Law of Motion to explain Kepler’s Laws of planetary motion

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Newton’s Law of Gravitation

    •  The gravitational force is always attractive and its magnitude depends on both the masses of the bodies involved and their separations.

    !

    ! F g =

    Gm1m2

    r2

    G = 6.6742x10"11

    N • m2/kg

    2

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Superposition principle

    •  The total force on m1= force on m1 by m2 + force on m1 by m3

    m1 m2

    m3

    !

    Find ! F 1,! F 2and the net force

    ! F

    Express the forces in ˆ i , ̂ j .

  • 2

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Compare little “g” with big “G”

    Consider the gravitational force on a mass (m) by the Earth:

    F =GmME

    r2=ma

    ! a =GME

    r2

    Near Earth's surface ! r~RE

    ! a =GME

    RE2" 9.8

    m

    s2

    #

    $%&

    '(= g (free) fall acelleration)

    The magnitude of "g" decreases as the object gets

    further away from the Earth.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Satellite motion

    Orbital motion = projectile motion with very large initial velocity How large an initial velocity is required? Answer on next slide

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Calculate satellite orbits - assume circular orbit

    !

    This example consider satellite around the Earth

    (but the same method applies to motion of satellite around

    any large mass even for a planet around the Sun)

    (1) Draw free diagram on the satellite

    (2) Set up Newton's 2nd Law F = ma equation (a = v2 /r)

    " F = mv

    2

    r

    (3) Apply Newton's Law of Gravitaion : F =GmM

    E

    r2

    (4) Combine "GmM

    E

    r2

    = mv

    2

    r

    (5) Solve for v in terms of r

    " v =GME

    r

    For an orbit near the surface of the Earth

    (r # RE = 6.4x106m),

    v # 8x103m /s ~ 18,000mi /h

    m

    ME

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Calculate planetary orbits - assume circular orbit

    mp

    MS

    !

    GmpMS

    r2

    = mpv

    2

    r

    v2 =GMS

    r

    v =2"r

    T; T = orbital period

    #2"r

    T

    $

    % &

    '

    ( )

    2

    =GMS

    r

    #r

    3

    T2

    =GMS

    4" 2= same number for all

    planets in the solar system.

    (This called Kepler's 3rd Law)

  • 3

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Gravitational potential energy

    !

    The expression for the gravitaitonal

    potential energy for a mass at height

    h from the surface of the Earth :

    U = mgh is only valid for an object

    near the surface of the Earth.

    The more general expression is :

    U = " GmME

    r= potential energy for the

    mass - Earth system separated by a distance of r.

    Near the surface of the Earth r = RE + h

    U(RE + h) "U(RE ) = "GmME

    RE + h

    #

    $ %

    &

    ' ( " "

    GmME

    RE

    #

    $ %

    &

    ' ( ) m

    GME

    RE2h = mgh

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Calculate escape velocity

    •  What is the minimum initial velocity of a rocket for it to escape from the gravitational effect of the Earth?

    !

    Ki +Ui = K f +Uf

    1

    2mvi

    2+1

    2MEVi

    2"GmME

    RE= 0 +

    1

    2MEVf

    2"GmME

    #

    Vf $Vi

    %1

    2mvi

    2"GmME

    RE= 0

    % vi =2GME

    RE& 25,300mi /h

    !

    Note : If a star is very dense such that it has a large mass

    but a small radius, then vescape > speed of light!!

    Such a star is a "black hole".

    However, correct treatment of black hole requires

    Einstein's theory of Gravitation (General Relativity).

    Newton's theory of graviation is only approximate.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Newton’s Law of gravitation

    How did Newton come up with the Law of Gravitation?

    !

    ! F =

    Gm1m2

    r2

    Newton relied on the astronomical data of Kepler.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Kepler’s laws for planetary motion

    •  1st Law: Each planet moves in an elliptical orbit with the sun at one of the foci

    •  2nd Law: A line connecting the sun to a given planet sweeps out equal areas in equal times.

    •  3rd Law: The periods of the planets are proportional to the 3/2 powers of the major axis lengths in their orbits. (Planets further away from the Sun takes longer to go around the Sun)

  • 4

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Kepler’s 1st Law - elliptical orbits

    Elliptical orbits are consequence of the 1/r2 dependence of the gravitational force.

    The proof requires advanced calculus.

    Circular orbit is a special case of elliptical orbit.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Kepler’s 2rd Law - Conservation of angular momentum • 2nd Law: A line connecting the sun to a given planet sweeps out equal areas in equal times => a planet moves faster when it is closer to the sun.

    !

    Kepler's 2nd Law : dA

    dt= constant

    Relate dA

    dt to angular momentum (

    ! L ) of the planet about the Sun

    dA

    dt=

    1

    2

    ! r "! v =

    1

    2m

    ! r "m

    ! v =

    ! L

    2m

    dA

    dt= constant #

    ! L = constant # small r means larger v.

    ! L = constant when torque = 0.

    Newtons concluded that the gravitaional force must be directed

    along the "radius" from the Sun to the planet.

    Since ! F and

    ! r are along the same direction,

    ! r "! F = 0.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Kepler’s 3rd Law and Newton’s Law of Gravitation

    !

    Derived in an earlier slide, repeat here :

    Newton's 2nd Law : F = ma = mpv2

    r

    Newton's Law of gravitation : F =GmpMS

    r2

    = mpv2

    r

    " v2 =GM

    S

    r

    Assume circular orbit " v =2#r

    T, T = orbital period

    "2#r

    T

    $

    % &

    '

    ( )

    2

    =GM

    S

    r

    " T =(2# )r3 / 2

    GMS

    * r3 / 2 (Kepler's 3rd Law)

    Newton did this backward, he started with Kepler's 3rd Law and

    deduce his law of gravitaiton.

    Note : We can use Kepler's 3rd law to calculate the mass of the Sun

    based on the radius and the period of the Earth's orbit around the Sun.

    Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley

    Sun and planet revolve about the center of mass.

    We have assumed the Sun was stationary while the planet orbits around the Sun.

    In fact, a better way to view their motion is that they both revolve about their center of mass.


Top Related