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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 .
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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.
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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)
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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.
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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)
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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.