Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-15
Course Announcements
•
How is the sunset/sunrise observing going?
•
Dark Sky nights – Mon. 10/5 & Wed. 10/7 starting at
7:30pm – at the Observatory.
•
Exam-2 will be Friday, Oct. 9
•
SW-chapter 4 posted: due Mon. Oct. 5
Newton’s First Law of Motion
• A body remains at rest or moves in a straight line at
a constant speed unless acted upon by an outside
(net) force.
• A rockets will coast in space along a straight line at
constant speed.
• A hockey puck glides across the ice at constant
speed until it hits something
Newton’s Second Law of Motion
• (net)Force = mass x acceleration or
Fnet = m x a
• Acceleration is the rate of change in velocity – or
how quickly your motion is changing.
• Three accelerators in your car!!
Newton’s Third Law of Motion
• Whenever one body exerts a force on a second
body, the second body exerts an equal and opposite
force on the first body.
• Don’t need a rocket launch pad!
• The Bug and the Windshield – who is having the
worse day?
i-Clicker Question
Renaissance Astronomy: Applying Newton's Laws 1
Newton’s Law of Gravitation
• Newton’s law of gravitation states: Two bodies
attract each other with a force that is directly
proportional the product of their masses and is
inversely proportional to the square of the distance
between them.

Gm
m
1
2
F

grav
2
d
What the ….? I thought I understood gravity?
Newton’s Law of Gravitation
• To figure out the gravitational force just multiply the
mass of the two things together then divide by the
distance they are apart (squared).

Gm
m
1 2
F

grav
2
d
m1
d
m2
i-Clicker Question
Renaissance Astronomy: Gravity Fundamentals
Newton’s Law of Gravitation
g ~ 10 m/s2 “the acceleration of gravity” & g x m is your weight!
• Newton’s law of gravitation states: Two bodies attract each other with a
force that is directly proportional the product of their masses and is
inversely proportional to the square of the distance between them.
Gm
m
1 2
F
 2
grav
d
2


Nm

11
24


6
.
67

10
5
.
97

10
kg
m
object
2


kg

F

grav
6 2
6
.38

10
m
F
gm
grav




i-Clicker Question
Renaissance Astronomy: Gravity Direction; option 2
MATH TOOLS 4.1
 The gravitational acceleration at the
surface of Earth, g, can be solved for by
using the formula for the gravitational force
and Newton’s second law.
 The m cancels.
 g is the same for all
objects at the same R.
CONNECTIONS 4.1
 Gravity works on every part of every body.
 Therefore, self-gravity exists within a planet.
 This produces internal forces, which hold the
planet together.
CONNECTIONS 4.1
 There’s a special case: spherically
symmetric bodies.
 Force from a spherically symmetric body is the
same as from a point mass at the center.
Newton’s Laws and Kepler's Laws
• Newton’s law of gravitation and
his three laws of motion prove all
of Kepler’s laws
Newton's friend Edmund Halley
predicted the comet would return in
1758 and it did!
Lecture – Tutorial
Newton’s Law of Gravity: pg 29
 Work with a partner!
 Read the instructions and questions carefully.
 Discuss the concepts and your answers with one
another.
 Come to a consensus answer you both agree on.
 If you get stuck or are not sure of your answer, ask
another group.
 If you get really stuck or don’t understand what the
Lecture Tutorial is asking, ask one of us for help.