Galileo’s experiments on falling objects
Apollo 15 demonstration
Feather and Hammer Drop
https://en.wikipedia.org/wiki/Galileo's_Leaning_Tower_of_Pisa_experiment
Assignments
•  Read/Study Ch. 4, “Making Sense of the Universe:
Understanding Motion, Energy & Gravity”
•  Do Online Exercise 05 (including “Energy” tutorial)
Happy Fall Equinox!
•  at 4:22 a.m. today, the Sun crossed from
north to south of the celestial equator
•  12 hours night = 12 hours day (almost)
http://www.timeanddate.com/astronomy/usa/philadelphia
Lunar Eclipse!
•  This Sunday (Sept. 27)
–  Starts around 8 p.m.
–  Max around 10 p.m.;
–  umbral eclipse lasts about an hour
•  Also a “SuperMoon”
–  14% closer than avg, => 14% larger
–  last supermoon eclipse was 1982; next 2033
•  Hope you will all view, take pics
–  send to me and I’ll post the best on class webpage
Chapter 4
Making Sense of the Universe:
Understanding Motion, Energy, and Gravity
“The eternal mystery of
the world is its
comprehensibility.
The fact that it is
comprehensible is a
miracle.”
Albert Einstein (1879 – 1955)
How do we describe motion?
Precise definitions to describe motion:
•  Speed: Rate at which object moves
%
speed = distance "$#units of m
'
s
&
time
€
Example: speed of 10 m/s
•  Velocity: Speed and direction
Example: 10 m/s, due east
•  Acceleration: Any change in velocity;
units of speed/time2 (m/s2)
Which of the following is not
changing for someone driving a car
in a circle at 30 miles per hour?
• 
• 
• 
• 
• 
position
velocity
speed
momentum
none of the above (all are changing)
Which of the following is not
changing for someone driving a car
in a circle at 30 miles per hour?
• 
• 
• 
• 
• 
position
velocity
speed
momentum
none of the above (all are changing)
Acceleration of Gravity
•  All falling objects
accelerate at the
same rate
(not counting air
resistance).
•  On Earth, g ≈ 10 m/
s2: speed increases
10 m/s with each
second of falling.
v=gt
Note: independent of mass m
How is mass different from weight?
•  Mass—the amount of matter in an object
•  Weight—the force that acts on an object
Weight = mass x gravity
W= m g
Thought Question
On the Moon, which has 1/6 Earth’s gravity:
A.  your weight is the same; your mass is less.
B.  your weight is less; your mass is the same.
C.  your weight is more; your mass is the same.
D.  your weight is more; your mass is less.
Clicker Question
On the Moon, which has 1/6 Earth’s gravity:
A.  your weight is the same; your mass is less.
B.  your weight is less; your mass is the same.
C.  your weight is more; your mass is the same.
D.  your weight is more; your mass is less.
Clicker Question
On the Moon, which has 1/6 Earth’s gravity:
A.  your weight is the same; your mass is less.
B.  your weight is less; your mass is the same.
C.  your weight is more; your mass is the same.
D.  your weight is more; your mass is less.
gmoon = gearth/6
mgmoon = mgearth/6
Wmoon = Wearth/6
Newton changed our view of universe
•  Realized same physical laws
that operate on Earth also
operate in the heavens.
•  Discovered laws of motion
and gravity.
•  Much more: Experiments
with light; first reflecting
telescope, calculus…
Sir Isaac Newton
(1642–1727)
Newton's three laws of motion
1.  Object moves at constant velocity unless a net force
acts to change its speed or direction.
2.  Force = mass x acceleration: F=m a
3.  For every force, there is always an equal and opposite
reaction force: F12 = - F21
Newton's first law of motion
An object moves at
constant velocity unless a
net force acts to change its
speed or direction.
Newton's second law of motion
Force = mass x acceleration.
Newton's third law of motion
For every force, there is
always an equal and
opposite reaction force.
Clicker Question
Is the force that Earth exerts on you larger,
smaller, or the same as the force you exert on it?
A.  Earth exerts a larger force on you.
B.  You exert a larger force on Earth.
C.  Earth and you exert equal and opposite
forces on each other.
Clicker Question
Is the force that Earth exerts on you larger,
smaller, or the same as the force you exert on it?
A.  Earth exerts a larger force on you.
B.  You exert a larger force on Earth.
C.  Earth and you exert equal and opposite
forces on each other.
Newton’s law of Gravity
GMm
F = _____
2
d
d
M
m
Gravity g on a planet with
Mass M and Radius R
GMm
F = _____ = W = m g
R2
M
R
m
GM
g = ____
R2
Mass vs. Weight?
•  mass – the amount of matter in an object
•  weight – a measurement of the force that acts on an object
•  When in “free-fall,” you are weightless!!
Relationship Between Cannonball's Speed and Orbital Trajectory
Launching cannonball from earth