Download Physics 117

Physics 117
Chapter 2
Explaining motion
Extra Material
Heavier bodies go down faster
 It is common
experience that
heavier body go
down faster.
 This one of the
reason why you
have the other
three guys…
Aristotle
 Aristotle lived from 384-322BC, and
is universally considered as one of the
great thinkers of the ancient world.
He was born in the city of Stagira in
Macedonia.
 Aristotle was a student of Plato and
world-class philosopher in his own
right.
 Aristotle had a tremendous influence
on western culture. His work
covered: Logic,Physics,Psychology,
Natural History, Philosophy, Politics.
Galileo Galilei
 Galileo was born in Pisa, Italy on February 15,
1564. His father, Vincenzo Galilei, was a
musician.
 At the University of Pisa, Galileo learned the
physics of the Ancient Greek scientist,
Aristotle. However, Galileo questioned the
Aristotelian approach to physics.
 Galileo eventually disproved this idea by
asserting that all objects, regardless of their
density, fall at the same rate in a vacuum.
 To determine this, Galileo performed various
experiments in which he dropped objects from
a certain height.
 He wrote down his discoveries about motion in
his book, De Motu, which means "On
Motion."
The leaning tower of Pisa
Would you go up there to throw rocks
down?
Do you believe
Galileo really did? Did he really need to?
Free fall law
In the absence of friction (e.g. by air) all the
bodies fall at the same rate independently of
their mass
Summary of Lecture 2:
Falling Bodies


Aristotle’s view: heavier bodies fall faster
Galileo’s observations led him to a new word view. Using
the inclined plane he studied how bodies fall under the
influence of gravity. He observed that
1.
Bodies fall with a constant acceleration independent of
their mass (but which was increasing with the
increasing steepness of the inclined plane)
2.
In the absence of friction bodies tends to remain in
their state of motion which can be at rest or at
constant speed along a straight line (constant velocity)
3.
The distance, d, covered by a falling body was
proportional to the the square of the time lapsed.
Indeed:
v  at,
1
d  at 2
2
(a  const)
In the case of vertical inclination (free fall)
m
a  9.8 2  g
s
Inertia
QuickTime™ and a Sorenson Video decompressor are needed to see this picture.
Newton




Isaac Newton was born on Xmas day of 1642 in
Woolsthorpe, England.
As a child he did not show particular signs of brightness
but he managed to graduate at the University of
Cambridge (although without particular distinction).
When he was 23 the plague swept trough London and
Newton retreated to his mother farm. Here in few
months he laid the foundations for the work that made
him immortal among men.
Newton is responsible (among other things) for:




The discovery of the law of universal
gravitation.
The invention of calculus
The first theory of light
The formulation of the three laws of
dynamics
Newton’s First Law of
Dynamics
“Every object continues in a state of rest, or of
uniform motion on a straight line (constant
velocity), unless it is compelled to change that
state by an unbalanced force impressed upon it.”
The property of material objects to resist changes in their
state of motion is called inertia
Newton’s First Law of
Dynamics
“Every object continues in a state of rest, or of
uniform motion on a straight line (constant
velocity), unless it is compelled to change that
state by an unbalanced force impressed upon it.”
The property of material objects to resist changes in their
state of motion is called inertia
Newton’s Second Law of
Dynamics
“The net force on an object is equal to its
mass times its acceleration and points in the
direction of the acceleration
F  ma
The acceleration is inversely
proportional to the mass
Newton’s Second Law of
Dynamics
“The net force on an object is equal to its
mass times its acceleration and points in the
direction of the acceleration”
F  ma

Summary of the previous
lecture
• Newton’s second law: F  ma
• Units: [m]=kg, [F]=N=kg·m/s2
• Gravitational force:
Fgrav  W  mg
 R  v
• Air resistance:
(viscous media, no turbulence) or
R  gv2 (turbulence, generally fast fall)

• Terminal velocity:
W  R(v)
a

0

it is velocity
 such that
m
•For skydivers once the chute is opened R=gv2
Newton’s Third Law of
Dynamics
“If an object exerts a force on a second
object, the second object exerts an equal
force back on the first object”
Engine pushed forward
Flow pushed backward