Transcript Lecture1_Inertia

Galileo Galilei
(1564-1642)
Isaac Newton
(1642-1727)
Albert Einstein
(1879-1955)
How Things Work
The Physics of Everyday Life
3rd Edition
By Louis A. Bloomfield
(ISBN 0-471-46886-X)
John Wiley & Sons, Inc.
The working of familiar objects and
apparatus will illuminate a number of
fundamental principles in Physics.
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We’ll discuss the concept of FORCE
as a demonstration of the importance
of both
precise DEFINITIONS and
careful OBSERVATIONS
in science.
and
study falling objects as an example
of a scientific investigation
a combination of
Experience
to frame the basic
description
Observation
Intuition using common sense
Logic to apply these ideas to
new situations…draw conclusions
…make predictions
Imagine set
down at rest:
a glass on
the counter
a ball on the
billiards table
a book on a desk
if left undisturbed No poking, prodding,
pushing, or pulling
would remain at rest!
Just as obviously,
common experience
shows us that
we move objects
by force.
An object subject to NO
external forces will
A. be at rest.
B. eventually come to rest.
C. move at constant speed.
D. move with increasing speed.
We tend to take the force of friction for granted
It is an
external force
and in
its absense
it is natural
to COAST!
When released and no
longer in contact with
the bowstring (and
propelled by its tension)
the arrow does not stop!
An object subject to NO
external forces will
A. be at rest.
B. eventually come to rest.
C. move at constant speed.
D. move with increasing speed.
An object subject to a constant
net external force will
A. resist moving and remain at rest.
B. move but eventually come to rest.
C. move at constant speed as long as
the force continues to be applied.
D. move with increasing speed.
In an evenly matched arm-wrestling bout
or tug of war match
enormous forces may be involved,
but little motion!
Tension in
supporting cable
Weight, the
force due to
gravity
The tension
and weight
are exactly
equal and
opposite!
T
T = -W
W
They
balance!
T+W=0
In this case there
is no NET force!
Newton’s 1st Law
“The Law of Inertia”
An object at rest tends to remain at rest
constant
velocity of ?
and
an object in motion tends
to remain in motion
with the same speed constant
and in the same direction velocity
unless acted upon by an unbalanced force.
In the absence of a net force, a
object’s velocity remains constant!
A hockey puck slides across the
ice at constant velocity. The total
force acting on the puck must be
A) zero
B) greater than zero
Since the puck moves with constant velocity, the net force
on the puck must be zero. This is just Newton’s First Law!
You set your book on the bus seat
next to you. When the bus stops
suddenly, the book slides forward
off the seat. Why?
A) a net force acted on it
B) no net force acted on it
The action of the brakes acted to stop the bus itself.
Nothing other than the tiny friction with the plastic seat
upholstery acted to slow or stop the book.
A brief push
starts this
kid off from rest.
An unbalanced force changes his
tangential speed from 0 to v.
introducing the
mathematical convention
of “delta” representing
a small change.
If each time
he swings past
you give
about the
same PUSH
you will boost his speed
by an additional v.
His speed will grow and grow with
each successive push.
What if you keep up by
racing alongside
and continue to push?
Isn’t that like a bunch
of successive pushes
following uninterrupted?
Easy to
push
Hard to
push
To produce the same amount of acceleration,
need to push the car much harder!
How much?
Twice as much mass requires twice the force!
100 as much mass requires 100 the force!
Fm
If a lot of others join in, each contributing
an additional force, the net force will
be greater, and the acceleration greater.
Fa
together with F
m
can be combined consistently to give
Newton’s 2nd Law
F = ma
Unveiled at last spring’s
International Auto Show in Detroit:
The Toyota FT-HS is a
400-horsepower hybrid sports car
its 3.5-liter V6 engine
provide an acceleration of
0-to-60-miles-an-hour in 4 seconds
v
acceleration, a =
t
+60 miles/hour
4 seconds
= 15 miles/hour
sec
If X = Y then
A. 0
B. ½
X = ?
Y
C. 1
D. 2
E. XY
F. infinity
If 1 mile = 5280 ft
1 mile
then
= ?
5380 ft
+60 miles/hour
4 seconds
miles/hour
= 15
sec
Notice 60 miles/hour is the same as
miles 5280 feet
feet
60

= 316800
hour
1 mile
hour
feet
316800
hour
1
hour
feet

= 88
3600 seconds
sec
So alternately
+60 miles/hour
4 seconds
= 22 feet/sec
sec
or 22 ft/sec2
More generally, if an object is already
in motion before it starts accelerating:
present velocity = initial velocity + at
v  v0  at
starting velocity
at time=0
A car traveling 35 mi/hr accelerates
4
mi/hr
sec
over the next 5 seconds.
What final speed does it attain?
v = 35 mi/hr +
mi/hr
(4 sec )
5sec
= 35 mi/hr + 20 mi/hr
= 55 mi/hr
The Space Shuttle touches down
with a landing speed of ~100 m/sec
(224 mi/hour), and comes to rest at
the end of its runway within 40 seconds.
What average acceleration does it undergo?
v  v0  at
starting velocity
at time=0
0  100m / sec  a(40sec)
- 100m / sec  a(40sec)
- 100m / sec
 a -2.5m/sec2
40 sec
14 lb. medicine ball
Tennis ball Softball
2 ounces 6.5-7.0 oz.
(57 grams)
112 times
heavier
than a
tennis
ball!
32 times
heavier
than a
softball!
If all 3 balls were released simultaneously,
from the same height, they would reach ground
A. tennis ball first, medicine ball last.
B. medicine ball first, tennis ball last.
C. together at about the same time.
A large boat whose maximum speed
in still water is v1, tows a smaller boat
whose maximum speed is the smaller
v2.
across the lake. If both outboard motors
run together at full bore, the speed that
they travel together with will be
A. the smaller value v1.
B. the larger value v2.
C. somewhere between v1 and v2.
Some answers:
An object subject to NO external forces will
C. move at constant speed.
Notice that being at rest is just a special case of constant
velocity (= zero). An object’s inertia will keep it from
spontaneously changing its velocity: whether that’s
starting to move from rest, or slowing down from a nonzero speed. It takes force to start something moving, but
also to slow or stop it once its moving!
An object subject to a constant net external force will
D. move with increasing speed.
If any unbalanced force can start an object moving…then
a continuously applied force can only make it move faster
and faster.
People are confused when friction is high enough that an
object slows to rest shortly after they stop pushing. In
that case, the unbalanced force (friction alone) slows it to
a stop. Pushing hard enough to just make up for (match)
friction means all the forces have been balanced, and
then there is no net (or unbalanced) force left over to
speed the object up or slow it down.
Any push beyond the amount needed to overcome friction
will cause the object to accelerate. The net force would
be your push minus the pull of friction.