Download Newton`s First Law of Motion

Newton’s First Law of
Motion—Inertia
Chapter 4
Aristotle (4th Century B.C.)
 Believed
in two different types of motion:
natural motion & violent motion
 Natural Motion – either straight up or
straight down, objects would seek out their
natural resting places (it was “natural” for
heavy things to fall and light things to rise)
 Violent Motion – imposed motion, result of
forces that pushed or pulled
 The proper state of objects was that of
rest.
Aristotle
Copernicus (1473-1543 A.D.)
 Because
Earth was already in its natural
resting place, it could not move (there was
no force big enough to move Earth)
 Copernicus concluded that the only way to
make sense of the way the planets move
is to assume that Earth and the other
planets move around the sun
 He worked in secret to avoid being
persecuted by the church
Copernicus
Galileo
 The
foremost scientist of the lateRenaissance in Italy
 Outspoken in his support of Copernicus,
later put on trial and confined to house
arrest
 A force is any push or pull
 Friction – the force that acts between
materials that touch as they move past
each other
 If friction were absent, an object would
need no force to remain in motion
Galileo
Galileo’s Inclined Planes
 Noted
that a ball that rolls down an
inclined plane picks up speed
 When the same ball rolls up the inclined
plane, it slows down
 When a ball rolls on a flat horizontal
surface, it rolls at near constant velocity
 Stated that with the absence of friction, the
ball would roll forever
Galileo’s Inclined Planes Cont.
 If
you have two inclined planes face each
other, a ball rolled down one plane will
reach nearly the same height as it rolls up
the other plane
 He noticed that the ball ended up at the
same height, even is the plane was
elongated or at a different angle
 Inertia – the property of a body to resist
change (the tendency of a moving body to
keep moving and every material object
resists change to its state of motion)
Inclined Planes
Newton’s First Law
 Every
object continues in a state of rest, or
of motion in a straight line at constant
speed, unless it is compelled to change
that state by forces exerted upon it.
 An object will keep doing what it’s already
doing
 Toss an object out the door of the
International Space Station, and it will
keep moving at the speed you threw it at
forever (no friction in space)
Newton’s 1st Law
The Law of Inertia
The Law of Inertia
Pioneer and Voyager

After they initially
were sent into space,
these two spacecraft
utilize Newton’s Law
of Inertia to move
through space
 They are now cruising
outside of our solar
system
Mass – A Measure of Inertia

The amount of inertia an object has depends on its
mass—which is roughly the amount of material
present in the object
 Mass is NOT volume, the measure of space that an
object takes up
 Mass is NOT weight, the force of gravity on an object
 Mass is a measure of the inertia that an object exhibits
in response to any effort made to start it, stop it, or
otherwise change its state of motion
 Mass and weight may not be the same, but they are
proportional to each other
Weight = mass x acceleration due to gravity
Measured in Newtons (N)
A 1-kg bag of nails weighs 9.8 N on the surface of Earth
(2.2 lbs.)
Net Force
 In
the absence of a net force, objects do
not change their state of motion
 If you push with equal and opposite forces
on opposite sides of an object at rest, it
will remain at rest
 Net Force – the combination of all forces
acting on an object
 We use force diagrams to figure out the
net force
Force Diagrams
Equilibrium
 If
only the force of gravity was acting on an
object at rest, the object would constantly
be in free fall
 The fact that the object is at rest means
that another force must be acting upon it
 The other force exactly balances out the
weight of the object and produces a net
force of zero
 The other force is called the support
force, or normal force
 Equilibrium – the net force on an object is
zero
Equilibrium
Vector Addition of Forces
 We
can use the same vector addition
techniques on forces as we did on
velocities!
 Force, like velocity has a magnitude and a
direction
 For any pair of scales, ropes, or wires
supporting a load, the greater the angle
from the vertical, the larger the tension
force in them
 The resultant of the tension forces in a
rope must be equal and opposite to the
load being supported
Vector Addition of Forces
Assignment (Due Wednesday 10/8)
 Read
Chapter 4 (pg. 43-55)
 Do Ch. Assessment #21-39 (pg. 57-58)
 Appendix F #1-10 (pg. 665-666)