Transcript Newton`s 1st Law of Motion

NEWTON’S 1ST LAW OF
MOTION - Inertia
Chapter 3
Objectives






Describe Aristotle’s concept of motion.
Describe Copernicus’ idea about Earth’s motion.
Describe Galileo’s idea about when a force is
needed to keep an object moving.
State Newton’s first law of motion.
Describe the relationship between mass & inertia.
Explain how the law of inertia applies to objects in
motion.
3.1 Aristotle on Motion

Aristotle believed there were 2 types of motion.
•
•
Natural motion on Earth was straight up & down;
circular motion was natural for the “heavens”. Natural
motion was not thought to be caused by forces.
Violent motion was the result of forces. Objects in their
natural resting places could not move by themselves;
they had to be pushed or pulled.
3.2 Copernicus and the Moving Earth



According to Aristotle, the Earth was too massive to
be moved by an outside force. It was believed then
that the planets and stars moved in perfect circles
around the Earth.
Copernicus interpreted astronomical observations in
another way: Earth and the other planets moved
around the sun.
Copernicus was persecuted because he did not
believe that the Earth was the center of the
universe.
3.3 Galileo on Motion



Galileo supported Copernicus and did not believe
Aristotle’s theory of motion.
Galileo believed a moving object did NOT need a
force to remain in motion.
He argued that a force was needed to keep an
object moving only when friction was present.
Friction
Forward motion (push)
3.3 Galileo on Motion

Recall that friction acts between materials as they
move past each other. It is caused by irregularities
(sometimes microscopic irregularities) in the surfaces
of the objects that are touching.
3.3 Galileo on Motion
Galileo rolled balls along surfaces tilted at different angles.
a. As the ball moves
downward, it moves
with gravity, so its
speed increases.
Initial position
Final position When it moves
upward against
gravity, its speed
decreases. The ball
reaches its initial
height before stopping on
the 2nd incline.

3.3 Galileo on Motion
b. As the angle of the
incline is reduced,
the ball rolls a
greater distance
before reaching its
initial height.
Initial position
Final position
3.3 Galileo on Motion
c. What about a ball rolling along a level surface?
Since it would not be moving with or against
gravity, it would have a constant velocity. Galileo
stated that if friction were absent, the ball would
move horizontally forever.
Initial
position
Final
position?
3.3 Galileo on Motion
•Galileo believed moving objects
tended to "keep on doing what they
were doing.“
•He developed the concept of inertia.
•Inertia is the resistance an object has to
a change in its state of motion.
3.4 Newton’s Law of Inertia
Newton’s first law is also called the law of
inertia.
□ An object at rest tends to stay at rest and an
object in motion tends to stay in motion with the
same speed in a straight line unless acted on
by a nonzero net force.
□ In other words, as Galileo maintained, things
“tended to keep on doing what they were
doing.“

Applications of the

The head of a hammer
can be tightened onto
the wooden handle by
banging the bottom of
the handle against a
hard surface. Why?
st
1
Law
What happens ….?
•To the car?
•To the man (without seatbelts)?
•To the man (with seatbelts)?
3.5 Mass – A Measure of Inertia
• The tendency of an
object to resist changes
in its state of motion is
dependent upon its
mass.
• If you kick an empty can
and a can filled with
sand, the empty can will
move easier.
• The can filled with sand
has more mass and
inertia.
3.5 Mass – A Measure of Inertia


Mass is a measure of the actual material in an
object. It depends only the number and kinds of
atoms that compose it.
The more mass an object has, the greater its inertia.
This means it will require more force to change its
motion.
Mass and Volume





Mass is not volume.
Mass is measured in kilograms or grams.
Volume is measured in liters, milliliters, cm3 or m3.
Volume is a measure of space – how much space
something occupies.
Mass is how much STUFF something has; volume is how
much space the stuff takes up.
Mass is NOT Volume
A one liter can of gravel and a one liter can of ping
pong balls both have the same volume but have
very unequal masses and inertias.
Suppose you have a pillow and a book. Which has
a greater volume? Which has a greater mass?
Mass and Weight


A heavy object has a lot of matter. The amount of
matter (its mass) is a fundamental property of the
object.
If we measure the gravitational attraction of the
object to Earth, we determine its weight. Weight is
defined as the force on a body due to the
gravitational attraction of another body (Earth).
Weight is a force on an object.
Weight



Weight is proportional to mass. Objects with
greater mass have greater weight. If you double
the mass, you double the weight.
Weight, unlike mass, however, depends on location.
That is, the strength of the gravitational force on a
mass depends on where it is measured.
For example, a person who weighs 120 lbs. on
Earth will only weigh 20 lbs. on the moon and 0 lbs.
in outer space. The mass of the person would not
change, however.


The person’s mass will NEVER change – it would remain the
same regardless of the person’s location.
The person’s weight will ALWAYS change with location.
Mass and Weight





In the U. S., we commonly describe objects by their
weight. The unit we use is the pound (lb.).
Most other countries describe things by mass, using
the unit kilogram or gram.
The SI community uses the NEWTON (N) as the unit
for ANY force (like weight).
1 kg = 9.8 N = 2.2 lbs.
(Your text rounds it off: 1 kg = 10 N)
3.6 The Moving Earth Again


Recall that Copernicus believed the Earth revolved
around the sun.
An example of one of the arguments against a
moving Earth was as follows.
 Why
is it possible for a bird to leave its perch and
drop to the ground to catch a worm?
 The worm, on the Earth’s surface, should have moved 30
km in the 1 sec. it would take the bird to drop to the
Earth.
 (To circle the sun in 1 yr., the Earth would travel at 1.07
x 105 km/hr. or 30 km/s.)
3.6 The Moving Earth Again



It would be impossible for
the bird to catch a worm
30 km away in 1 sec.
Since birds DO catch
worms by dropping to
Earth from their perches,
the Earth MUST be at rest?
Right?
Hint: Think about inertia.
Objects Move With Earth



The law of inertia says objects in motion remain in
motion if no outside forces act on them.
So the Earth, and objects on the Earth (like a tree, a
bird on the tree, a worm, and the air all around
them) are ALL moving at 30 km/s.
As a bird drops to the Earth, there are no sideways
forces acting on it, so its sideways motion remains
unchanged. It travels WITH the Earth and the
worm.
Objects move with Vehicles



If you flip a coin while
traveling in a car (or train or
plane), you would catch it as
if the vehicle were at rest.
The horizontal (sideways)
motion of the coin remains the
same. The coin keeps up with
you.
This is another example of the
law of inertia.
Check Your Understanding
1. Imagine a place in the cosmos far from all
gravitational and frictional influences. Suppose
an astronaut in that place throws a rock. The
rock will:
a) gradually stop.
b) continue in motion in the same direction at
constant speed.
Check Your Understanding
2. An 2-kg object is moving horizontally with
a speed of 4 m/s.
How much net force is required to keep the
object moving with the same speed and in the
same direction?
Check Your Understanding
3. Mac and Tosh are arguing in the cafeteria.
Mac says that if he throws his jello with a
greater speed it will have a greater inertia.
Tosh argues that inertia does not depend
upon speed, but rather upon mass.
With whom do you agree? Why?
Check Your Understanding
4. If you were in a weightless
environment in space, would it
require a force to set an object
in motion?
Check Your Understanding
5. Mr. Wegley spends most Sunday
afternoons at rest on the sofa,
watching pro football games and
consuming large quantities of food.
What effect (if any) does this practice
have upon his inertia? Explain.
Check Your Understanding
6. Ben Tooclose is being chased through the
woods by a bull moose which he was
attempting to photograph.
The enormous mass of the bull moose is
extremely intimidating.
Yet, if Ben makes a zigzag pattern through
the woods, he will be able to use the large
mass of the moose to his own advantage.
Explain this in terms of inertia and Newton's
first law of motion.
Check Your Understanding
7. Several physics teachers are
taking some time off to play a
little mini golf. The 15th hole
has a large metal rim which
putters must use to guide their
ball towards the hole.
Mrs. M guides her golf ball
around the metal rim. When
the ball leaves the rim, which
path (1, 2, or 3) will the golf
ball follow?
Check Your Understanding
8. A 4.0 kg object is moving across a frictionless
surface with a constant velocity of 2 m/s. Which
one of the following horizontal forces is necessary
to maintain this state of motion?
a) 0 N
b) 0.5 N
c) 2.0 N
d) 8.0 N