1. NEWTON’S 1ST LAW OF MOTION - Inertia
Chapter 3

2. 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.
Isaac Newton (a 17th century scientist) put forth three laws which explain why objects move (or don't move) as they do and these three laws have become known as Newton's three laws of motion.

3. 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.

4. 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.

5. 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)

6. 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.

7. 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.
When it moves
upward against
gravity, its speed
decreases. The ball
reaches its initial
height before stopping on
the 2nd incline.
Initial position Final position

8. 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

9. 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?

10. 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.
Seat belts are used to provide safety for passengers whose motion is governed by Newton's laws.
The seat belt provides the unbalanced force which brings you from a state of motion to a state of rest.

11. 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.“

14. Applications of the 1st Law
The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. Why?
Blood rushes from your head to your feet when riding on a descending elevator which suddenly stops.
To dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

15. What happens ….? To the car? To the man (without seatbelts)?
The law of inertia is most commonly experienced when riding in cars and trucks. In fact, the tendency of moving objects to continue in motion is a common cause of a variety of transportation accidents - of both small and large magnitudes.
Upon contact with the wall, an unbalanced force acts upon the car to abruptly decelerate it to rest.
Any passengers in the car will also be deccelerated to rest if they are strapped to the car by seat belts.
Being strapped tightly to the car, the passengers share the same state of motion as the car.
As the car accelerates, the passengers accelerate with it; as the car decelerates, the passengers decelerate with it; and as the car maintains a constant speed, the passengers maintain a constant speed as well.
But what would happen if the passengers were not wearing the seat belt? What motion would the passengers undergo if they failed to use their seat belts and the car were brought to a sudden and abrupt halt by a collision with a wall? Were this scenario to occur, the passengers would no longer share the same state of motion as the car. The presence of the strap assures that the forces necessary for accelerated and decelerated motion exist. Yet, once the strap is no longer present to do its job, the passengers are more likely to maintain its state of motion. The animation below depicts this scenario.
To the car?
To the man (without seatbelts)?
To the man (with seatbelts)?

17. 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.
All objects resist changes in their state of motion.
All objects have this tendency – they have inertia.
But do some objects have more of a tendency to resist changes than others?
Yes, absolutely!
The tendency of an object to resist changes in its state of motion is dependent upon its mass.
Inertia is a quantity which is solely dependent upon mass.
The more mass an object has, the more inertia it has – the more tendency it has to resist changes in its state of motion.

18. 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.

19. 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.
People think that if an object has a large mass it has a large volume. But volume is a measure of space and is measured in units like cubic centimeters or mL.

20. Mass is NOT Volume GRAVEL
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?

21. 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.

22. 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.

23. 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.

24. 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)

25. 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 x 105 km/hr. or 30 km/s.)

26. 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.

27. 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.

28. 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.

29. 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:
gradually stop.
b) continue in motion in the same direction at constant speed.
The answer is B

30. Check Your Understanding
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?
None
An object in motion will maintain it’s state of motion

31. 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?
Tosh
Inertia is a quantity that depends solely on mass. More mass more inertia. Momentum depends on both mass and speed.

32. Check Your Understanding
4. If you were in a weightless
environment in space, would it
require a force to set an object
in motion?
Yes, absolutely.
Even in space objects have mass. And if they have mass then they have inertia

33. 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.
His inertia will increase.
If he continues this behavior his mass will increase therefore his inertia will increase.

34. 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.
The large mass of the moose means that the moose has a large inertia. Ben will be able to change his own state of motion easier than the moose will.

35. 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?
Answer 2

36. 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
0 N
If an object is in motion it will not require another force to keep it going but will require a force to change its motion.