NEWTON’S 1 ST LAW OF MOTION - Inertia Chapter 3. Objectives Describe Aristotle’s concept of motion. Describe Copernicus’ idea about Earth’s 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)


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. 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


b. As the angle of the incline is reduced, the ball rolls a greater distance before reaching its initial height.

Initial position Final position


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?


•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 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?

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 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 and Volume

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.


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?


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?


4. If you were in a weightless environment in space, would it require a force to set an object in motion?


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.


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.


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?


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

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NEWTON’S 1 ST LAW OF MOTION - Inertia Chapter 3. Objectives Describe Aristotle’s concept of motion. Describe Copernicus’ idea about Earth’s motion