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 * Chapter 4 Review Page**

Zach Rosinger

Jon DiFiore


 * Chapter 4: Newtons First Law of Motion--Inertia

4.1 Aristotle and Motion:** Aristotle divided motion into two simple categories, one being //natural motion// and the other //violent motion.// Natural motion on the earth was either straight up or straight down, an example of this would be a boulder falling downward in a straight line. It was thought that objects would seek a natural resting place, like a rock on the ground, or smoke in the air. Aristotle thought that circular motion was natural for the heavens because it had no beginning or end. Violent motion was also known as imposed motion. It was the result of forces that pushed or pulled. An example was that a ball rolled if a person kicked or pushed it. Tug-of-war was won by pulling on a rope harder than the other side. Objects in their natural resting places could not move by themselves; they had to be either pushed or pulled to change. People thought that earth was always in its natural resting place, and therefore didn't realize that it was in orbit for nearly 2000 years. Copernicus believed that the earth and other planets revolved around sun in their designated orbits. This belief was highly controversial at the time, so he developed his ideas in secret in order to escape persecution. His works were published in De Revolutionists. **4.3 Galileo on Motion:** Galileo greatly supported Galileo and as a result was put on trial and house arrest. Galileo disproved the notion that a force is required to keep an object moving. He was more concerned for //how// things move rather than //why// they move. A **force** is any push or pull on an object, where as **friction** is the force that acts between materials as they touch. Galileo’s argument was that only when friction is present, will an object need a constant force to stay moving. He experimented this by rolling a ball down different slopes: Downward slope - speed increases Upward slope - speed decreases No slope - speed doesn't change A ball dropped from an initial height down an incline plane will travel up another incline plane, which is facing the opposite direction, to a final height equal to the initial height. Smoother the surfaces, the more equal the initial and final height will be. If the slope of the second incline plane were to be reduced to zero, then friction would be the only force acting on the ball keeping it from moving forever. In the absence of friction, the ball would keep on naturally moving forever. **Inertia**, however, is an objects resistance to change.
 * 4.2 Copernicus and the Moving Earth:**

By the age of 24 Isaac Newton developed his famous laws of motion, all of which rejected and discredited Aristotle's beliefs which had been believed by all for over 2000 years prior to Newton. Newton's first law was called the **Law of Inertia** and was basically a restatement of Galileo's idea.
 * 4.4 Newtons Law of Inertia:**

"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" ~Law of Inertia

What this law is saying, is that if an object is at rest, it will stay at rest unless an outside force acts upon it to change its state of being. Likewise for the object in a straight line motion, if it is moving in a straight line, it will continue to do so at a constant speed unless an outside force acts upon it to change its momentum. These objects move or remain at rest because of their inertia. Since we don't live in a place where friction and gravity are absent, it is necessary to overcome the two in order to move an object, and because of the two, the objects don't continue on forever in a straight line if pushed or pulled.

The amount of inertia in an object depends on its mass, which is simplified to being a general amount of material present in the object. An example of this would be a bag of feathers have less inertia than a bag of bricks. Mass is NOT volume. Volume is a measure of space and is measured in units like cubic centimeters, cubic meters, or liters. Mass is in kilograms. Mass and volume are not dependent on one another, for example, an object with a large amount of mass might not have a large amount of volume, like a brick, which might be massive, but it has less volume than a large bag full of feathers. The bag of feathers has less mass, but a larger volume. Mass is NOT weight. Weight is the amount of force in which gravity pulls down on an object, and mass is the amount of material in an object. Mass does NOT change wherever you are, but weight changes depending on the gravitational pull at different points. If an object has a mass of 3 kilograms, its mass is 3 kilograms anywhere in the universe, whereas its weight would change dependent on the planet. Mass and weight are proportional to each other in a given place. One Kilogram is 9.8 Newtons. A newton is the SI unit of force. Weight=mg in other words Weight=massXacceleration of gravity Acceleration of gravity=9.81 m/s^2
 * 4.5 Mass--A Measure of Inertia:**

Without force, objects stay at rest or continue in motion, depending on what they were doing the whole time. If the object is in motion, obviously, it will stay in motion, and the same goes for an object that is at rest, it will stay at rest unless there is an outside force. The same goes for an absence of a //Net Force.// Net force what changes an object's state of motion. When you pull horizontally with a force of 10N on an object resting on a frictionless surface, the net force acting on the object is in turn 10N. If you and another person pull in the same direction, one with 5N and one with 10N, then the total net force is 15N. If one pulls with 5N to the left, and one pulls 5N to the right, then the net force is 0N.
 * 4.6 Net Force:**

If your notebook is on a table, what forces act on it while its sits motionless? Its not just weight that acts on the notebook becuase if something is motionless there has to be a pair of forces that keeps it in this state. This other force is called a **support force (normal force)** becase it balances the book's weight and produces a net force of zero. The support force will always act on an object with a force equal to that object's weight. This state, where an object is at rest and the net force is zero, is called **equilibrium**. If you hang from a bar that is suspended by two ropes, the sum of the tension of the two ropes must equal your weight. Tension occurs when atoms within an object are stretched apart. A spring scale can be used to measure tension since it will measure the weight of the object first, which then shows tension of the spring needed to achieve equilibrium.
 * 4.7 Equilibrium-When Net Force Equals Zero**

Since force has magnitude and direction, like velocity, it can be measured using vectors. Tension, however, is greater in a pair of nonvertical spring scales and depends on the angle from the vertical. The greater of an angle from the vertical will cause greater tension in the scales. The sum of tension in both non vertically hanging scales, hanging at a certain angle from the vertical, have to support the downward weight. Again tension in both non vertically hanging scales will depend on the angle from the vertical and weight of a vertically hanging object.
 * 4.8 Vector Addition of Forces**

The idea of the earth being in motion was introduced by Copernicus in the sixteenth century. People argued at the time that if a bird were to attempt to catch a worm on the ground if it was in a tree vertically above it, then if the earth were moving, the worm would be swept away by the time that the bird would get to the ground. People debated this issue for a long period of time. What people didn't have the knowledge of was inertia, which accounts for the fact that all of the objects on the earth are moving just as fast as the earth is, and, in turn would stay in the same spot unless acted on by an outside force. A way of proving this inertia is to stand against a wall, and jump straight up. Once you jump straight up, the wall doesn't slam into you, which means that you are moving just as fast as the earth is. People were not accustomed to the idea of inertia, but also, they didn't experience the idea of fast-moving vehicles, which would be a more concrete piece of evidence pointing to the concept of inertia. The whole idea of inertia that we have nowadays is completely different from how ancient scientists once thought of them, including some of the most well-known ones in history, like Aristotle. Before, they did not recognize the fact that the earth was in motion, or the idea of inertia at all, but now we know that the earth is actually moving at a very fast speed around the sun and that there is most definitely a concept known as inertia, which explains many different scientific happenings.
 * 4.9 The Moving Earth Again**

Conceptual Physics--Third Edition Written and illustrated by Paul G. Hewitt Addison Wesley Longman, Inc. 1999 A Pearson Company Menio Park, California 94025
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