Chapter 4 Newton’s First Law: The Law of Inertia

1.ARISTOTLE ON MOTION Aristotle attempted to understand motion by classification. Two Classes: Natural and Violent

Ancient Greek philosopher and scientist Aristotle developed the earliest theory of how things move. natural motion – motion that could maintain itself without the aid of an outside agent. (pushing a rock off the ledge and it falls to the ground, liquids falling or running downhill, air rising, flames leaping upward) Aristotle believed everything was made of four elements Fire Air Water Earth Aristotle's Periodic Table “Natural Motion” (vertical) “Violent Motion” (horizontal)

Aristotelian Universe Terrestrial Realm Earth and Water tend to sink  Composition predicts motion natural tendencies  Overall tendency to seek rest o Objects are corruptible (changing) Fire and Air tend to rise

How things Move Fire Air Water Earth Aristotle's Periodic Table “Natural Motion” (vertical) “Violent Motion” (horizontal) Each element has its own natural motion, and its own place that it strives to be. Aristotle believed an object’s natural motion was determined by how much of each element the object contained (rocks sink in water because they contain mostly earth, wood floats because it contains mostly air) earth moves downward because Earth’s center is its natural resting place water’s natural resting place is on top of earth Violent Motion – motion that forced objects to behave contrary to an objects natural motion, meaning an external push or pull was needed

Aristotle’s Laws of Motion –Summary 1.Natural Motions are determined by the innate “desire” of the moving object to return to it’s “natural resting place “. The proper state was “at rest”. 2.Violent Motions occur because outside influences give motion to the object. As soon as force “runs out”, the object stops. This was the belief for 2000 years. 3.Objects composed of ether travel in gigantic, perfect circles centered on the Earth which was assumed to be at rest.

Stationary Earth Earth is heavy. – Easier to imagine the sky can move It looks like the sky is moving. – We have no sensation of our motion. A rotating Earth would make objects fly off of the surface. It would take a huge force to move the Earth so the Earth must not be moving. The Earth must be the center of the universe.

Nicholas Copernicus (Niklas Koppernigk) 2. Nicholas Copernicus developed a mathematical model for a Sun-centered solar system based on astronomical observations.

Galileo Galilei He was among the first to turn a telescope to the sky. He developed the Scientific Method. He believed in the popularization of science. He defined inertia as resistance to change in motion.

3. Galileo Galilei helped overthrow Aristotelian physics. Because of his radical ideas he was confined to house arrest for the last 10 years of his life. Galileo argued that only when friction is present is a force needed to keep an object in motion. Friction is a force caused by irregularities in the surfaces of objects in contact. Friction acts in a way which opposes motion. I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use. -Galileo

As a ball rolls down an incline its speed increases. Push the ball up incline, it slows down, stops and rolls back down. Suppose the incline is nearly horizontal. Obviously, due to friction the ball would probably come to a stop. Galileo understood that this slowing is due to the roughness of the incline and ball. Let’s try to visualize the experiment and neglect friction. The ball would then speed up on any downward slope and slow down on any upward slope. Let’s look at the limiting case when the incline is perfectly horizontal. In the absence of friction, a ball that got started rolling horizontally would roll forever. This is contradictory to Aristotle’s view of motion

Aristotle What type of motion?All objects resist a change in their motion so what are the outside influences that cause motion? CelestialNaturalViolent Ether CirclesWhat isHow much it made ofmotion given? Fire Air WaterEarth Natural Resting Place? Galileo

Isaac Newton

Newton’s Laws Every object continues to move as it has been moving unless acted upon by an external force. The 1 st Law

Law of Inertia – An object will move in a straight line at a constant speed UNLESS acted upon by an external force - objects in motion stay in motion - objects at rest stay at rest UNLESS influenced in some way YOU PASSENGER Car begins turning here you start turning here outside influence – tires on car friction on tires outside influence – door on you

Another example: “Canis Insanis” you fastened to you seat belt Miss Pickles standing on the console Jeep apply brakes at red light you stop too! trip to the vet needed

Mass and Inertia Inertia is a property of a body that resists changes in motion. Mass is a measure of the amount of matter in a body. Inertia and Mass are the same concept. –Units – Kg or slugs Mass and weight are NOT the same concept but they are proportional. Mass and volume are NOT the same concept but they are proportional.

Weight Force of gravity pulling on the mass of the body. –Units – Newtons or Pounds A newton is about the weight of a small apple. –One kg weighs 2.2 lbs. W = mg –g = acceleration due to gravity at the surface of the Earth = 32 ft/s 2 = 9.8 m/s 2

6. Net Force A force is a push or a pull. Force is a vector –We must add all the forces acting on the body 5 N 10 N 5 N 0 N 5 N 10 N 5 N

7. Static Equilibrium Velocity is zero Examples: Hanging from a tree Weighing yourself on a set of scales Computer setting on a table Car parked on an incline Normal up Weight down Scales pushing up Tree pulling up Normal Friction

Dynamic Equilibrium When the net force is zero (  F = 0). This refers to all of the forces acting on an object, including friction.  Acceleration is zero.  Velocity may not be zero.

How things Move Gravity is the “outside influence” that causes the ball to drop In fact all falling objects fall at the same rate, called the acceleration of gravity (neglecting air resistance) Drop different objects their speed will increase at the same rate! Their speed will increase by ~ 10 m/s (32 ft/s) every second

Free Fall and Friction Air friction slows the ping pong ball down compared to the billiard ball. In the absence of air, Galileo predicted that they would fall at the same rate, like a wooden and iron ball.

How things Move Two things Galileo figured out as a result of the law of inertia 1.Assuming that the only “outside influence” has the same effect on all objects, all should fall at the same rate. [Galileo’s Law Falling] Galileo knew not of force or the law of gravity, these concepts came along later. Rather he took this “same effect” idea from his observations and idealizations 2.There is an outside influence called friction present in the “real world”. Friction acts differently on different objects. Galileo’s reasoning for “why moving things stop”

Uniform Circular Motion – acceleration by virtue of a change in direction “centripetal acceleration” – center pointing Imagine that a car travels with a constant speed around a circular track In this type of motion, the acceleration is always directed toward the center of the circular path R C instantaneous velocity at this point on the track where the car ends up a moment later if there is no acceleration the difference in velocity points in the direction of the acceleration……. toward the center instantaneous velocity of the car at a new point on the track a moment later….same speed – different direction Quantitative: travel at speed = s around circle of radius = R your “centripetal” acceleration is C = center of the circular path R = radius of the circular path

Motion not on earth followed a different set of rules. Motion in the heavens had no beginning or end. Circular paths were considered natural for the stars and the planets. 5 th element – ether (from the Greek word for to kindle or blaze) – had no weight and was unchangeable, and perfect in every way Moon, sun, planets and stars were made of ether celestial motion “perfect circles” Ether’s natural place was in the “heavens” and it moved in perfect circles. Objects on earth could not move the way the stars did because they did not contain ether. Aristotle's physics governed science until about the mid 16 th century. It was popular because it reinforced religious beliefs.