Physics Lesson 7 Newton's First Law of Motion - Inertia Eleanor Roosevelt High School Chin-Sung Lin

History of Motion

Aristotle on Motion Natural Motion & Violent Motion  In fourth century B.C.  Four elements of the world: earth, water, air, and fire  Each element had its own natural place in the hierarchy of the universe  Natural motion would be to return to its natural place

Aristotle on Motion Natural Motion  Straight up or down motion  Heavy things fall (such as rocks) and light things rise (such as smoke)  Circular motion was natural for the heavens (such as stars), and they were not thought to be caused by forces

Aristotle on Motion Violent Motion  Imposed motion with external cause. It was the result of forces that pushed or pulled  If an object was moving “against its nature”, then a force of some kind was responsible

Aristotle on Motion Rest is the Proper State  If there were no force, there would be no motion  The proper state of objects was one of rest (except in the vertical direction)  It is a commonly thought for nearly 2000 years

Galileo on Motion Break from Aristotle  Aristotle’s thoughts dominated the world for nearly 2000 years  Galileo demolished the notion that a force is necessary to keep an object moving

Galileo on Motion Friction & Motion  Friction is the force acting between materials as they move past each other  Galileo argued that only when friction is present is a force needed to keep an object moving

Galileo on Motion Speed Up  A ball rolling down an inclined plane picked up speed

Galileo on Motion Slow Down  A ball rolling up an inclined plane slowed down

Galileo on Motion What will happen?

Galileo on Motion Constant Speed  A ball rolling on a level surface has almost constant speed  If there is no friction, the ball moved horizontally would move forever

Galileo on Motion Two Inclined Planes Experiment  A ball released to roll down on one plane would roll up to the other to reach nearly the same height  The smoother the planes were, the closer the heights would be

Galileo on Motion Two Inclined Planes Experiment  If the angle of the second plane was smaller, the ball would roll farther to reach the same height

Galileo on Motion Two Inclined Planes Experiment  If the angle of the second inclined plane reduced to zero, only the friction would keep the ball from rolling forever

Galileo on Motion Inertia  Galileo was concerned with how things move rather than why they move  He stated that this tendency of a moving body to keep moving is natural and that every material object resists change to its state of motion  We call this property of a body to resist change inertia

Newton’s Law of Inertia

Newton’s First Law – 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 an unbalanced force exerted upon it

Newton’s Law of Inertia Newton’s First Law Example  Place a cardboard on an empty tumbler and a coin on the cardboard  Coin drops into the tumbler as the cardboard is flicked

Newton’s Law of Inertia Newton’s First Law Example  Snap tablecloth from beneath dishes, and dishes stay

Newton’s Law of Inertia Newton’s First Law Example  Tighten the head of a hammer by banging the handle

Newton’s Law of Inertia Newton’s First Law Example  A passenger standing in a moving bus leans forward when the brakes are applied all of a sudden

Newton’s Law of Inertia Newton’s First Law Example  When the string is released, the ball will fly straight away, not along the curve

Newton’s Law of Inertia Newton’s First Law Example  Roller coaster is the driven by inertia

Mass

Newton’s Law of Inertia Measure of the Inertia  The amount of inertia an object has depends on its mass— the amount of material present in the object  The more mass an object has, the greater its inertia and the more force it takes to change its state of motion  Mass is measured in kilograms (kg)

Newton’s Law of Inertia Mass is not Weight  We often determine the amount of matter in an object by measuring its gravitational attraction to Earth  Mass is more fundamental than weight. Mass is the quantity of matter in an object and only depends on the number of and the kind of atoms that compose it  Weight is the force of gravity on an object. They are proportional to each other

Newton’s Law of Inertia Mass is not Weight  The weight unit in U.S. is pound  The SI unit of mass is the kilogram (kg)  At Earth’s surface, an 1-kg object has a weight of 2.2 pounds. The SI unit of force is the newton (N), an 1-kg object has a weight of 9.8 N  Weight = mass x acceleration due to gravity or F g = mg = 9.8 m

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