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 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 Fg = mg = 9.8 m

The End