Chapter 2Chapter 2

Newton's First Law of Motion

- Inertia

1. ARISTOTLE ON MOTION1. ARISTOTLE ON MOTION

Aristotle attempted to understand motion by

classification.

Two Classes:

Natural and Violent

NaturalNatural

Natural motion depended on nature of the object.

Examples:A rocks falls

Smoke rises

The falling speed of an object was supposed to be proportional to its weight.

Natural motion could be circular (perfect objects in perfect motion with no end).

ViolentViolent

Pushing or pulling forces imposed motion.

Some motions were difficult to understand.

Example: the flight of an arrow There was a normal state of rest

except for celestial bodies.

Aristotle was unquestioned for 2000 years.

for it was in its normal state.

No one could imagine a force that could move it.

Most thought that the Earth was the center of everything

2. COPERNICUS ANDTHE MOVING EARTH

2. COPERNICUS ANDTHE MOVING EARTH

Sun was center, not earth.He was hesitant to publish because he didn't really believe it either.

De Revolutionibus reached him on the day he died, May 24, 1543.

3. GALILEO AND THE LEANING TOWER

3. GALILEO AND THE LEANING TOWER

17th Century scientist who supported Copernicus.

He refuted many of Aristotle's ideas.

Worked on falling object problem - used experiment.

4. GALILEO'S INCLINED PLANES4. GALILEO'S INCLINED PLANES

Knocked down Aristotle's push or pull ideas.

Rest was not a natural state. The concept of inertia was

introduced. Galileo is sometimes referred to as

the

“father of experimentation.”

He tested with planes.

He tested with planes.

Demo - Ball and incline planeDemo - Ball and incline plane

The change in speed depended

on the slope of the incline.

5. NEWTON’S FIRST LAW OF MOTION

Newton finished the overthrow of Aristotelian ideas.

Law 1 (Law of Inertia) Every object continues in its state of rest,

or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

“If you leave an object alone, it has constant velocity.”

DemonstrationsDemonstrations

Demo - Weight and stringDemo - Weight and string Demo - Card, cup, and coinDemo - Card, cup, and coin Demo - Swinging RocksDemo - Swinging Rocks Demo - Coins on elbowDemo - Coins on elbow Demo - Table settingDemo - Table setting Demo - Bottle, hoop, and chalkDemo - Bottle, hoop, and chalk Demo – Lead brick and hammerDemo – Lead brick and hammer

10 N

6. NET FORCE A force or a combination of forces

produces changes in motion (accelerations).

10 N m10 N

= 20 N m

10 Nm10 N = 0 N m

10 Nm20 N = m

7. THE EQUILIBRIUM RULE

Examples of MechanicalEquilibrium:

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

Weight down

Weight downWeight down

Scales pushing up

Tree pulling up

NormalFriction

The Equilibrium Rule

0F

Scales pushing up

Weight down

8. SUPPORT FORCE In the first example of

mechanical equilibrium the table supplied a force upward that was called the normal force. It is a support force.

Consider the second example of mechanical equilibrium. The scales supply a support force on the man.

Normal up

Weight down

Equilibrium is a state of no change. If an object moves in a straight line with no

change in speed, it is in equilibrium.

9. EQUILIBRIUM OF MOVING THINGS

Examples:Driving at constant velocity

Force from road

AirResistance

Normal up

Weight down

Terminal velocity in parachuting

Weight down

Air resistance

It is hard to detect the motion of the earth because we are moving with it.

Early science could not predict large enough forces to move the earth.

Can Hewitt’s bird drop down and catch the worm if the Earth moves at 30 km/s?

Demo - Cart and ball launcherDemo - Cart and ball launcher

10. THE MOVING EARTH

Chapter 2 Review Questions

Chapter 2 Review Questions

Which person below is most highly recognized for proposing a heliocentric universe?

Which person below is most highly recognized for proposing a heliocentric universe?

(a) Aristotle(b) Newton(c) Galileo(d) Copernicus

(d) Copernicus

If an object weighs 10 lb, what must the air resistance force be if the object is falling and has reached terminal velocity?

If an object weighs 10 lb, what must the air resistance force be if the object is falling and has reached terminal velocity?(a) 10 lb(b) 32 lb(c) there is no way of telling without

knowing what the value of the terminal velocity is

(a) 10 lb

Newton's concept of motion said that the natural state of an object was

Newton's concept of motion said that the natural state of an object was

(a) constant velocity(b) constant acceleration(c) constant net force

(a) constant velocity