Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 2 Newton’s First Law

of Motion

Part I: Mechanics

Apr 18, 2023 Physics 1 (Garcia) SJSU

Aristotle & Galileo

• Aristotle was great philosopher but not such a good scientist.

• Aristotle’s theory of motion is wrong.

• Took 2000 years before Galileo got motion right.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Motion according to Aristotle (I)

• Every object has a “natural” state.

• In “natural motion”, “Earth” elements (stone, apple, you, etc.) are drawn to the Earth.

• Heavier objects are more strongly attracted so they fall faster (stone falls faster than a feather).

Aristotle

Reality

Important: These are Aristotle’s ideas, but he’s wrong!

Apr 18, 2023 Physics 1 (Garcia) SJSU

Motion according to Aristotle (II)

• Pushing or pulling an object causes “unnatural” motion (or “violent” motion).

• If cause is removed (stop pushing) then object returns to “natural” state and stops moving.

BRICK BRICK

Pushed brick slides but then comes to a stop

Apr 18, 2023 Physics 1 (Garcia) SJSU

Natural & Unnatural MotionAristotle thought objects in

unnatural motion would first stop, then fall.

He was wrong!

“Unnatural”

“Natural”Reality

Apr 18, 2023 Physics 1 (Garcia) SJSU

Galileo & Falling

• Pull of gravity (weight) obviously larger for a bowling ball than for a baseball.

• YET the two balls fall with the same speed.

• What about a feather?(Answer in Chapter 4)

Apr 18, 2023 Physics 1 (Garcia) SJSU

Galileo’s Inclines (I)

Downhill: Speed increases

Uphill: Speed decreases

Flat surface: Speed increases, decreases,or constant?

Questions existence of “natural Earth” state of not moving.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Galileo’s Inclines (II)

Again, questions existence of “natural Earth” state of not moving.

Imagine ball rolling down hill then back uphill

Apr 18, 2023 Physics 1 (Garcia) SJSU

Newton’s First Law of Motion

An object at rest remains at rest &

an object in motion remains

in uniform motion*,

unless a force acts on the object.

*Moving in a straight line with constant speed.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Newton’s First Law of Motion

Object at rest

(not moving)No force on

object

Uniform motion

Note: Object at rest is just uniform motion at zero speed.

then

then

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Riding Light Rail

When a moving train stops, you continue moving forward.

When the stopped train starts moving again, you remain stationary and are thrown backwards

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Rolling, rolling, rollingW

ALL

All moving together

Bottom car stops.Other two continueWith same speed.

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Tablecloth Pull

Tablecloth

FlowerVase

Yank quickly

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Inertia Balls

HeavyCannonBalls

Upper String

Slowpull

Quickyank

Lower String

Which string breaks in each case?

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Paper Pull

Quickyank

What happens in each case?

Which roll has more inertia?

Apr 18, 2023 Physics 1 (Garcia) SJSU

Net Force

same as

The Newton is metric unit of force (about 1/5 pound).

When several forces act on an object, the forces add together.

Sum of forces called net force or total force

BRICK

3 Newtons5 Newtons

8 Newtons

Apr 18, 2023 Physics 1 (Garcia) SJSU

Check Yourself

?

Apr 18, 2023 Physics 1 (Garcia) SJSU

Equilibrium Rule

same as

If an object is at rest then the net force must be zero. Similarly if in uniform motion.

BRICK3 Newtons 3 Newtons

Zero Newtons(No Force)

When this happens we say that forces “balance.”

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Up+Up=Down+Down

STICK (2 Newtons)

10 NewtonIron mass

Scale reads5 Newtons

Scale reads7 Newtons

The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass).

Stick is at rest.

5+7=10+2

Apr 18, 2023 Physics 1 (Garcia) SJSU

Support Force

Solid surfaces exert a force, called a support force, on objects pressed against them.

100 NewtonGold Brick

100 NewtonSupport force

Downward force (weight) balanced by upward force (support).

How much is the net force on the brick?