lecture ch 2

7/29/2019 Lecture Ch 2

1/26

Newtons First Law of Motion

The Law of Inertia

Conceptual Physical ScienceChapter 2


2/26

Aristotles Classification of Motion

Greek scientist (Born384 BCE)

Classified motion into

two categories Natural motion Unnatural motion

Natural motion occurs

without force. Unnatural motion

required a force.


3/26

Aristotles Classification of Motion

Aristotle believed that All objects have resting places that they naturally seek.

All motion on the Earth is linear.

All motion in the heavens (outer space) is curved.

The speed at which an object falls is directly related to the massof an object.

Motion continues so long as there is only an applied motion(force) to an object. Removing the motion (force) stops theobject.

Aristotles ideas lasted almost 2000 years It would take a man named Galileo to start the wheels of

change in this field of knowledge called physics.


4/26

Concept Check:

According to Aristotle, what tendency of

moving objects governed their motions?

According to Aristotle, what kinds of

motion required no forces?


5/26

Galileos Concept of Inertia

Galileo Galilei was an

Italian scientist (1564-

1642 CE).

Believed inexperimentation

Blew away Aristotles

notions of motion


6/26


Galileo found that

Except for the effects of friction, objects fall at

the same rate regardless of size.

Force is required to start an object moving,

but not to keep it moving.

Tested with this experimental design.


7/26


The tendency of things is to remain as theyare If moving, they tend to keep moving.

If at rest, they tend to stay at rest.

is called inertia.


8/26

Concept CheckA ball rolling on a pool table

slowly comes to a stop.

How would Aristotle explainthis behavior?

How would Galileo explain it?

How would you explain it?

What two main ideas of Aristotledid Galileo discredit?

What is the name of theproperty of objects tomaintain their states ofmotion?


9/26

Galileo Formulates

Speed & Velocity

Until Galileo motion was described as

eitherfastorslow.

Galileo measured speed by considering

distance and the time it took to cover it.

Distance

Speed = Time


10/26

Galileo Formulates

Speed & Velocity

Approximate speeds in different units

12 mph = 20 km/h = 6 m/s (bowling ball)

25 mph = 40 km/h = 11 m/s (super sprinter)

37 mph = 60 km/h = 17 m/s (sprinting rabbit)

50 mph = 80 km/h = 22 m/s (tsunami)

62 mph = 100 km/h = 28 m/s (cheetah)

75 mph = 120 km/h = 33 m/s (batted softball)

100 mph = 160 km/h = 44 m/s (batted baseball)


11/26

Galileo Formulates

Speed & Velocity

Velocity differs from speed in that we also

know the direction of the moving object.

Velocity is both speed and direction.

Velocity is a vector quantity.

Speed is a scalar quantity.

Constant speed doesnt mean constantvelocity the opposite is true though.


12/26

Concept Check

What is the average speed ofcheetah that sprints 100 m in 4 s?

The speedometer on a bicyclemoving east reads 50 km/h. Itpasses another bicycle movingwest at 50 km/h. Do both bikeshave the same speed? Do theyhave the same velocity?

She moves at a constant speed

in a constant direction. Say thesame sentence in fewer words.


13/26

Motion is Relative

Everythingis alwaysmoving!

Our speed relative to the sun is 100,000

km/h. Faster relative to center of our

galaxy!

Discussing motion

always involves a

reference point.


14/26

And then there was Newton!

Aristotle valued logic to figure things out.

Galileo used experiments.

Galileo showed that experiments arebetter than logic in testing knowledge.

The path was clear then for Isaac

Newton


15/26

Newton

Sir Isaac Newton (1642-1727)

Mathematician and physicist

Discovered many things: Laws of motion

Optics

Gravity

Calculus


16/26

Newtons 1st Law of Motion

An object tends to remain in a constant

state (at rest or in motion) until an

unbalanced force acts upon it.

The larger the mass, the larger the inertia.

Mass and inertia are similar, but not

equivalent.

Inertia depends on the distributionof

mass.


17/26

Concept Check

How does the law of inertia account for

removing dirt from your shoes by stamping

on the porch before entering a house or

removing dust from a coat by shaking it?


18/26

Net Force

The Net Force (Fnet) is the sum of all the

forces acting on an object.

Force is measured in Newtons (N).

5 N

5 N

10 N


19/26

Equilibrium for Objects at Rest

Objects at rest are said to be in staticequilibrium.

Static objects have Fnet = 0 N. Even

though there may be many forces actingon that object they all cancel each otherout.

5 N 5 N

Fnet = 0 N


20/26

Equilibrium rules

Forces are vector so they have direction

and magnitude.

Upward forces +, downward forces -,

forward forces +, backward forces -.


21/26

Equilibrium

Weight is a force (-) that acts in a

downward direction (towards the center of

the Earth).

For an object at rest, there must be an

opposite but equal force acting on it to

have a net force = 0 N.

That force is called the Normal force (Fn).

Its always perpendicular to the surface.


22/26

Concept Check

As you stand at rest

on a floor, does the

floor exert an upward

force on your feet? Ifso, what exactly is

this force?


23/26

Equilibrium for moving objects

Moving objects are in dynamic

equilibrium, not static.

Sum of forces must be = 0 N if

its velocity is constant.

Remember: Newtons First Law!

The balancing a deck of cards

in your hand while on a moving

train, example cards dont

know the difference.

Am I

moving or

standing

still?


24/26

Earths Rotation

If the Earth came to a sudden stop, and I

jumped up into the air at that exact

moment, wouldnt I land safely back on the

ground?


25/26

Earths Rotation & Inertia

The Earth is rotating very rapidly (even as youwrite this down). Do you feel it?

Could you travel from San Francisco to New

York by staying stationary in the air for 3 hours?


26/26

Hard Work!

Humankind struggled

for nearly 2000 years

in developing the

ideas of this section.You should be patient

with yourself if it takes

a few days or weeks

to achieve as much!

Thinking by Broken Persona

lecture ch 2

Documents