physics 101 chapter 2

8/12/2019 Physics 101 Chapter 2

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Chapter 2Newtons 1st

Law ofMotion: The Law of Inertia

Inertia, Net Force, the EquilibriumRule and Mechanical Equilibrium


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OR

Why you should alwayswear your seat belt!


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Aristotle vs. Galileo

Falling Objects

Aristotle - Greater Mass Faster it Falls

GalileoHow fast an object falls independent of

mass

Natural Motion

Aristotle -earth, water, air and fire.

Natural motion straight up or straight down

Rest is the natural state Violent motionimposed motion from push or pull

Geocentrismdifferent rules apply to heavens

Unchanging substance (quintessence)

Perfect spheres; perfect circles


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Aristotle vs. Galileo (Cont.)

Copernicus

Heliocentrism: Sun-centered solar system

Galileoconstant motion is just as natural

as the state of rest

Constant motion means unchanging speed and direction

(straight-line).

He called this property of an object inertia (more on this

later)


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Newtons First Law of Motionor the Law of Inertia

Every object continues in its state ofrest, or uniform motion in a straightline, unless compelled to change thatstate by forces impressed upon it.

Every object at rest stays at rest, andevery object in motion stays in motionat constant speed and direction unlessacted on by a net force.


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Weight and Force

Same thing; same units.

Your weight is the force of the earth on

you.

We call it the force of gravity.Newtons or pounds.


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Inertia Revisited

The resistance to change in the state of

motion of an object.

The measure of the inertia of an object is

its mass.

Orthe sluggishness an object exhibits in

response to an effort (force) to change its

motion (or non-motion).


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Net Force

Forces are vector quantities

A vector is a quantity in physics that has

both magnitude (size), and direction.

Represented by arrows

Often there are many applied forces.

Their vectorsum (F) is the net force.


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Net Force

F

is the symbol for sum of

Read the sum of the forces or The net force.


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Equilibrium Rule

Another way of viewing the 1stLaw

Rule: If the net force (F) on something is

zero, then that thing is in mechanical

equilibrium.

F = 0 is a necessary condition for

mechanical equilibrium.

CAUTION: Mechanical equilibrium does not

imply the object is at rest!


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Mechanical Equilibrium

State of an object (or system of objects) for

which the motion does not change.

If moving, it does not speed up, it does not

slow down, and it does not change

direction. (dynamic equilibrium)

If not moving (at rest), it does not begin to

move. (static equilibrium)


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Support Force

Normally called the normal force.

Why? Because it is perpendicular (normal)

to the supporting surface.

Example: book on table.


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Equilibrium of Moving Things


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Equilibrium of Moving Things


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Motion and Reference Frames

Our moving earth.

Example: throw a ball straight up while

riding (not driving!) in a car going straight

at 60 mph.

What is our approximate speed of rotation

about the earths axis?

What is our approximate speed around the

sun?


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physics 101 chapter 2

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