Transcript Newton`s Laws

Newton’s Laws of Motion
Beginning with:
The Law of Inertia
Force

A force is a push or a pull on an object.

Forces are measured in Newtons (N).

1 N = 1 kg  m/s2
Net Force



External force – acts from outside the system
or object
Internal force – acts from inside the system
A net external force is the sum of the
external forces acting of the object. Simply,
the net force is the overall force on an object.
Balanced vs. Unbalanced Forces

If the net force on an object
is zero, the forces are
balanced. (This condition is
called equilibrium.)

If the net force is not zero,
then the forces are
unbalanced.
A Historical Perspective
Aristotle – 4th century BC, Greece



The proper state of
objects is at rest.
Unless an object is
being pushed or pulled
(violent motion), it is
seeking its natural
resting place.
Earth did not move, it
was in its natural
resting state, of course.
http://library.thinkquest.org/11924/aristotle.html
Copernicus – 1540 AD



The Earth and planets move
around the sun.
The idea was very controversial
and Copernicus was reluctant to
publish his theory, in fear of
being prosecuted.
On the day of his death, the first
copy was delivered to him.
Galileo on Motion
1564-1642 AD
Galileo asked how things move, not why they
moved.
An object's velocity will not change all on its
own. Pushes, or pulls, are necessary to
change an object's velocity.
Galileo’s Contributions




Inertia is a property of matter. It is that property
of matter which opposes changes in velocity.
Simply stated, a common object will not change
its velocity spontaneously.
Friction is the name given to the force that acts
between materials that touch as they move past one
another.
Argued that when friction is present a force is
needed to keep an object moving.
http://library.thinkquest.org/11924/aristotle.html
Other Contributing Scientists

Ibn al-Haytham (965 – 1039)


lived in Persia (now Iraq)
Mo Tzu (circa 470 - 390 BCE)


a collection of philosophical texts
originated in China
Isaac Newton (1642-1727)
Issac Newton’s
monumental work,
Philosophiae Naturalis
Principia Mathematica,
was published in 1687.
It proposed scientific laws
that are still used today to
describe the motion of
objects. (credit: Service commun de
la documentation de l'Université de
Strasbourg)
The Law of Inertia

First Law of Motion: Every object continues
in a state of rest, or continues moving in a
straight line at constant velocity, unless it is
compelled to change that state by forces
exerted upon it.

Main Idea: There must be a cause (a net
force) for there to be a change in velocity.
Equilibrium

The Law of Inertia describes objects at
equilibrium. For an object to be at
equilibrium it must:
Be at rest
OR
Move at constant
velocity
AND
Have balanced forces (net force = 0)
First law in Motion
http://www.glenbrook.k12.il.
us/gbssci/phys/mmedia/newtl
aws/mb.html
MASS
A Measure of Inertia
Mass vs. Weight

They are NOT the same!

Mass = amount of stuff in an object,
measured in kilograms.

Weight = FORCE of gravity on an
object, measured in Newtons.
Mass is not Weight




The mass of an object does not change.
Weight can vary with location.
No gravity = no weight.
An object with a mass of 1 kg will have a
weight of 9.8 N.
W = Fg = m g
The Second Law (Normal Words)

Force causes acceleration.

For the same mass,
More force = more acceleration

For the same force,
Smaller mass = more acceleration
More mass = less acceleration
Newton’s Second Law of Motion
The fancy words version:
The acceleration produced by a net force on an
object is directly proportional to the magnitude
of the net force, is in the same direction as the
net force, and is inversely proportional to the
mass of the object.
The 2nd Law Equation
F = ma
The units are:



Force (F) is in Newtons (N)
Mass (m) is in kilograms (kg)
Acceleration is in meters/second2 (m/s2)
Newton’s Third Law
Fancy words:
 Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first.
Normal words: For every action, there is a equal
and opposite reaction.
3rd Law



The 3rd law always involves two objects.
The force are equal in size.
The direction of the force is opposite.
Example:
 An angry person punches the wall. (action
force)
 The wall punches back. (reaction force)
Result = sore knuckles