Transcript force

Forces
Today’s Goal:
1. Describe and give examples of the law of
inertia and understand that inertia is a
basic property of matter
Sir Isaac Newton
• English “natural philosopher” (what we
would call a scientist.
• Famous apple
• Published in 1687 The Philosophiae
naturalis principia mathematica, commonly
known as the Principia
• In the King’s English: The Mathematic
Principles of Natural Philosophy
The Principia
• First publication of its kind; nobody had ever
treated science so rigorously
• In order to adequately describe nature,
Newton invented Calculus at the same time
• Laid out the fundamental principles of
Classical Physics which dominated science
until the advent of Quantum Mechanics in the
1920’s
Newton
• Also developed the
science of optics
• Was knighted (the first
scientist to be so
honored)
• “If I have seen so far,
it is because I stood on
the shoulders of
Giants.”
Mass
• The quantity of matter – how much stuff
something is made of
• On a fundamental level dependent on the
number of atoms that make something up
• SI Unit : kilograms
• An object’s mass can only be changed by
changing the object (breaking it, cutting, etc)
Forces
• A force is any action that is capable of
accelerating an object – that is capable of
changing the motion of an object
• Push or pull
• Forces are vectors; meaning they have magnitude
and direction
• Symbol: F
• http://www.brainpop.com/science/motionsforce
sandtime/force/
Newton’s Second Law
• The acceleration of an object is
proportional to the net force acting
on the object and is inversely
proportional to the mass of the
object.
• Net force – sum of all forces acting
on an object.
F
a
m
F  ma
Newton’s First Law
• So what if no net force acts on an object?
– The object continues in motion in a straight line with
constant velocity (the acceleration is zero)
– If it’s at rest, it stays at rest.
• Inertia: the tendency of any object to resist
changes in its motion
• Equilibrium – state when the net force acting on
an object is zero
Newton’s First Law
• “Every body perseveres in its state of rest,
or of uniform motion in a right line, unless
it is compelled to change that state by forces
impressed thereon.”
Newton
Newton’s First Law
• The idea that an object keeps moving
forever in a straight line is contrary to what
you see. This is because every object that
you encounter on a daily basis is stopped by
friction or drag.
Newton’s Third Law
• Forces always occur in pairs
• When you push on something, it pushes
back. This force is called a reaction force.
• The reaction force is equal in magnitude to
the original force & opposite in direction.
Measuring Force
• Forces can be measured with springs or
other elastic objects (such as rubber bands)
• SI Unit: Newton (N)
• English Unit: Pound (lbs)
1 lb = 4.4 N
• Another unit: ton
1 ton = 2000 lbs
Force Magnitudes
Force (N)
Force (lbs)
Refrigerator magnet
10
2.3
Weight of a person
660
150
Force on jet pilot during
takeoff
Force generated during
space shuttle launch
2600
600
140,000
35,000
Two Kinds of Forces
• Long – range forces – act over distances
between two objects
• Contact forces – force between two objects
in direct contact
• What are some examples of each?
Four Fundamental Forces
1. Gravity – acts over distances between two
objects
2. Electromagnetic – responsible for all the
forces you encounter except gravity –
force between charged particles
3. Weak Nuclear Force
4. Strong Nuclear Force
Hold nucleus of
atoms together: very
short range
Forces You Should Know
• Gravity (Fg) – the force of attraction
between two objects: also known as weight
• Tension (FT) – the force that holds a string
or rope together
• Friction (Ff) – force caused by surfaces
rubbing together; acts to oppose relative
motion
• Normal (FN) – reaction force of a surface on
anything pushing on it
Forces You Should Know
• Drag (FD) – force of a fluid exerted on
anything moving through it (air resistance)
• Thrust (FT) – force from engines, rockets,
etc. that propel an object forward
• Lift (FL) – Upward force that causes
airplanes and other flying objects to fly
What is Gravity?
GRAVITY: An attraction force between all
masses
 Newton’s universal law of gravitation: Every
object in the universe exerts a
gravitational attraction to all other objects
in the universe
 The amount of gravitational force depends
upon the mass of the objects and the
distance between the objects
Mass vs. Weight
• Mass – the measure of how much matter an
object possess
• Weight – a measure of how strongly the
Earth (or whatever planet an object is on)
pulls the object down: same as saying
“force of gravity”
• Mass and Weight ARE NOT
THE SAME!!
Mass vs. Weight
• Mass can only be changed by adding or removing
matter from the object
• Weight can be changed by moving to a different
planet, or by changing the mass
• Weight is dependant on the mass of an object and the
acceleration due to gravity, which changes from
place to place.
Fg  mg