Transcript Newton`s First Law of Motion

Physical Science 2
Chapter 2.3 & Chapter 3 Forces
NEWTON
• If I have seen farther than
other men, it is because I
have stood on the
shoulders of giants."
-Isaac Newton
• When Newton made that famous
statement, he was referring to such
giants as Tycho Brahe, Johannes
Kepler, and, most specifically, Galileo
Galilei
History behind the
First Law of Motion
• Prior to Galileo and Newton's work on motion, it was
believed that the natural tendency of objects was to
come to rest.
– Sooner or later, moving objects would stop moving.
– Meaning that some force was necessary to keep an object in
motion.
– Aristotle and Ptolemy.
• Galileo boldly asserted the exact opposite.
• He reasoned that moving objects eventually stopped
moving due to a force called friction.
• Galileo's Experiment
• Galileo developed the concept of inertia.
Galileo developed the
concept of inertia
•
•
Galileo's Experiment
After placing a marble at any height
on one plane, Galileo observed that,
when released, the marble rolled
down that plane and up the opposite
plane to approximately the same
height. If he sanded the planes to be
smoother, he noted that the marble
rolled up the opposite plane even
closer to its original height
•
Galileo concluded that the difference
between the marble's initial and final
heights could be attributed to a force
he called friction. He theorized that
without this force, the marble would
reach its original height exactly.
Newton’s First Law of Motion - INERTIA
• Objects at rest stay at rest unless acted on by another force.
• Objects in motion stay in motion unless acted on by another force
• There is a natural tendency for objects to resist changes in their state
of motion.
• INERTIA – The resistance of an object to a change
in its state of motion.
• The more mass an object has, the greater inertia.
Shooting Objects at Rest
The object’s inertia causes it to remain still
even when a bullet passes through it.
FORCE
– is a push or a pull
– can give energy to an
object causing the
object to
• start moving,
• stop moving,
• change its motion.
Balanced vs. Unbalanced
• Balanced forces
– They are equal in size and
opposite in direction
– Do NOT cause a change in
motion.
– Result in a zero net force
• Net force is the result of
all the forces acting on the
object
• Unbalanced forces
– They are unequal in size
and or in the same
direction
– Cause a change in motion
Tugboat Problems
•
Two tugboats are moving a barge. Tugboat A exerts a force of 3000
Newtons on the barge. Tugboat B exerts a force of 5000 Newtons in the
same direction. What is the combined force on the barge? Draw arrows
showing the individual and combined forces of the tugboats.
•
Now suppose that Tugboat C exerts a force of 2000 Newtons on the barge
and Tugboat D exerts a force of 4000 Newtons in the opposite direction.
What is the combined force on the barge? Draw arrows showing the
individual and combined forces of the tugboats.
•
Could there ever be a case when Tugboat E and Tugboat F are both
exerting a force on the barge but the barge doesn't move? Draw arrows
showing the individual and combined forces in such a situation.
Newton’s Second Law
• The acceleration of an object is dependent upon
the force acting upon the object and the mass of
the object.
• Newton’s Second Law can be written as the
equation:
F=mxa
• F = force (N)
• m = mass (kg)
• a = acceleration (m/s/s)
Newton is the SI Unit of force
N = kg m / s2
NEWTON’S 3rd LAW
For every action, there is
an equal and opposite reaction
•
•
•
•
“Action” and “Reaction” are names of forces
Forces ALWAYS occur in pairs
Single forces NEVER happen
action reaction pairs
“Equal & Opposite”
• In Newton’s Third Law,
“equal” means:
• In Newton’s Third Law,
“opposite” means:
• Equal in size.
• Opposite in direction
– The action and reaction
– The action and reaction
forces are EXACTLY
forces are EXACTLY
the same size.
180o apart in direction.
• Equal in time.
– The action and reaction
forces occur at
EXACTLY the same
time.
Why don’t the forces
cancel each other out?
• Only forces pushing or pulling on an
object affect the object’s motion.
• Only forces that act on the same
object can cancel.
• Newton’s Third Law action and reaction
forces act on different objects, so
they don’t cancel.
Writing Action Reaction Forces
• Action force: “A action verb B”
• Reaction force: “B action verb A”
– ACTION
Bowling ball hits the pin to the left.
– REACTION
Pin hits the bowling ball to the right
• The action and reaction
forces don’t cancel since
they push on different
objects.
Friction
• A force that opposes motion between two
surfaces that are touching each other
• The amount of friction is dependant upon
– Surfaces
– Force pressing the surfaces together
Types of Friction
• Static Friction
– Friction between two objects that are
NOT moving.
• Sliding Friction
– Friction that between two surfaces moving
past each other.
• Rolling Friction
– Friction between a rolling object & the
surface it is rolling on.
• Fluid Friction
– Friction between an object and a gas or
liquid
• Encarta Friction Animation
Air Resistance
• Collisions of the object's leading surface with air
molecules.
• Dependent upon a variety of factors, most
importantly
– the speed of the object
• Increased speeds = increased air resistance.
– the cross-sectional area of the object
• Increased cross-sectional area =
increased air resistance.
FIND the Friction – 5 sentences
Gravity
• Universal law of Gravity
– Any two objects will exert
an attractive force on each
other
– The size of the force is
dependant on
• Mass of both objects
• Distance between the
objects
– The range of gravity never
disappears
• Therefore it is not possible
for an object with mass to
have no weight.
expanding universe
Weight vs. Mass
• Weight
– Force of gravity pulling you toward
the earth
– F = ma becomes W = mg
• W means weight
• g means acceleration due to gravity
~10m/s2
– SI Base Unit = Newtons
– Weight can change with a change in
location.
• Mass
– A measure of how much matter any
object has
– You know an object has mass
because it has inertia
– SI Base Unit = Kilograms
Falling Objects on Earth
• Near Earth’s surface
• Acceleration due to gravity
– 9.8 m/s2 or ~ 10 m/s2
• If a bowling ball and a
marble are dropped from
the same height, which
would hit the ground first?
Simultaneous drop of two objects with different masses
dropping objects on the moon
Momentum
• A property related to the how much force is
required to change the objects motion
• The product of the objects mass and velocity
• p = mv
– p = momentum
– m = mass
– v = velocity
• SI UNIT = Kg m / s
Law of Conservation of Momentum
• Momentum may be
transferred to another
object
• The total momentum
before a collision is
equal to the total
momentum after the
collision
• m 1 v 1 = m2 v 2
• interactive
Types of Collisions
• Elastic collisions
– the total kinetic energy
before the collision is equal
to the total kinetic energy
after the collision.
– Usually the objects bounce
off of each other.
• Inelastic collisions
– the total kinetic energy
before the collision is NOT
equal to the total kinetic
energy after the collision.
– Usually the objects stick
together
Monkey and the Zookeeper
•
The zookeeper must shoot the
banana from the banana cannon
to the monkey who hangs from the
limb of a tree.
•
This particular monkey has a habit
of dropping from the tree the
moment that the banana leaves
the muzzle of the cannon.
•
The zookeeper is faced with the
dilemma of where to aim the
banana cannon in order to hit the
monkey.
•
If the monkey lets go of the tree
the moment that the banana is
fired, then where should she aim
the banana cannon?
Monkey and the Zookeeper
Where should the zookeeper aim?
Taking Gravity into effect The zookeeper aims above the monkey
Does the speed of the banana change the effect?
The zookeeper aims at the monkey and shoots the banana very fast
The zookeeper aims at the monkey, yet shoots the banana very slow.
ZOO in SPACE
In a Gravity free environment, where should the zookeeper shoot the banana?