Download Forces

Newton’s 1st Law
2.3
Aristotle
• 4th Century
• Natural Motion
• Up or down
• Violent Motion
• Pushed or pulled
Newton’s 1st Law
2.3
Nicholas Copernicus
(Mikolaj Kopernik)
• 1473-1543
• De Revolutionibus
• Earth goes around the Sun
• Feared Persecution
Newton’s 1st Law
2.3
Galileo Galilei
• 1564-1642
• Force is a push or pull
•
Galileo
explains
his
discoveries
to the pope
• Friction is the force
between materials- caused
by irregularities in surface
• Inertia
Motion and Forces
2.3
Inertia
• Inertia (ih NUR shuh) is the tendency
of an object to resist any change in its
motion.
• It will keep moving at the same speed
and in the same direction unless an
unbalanced force acts on it.
Motion and Forces
2.3
Inertia
• The velocity of the object remains constant
unless a force changes it.
• If an object is at rest, it tends to remain at
rest. Its velocity is zero unless a force makes
it move.
Newton’s 1st Law
2.3
Galileo Galilei: Spheres on inclines
•Horizontal Surface: Without Friction
•How High:
With Friction
Motion and Forces
2.3
Newton's Laws of Motion
• The British scientist Sir Isaac
Newton (1642–1727) was
able to state rules that
describe the effects of forces
on the motion of objects.
• These rules are known as
Newton's law's of motion.
Motion and Forces
2.3
Newton's First Law of Motion
• Newton's first law of motion is that an
object stays at rest or stays at a constant
velocity unless acted upon by a net force
• This law is sometimes called the law of
inertia.
• ( you might know this as an object at rest
stays at rest, an object in motion stays in
motion unless acted upon by an
unbalanced force)
Motion and Forces
2.3
Newton's First Law of Motion
• Newton's first law of motion is that an
object stays at rest or stays at a constant
velocity unless acted upon by a net force
• Does this law pertain to moving objects,
objects at rest or both?
Motion and Forces
2.3
Forces
• Push or pull
• Can be
• At-a-distance
• Contact
Motion and Forces
2.3
Unbalanced Forces
• The forces are
considered to be
unbalanced forces
because the forces
do not cancel each
other perfectly.
Motion and Forces
2.3
Unbalanced Forces
• The students are pushing on the box in the
same direction.
• These forces are
combined, or added
together, because
they are exerted on
the box in the same
direction.
Motion and Forces
2.3
Balanced Forces
• The net force on the box is zero because the
two forces cancel each other.
• Forces on an object
that are equal in size
and opposite in
direction are called
balanced forces.
Motion and Forces
2.3
Possible Motions
• What are the possible motions for an
object with
• Unbalanced forces
Balanced Forces
Motion and Forces
2.3
Newton's First Law of
Motion
• Demonstrations
• Table cloth
http://www.stevespanglerscience.com/content/science-video/tablecloth-trick
http://images.google.com/imgres?imgurl=http://www.boingboing.net/images/ricardocloth.jpg&imgrefurl=http://www.boing
boing.net/2007/12/19/old-table-clothtric.html&usg=__wHCnH5iUI2NuMys86fXQ6wWUU4g=&h=200&w=296&sz=24&hl=en&start=6&tbnid=Y6wH3E
H6oHOE-M:&tbnh=78&tbnw=116&prev=/images%3Fq%3Dtablecloth%2Btrick%26gbv%3D2%26hl%3Den
• Seatbelts
•
http://www.youtube.com/watch?v=knCgF8K8xcY&feature=channel
Section Check
2.3
Question 1
A force is a __________.
Answer
A force is a push or pull. Forces, such as the
force of the atmosphere against a person’s body,
are not always noticeable.
Section Check
2.3
Question 2
When are forces on an object balanced?
Answer
When forces are equal in size and opposite in
direction, they are balanced forces, and the net
force is zero.
Section Check
2.3
Question 3
Inertia is __________.
A. the tendency of an object to resist any
change in its motion
B. the tendency of an object to have a positive
acceleration
Section Check
2.3
C. The tendency of an object to have a net
force of zero.
D. The tendency of an object to change in
speed or direction.
Section Check
2.3
Answer
Inertia is the tendency of an object to resist
any change in its motion. An unbalanced
force must act upon the object in order for
its motion to change.
•Newton’s Second Law
•3.1
Newton’s Second Law
• Newton’s second law of motion states that a
net force results in an acceleration
Motion and Forces
3.1
Unbalanced Forces
• net force is the combination of forces acting
on an object
• F Newtons
• When two students
are pushing with
unequal forces in
opposite directions, a
net force occurs in
the direction of the
larger force.
Motion and Forces
3.1
• NOT
Mass
• Volume
 IS
 Number of atoms
 Measured in kilograms
 Related to inertia
M
kg
•Newton’s Second Law
•3.1
Newton’s Second Law
• Newton’s second law of motion states that an
unbalanced force results in an acceleration
Fnet
m
a
Motion and Forces
3.1
Net Forces
Large mass
vs.
Small mass
•Gravity
•3.2
•What is gravity?
• Gravity is an attractive force between any
two objects that depends on the masses of
the objects and the distance between them.
•Gravity
•3.2
•Weight
• The gravitational force exerted on an object
is called the object’s weight.
•Symbol: W
•Unit : Newton
Motion and Forces
3.2
 IS
Mass
 Number of atoms
 Measured in kilograms
 Related to inertia
 Symbol: M
 Unit: kg
•Gravity
•3.2
•Earth’s Gravitational Acceleration
• Close to Earth’s surface, the acceleration of a
2
falling object in free fall is about 9.8 m/s .
• This acceleration is given the symbol g and is
sometimes called the acceleration of gravity.
•Gravity
•3.2
Weight
• Because the weight of an object on Earth is
equal to the force of Earth’s gravity on the
object, weight can be calculated from this
equation:
W
m g
•Gravity
•3.2
•Weight and Mass
• The table shows how various weights on
Earth would be different on the Moon and
some of the planets.
•Gravity
•3.2
•Hammer and Feather
• What happens on the Moon if you drop a
feather and hammer?
• http://er.jsc.nasa.gov/seh/feather.html
•Gravity
•3.2
•Weightlessness and Free Fall
• at 400-km altitude the force of Earth’s gravity
is about 90 percent as strong as it is at Earth’s
surface.
• So an astronaut with a mass of 80 kg still
would weigh about 700 N in orbit, compared
with a weight of about 780 N at Earth’s
surface.
•Gravity
•3.2
•Floating in Space
• If you and the scale were in free fall, then you
no longer would push down on the scale at all.
• The scale dial
would say you
have zero weight,
even though the
force of gravity on
you hasn’t
changed.
•Newton’s Second Law
•3.1
•Air Resistance
• The amount of air resistance on an object
depends on the speed, size, and shape of
the object.
• Air resistance, not
the object’s mass, is
why feathers, leaves,
and pieces of paper
fall more slowly
than pennies, acorns,
and apples.
•Newton’s Second Law
•3.1
•Terminal Velocity
• As an object falls, the downward force of
gravity causes the object to accelerate.
• However, as an object
falls faster, the upward
force of air resistance
increases.
• This causes the net
force on a sky diver to
decrease as the sky
diver falls.
•Newton’s Second Law
•3.1
•Terminal Velocity
• The terminal velocity is the highest
speed a falling object will reach.
• The terminal velocity depends on the size,
shape, and mass of a falling object.
• Happens when
• Weight = air resistance
•Section Check
•3.1
•Question 1
•Newton’s second law of motion states that
_________ of an object is in the same
direction as the net force on the object.
• A. acceleration
• B. momentum
• C. speed
• D. velocity
•Section Check
•3.1
•Answer
•The answer is A. Acceleration can be calculated
by dividing the net force in newtons by the mass
in kilograms.
•Section Check
•3.1
•Question 2
•The unit of force is __________.
• A. joule
• B. lux
• C. newton
• D. watt
•Section Check
•3.1
•Answer
•The answer is C. One newton = 1 kg · m/s2
•Section Check
•3.1
•Question 3
•What causes friction?
•Answer
•Friction results from the sticking together of
two surfaces that are in contact.
•Gravity
•3.2
•Centripetal Force
• An unbalanced force that
cause a change in direction is
a centripetal force.
•Gravity
•3.2
•Centripetal Force and Traction
• Anything that moves in a circle is doing so
because a centripetal force is accelerating it
toward the center.
•Gravity
•3.2
•Centripetal Force and Traction
• This centripetal force is the
frictional force between the
tires and the road surface.
• This centripetal
force is the sides
of the washer
•The Third Law of Motion
•3.3
•Newton’s Third Law
• Newton’s third law of motion describes
action-reaction pairs this way. When one
object exerts a force on a second object,
the second one exerts a force on the first
that is equal in strength and opposite in
direction.
•The Third Law of Motion
•3.3
•Action and Reaction
• When a force is applied in nature, a reaction
force occurs at the same time.
• When you jump on a trampoline, for
example, you exert a downward force on
the trampoline.
• Simultaneously, the trampoline exerts an
equal force upward, sending you high into
the air.
•The Third Law of Motion
•3.3
•Action and Reaction Forces Don’t
Cancel
• According to the third law of motion, action
and reaction forces act on different objects.
• Thus, even though the forces are equal, they
are not balanced because they act on
different objects.
The Third Law of Motion
•3.3
•Action and Reaction Forces Don’t
Cancel
• For example, a swimmer “acts” on the water,
the “reaction” of the water pushes the
swimmer forward.
• Thus, a net force,
or unbalanced
force, acts on the
swimmer so a
change in his or her
motion occurs.
•The Third Law of Motion
•3.3
•Rocket Propulsion
• In a rocket engine, burning fuel produces
hot gases. The rocket engine exerts a force
on these gases and causes them to escape
out the back of the rocket.
• By Newton’s third law,
the gases exert a force
on the rocket and push
it forward.
•Section Check
•3.3
Question 1
According to Newton’s third law of motion, what
happens when one object exerts a force on a
second object?
Answer
According to Newton’s law, the second object
exerts a force on the first that is equal in
strength and opposite in direction.
•The Third Law of Motion
•3.3
Force Pressure Area
• http://www.stevespanglerscience.com/content
/science-video/the-can-crusher
•Pressure = Force/ Area
F
P A