Transcript Ch 12 Notes – Teacher2 - Mona Shores Public Schools

A N D
1 2
M O T I O N
F O R C E S
c h a p t e r
What is a FORCE?
• A FORCE is a push or pull that acts on an
object.
• A force can cause a resting object to move
OR…Accelerate a moving object by:
– Changing the object’s speed
– Changing the object’s direction
How do we MEASURE force?
• Forces can be measured on a spring scale
– WEIGHT is a FORCE
• UNITS OF FORCE:
– Newton (N)
– A Newton is a force that causes a 1 kg mass to
accelerate at a rate of 1 meter per second each
second
• Which is written as: 1 N = 1 kg  m/s2
Representing Forces
• Forces are vectors & can be represented with
an arrow
– The length of the arrow shows strength or
magnitude (large force = long arrow)
– Direction of the arrow shows the direction of the
force
Combining Forces
• Net Force = the overall force acting on an
object after all forces are combined
Combining Forces
• Same Direction: add vectors
• Opposite Directions: subtract vectors
Net Force
Balanced Forces
• Balanced Forces = the net force is zero, there
is no change in the object’s motion
• Question: what are some examples where the
net force would equal zero?
ANSWER
• Net force would be zero when:
– You play tug-of-war and neither team moves
– You arm wrestle and neither person wins
– A car using cruise control – constant speed!
Unbalanced Forces
• Unbalanced Forces = the net force acting on
an object does not equal zero, the object
accelerates
• Question: What happens to the object when the net
force acting on an object is NOT ZERO, or
unbalanced?
Answer
• When an unbalanced force acts on an object,
the object accelerates
– 3 examples of a net force causing an object to
accelerate
• Pushing against the side of a book & getting it to move
• A team winning a game of tug-of-war & pulling the
other team towards them
• A person in freefall
Watch video clip: The Effects of Forces on Speed
• On video notes page, draw an example of when the
forces are balanced & when they are unbalanced.
Then describe the result of these forces.
FRICTION
FRICTION
• Friction: a force that opposes the motion of
objects that touch as they move past each
other
• Without friction, it would be a very different
world!!!
– Food would not stay on your fork!
– Cars would slide all over the road!
– Walking would be almost impossible!
• Friction acts at the surface where objects are
“in contact”
The 4 Types of Friction
• There are 4 main types of friction:
– Static Friction
– Sliding Friction
– Rolling Friction
– Fluid Friction
STATIC FRICTION
• friction force that acts on objects that are not
moving
• Examples:
– every time you take a step and push off
– glass of water sitting stationary on the table
SLIDING FRICTION
• A fricitonal force that opposes the motion of
an object as it slides over a surface
• Sliding friction is a weaker force than static
friction
– This is why less force is needed to keep an object
moving than it is to start it moving
ROLLING FRICTION
• The frictional force that acts on rolling objects
• Rolling friction is about 100-1000 times less
than static or sliding friction
– This is why we use wheeled dollies to move heavy
objects!
– In machines, ball bearings, are often used to
reduce friction between two surfaces
FLUID FRICTION
• Frictional force that opposes the motion of an
object through a fluid
– Water and a mixture of gases such as air are
known as fluids
– Example: a submarine moving through water
• The motion of the submarine is slowed by fluid friction
• Fluid friction increases as the speed of the object
moving through the fluid increase
– So the faster the sub goes, the greater the friction!!!
AIR RESISTANCE
• Air resistance is a type of fluid friction
– Remember…gases are considered “fluids”
GRAVITY
GRAVITY
• Gravity: a force that acts between any two
masses
– Gravity is an attractive force so it pulls objects
together
– Gravity does not require objects to be in contact
for it to act on them
• Gravity can act over large distances!!!
More GRAVITY!
• Earth's gravity acts downward toward the
center of the Earth.
• There is usually an upward force that acts
against gravity to balance out the forces and
allow objects to remain still.
FALLING OBJECTS
• QUESTION: What forces are acting on an
object as it falls?
• ANSWER: The only two forces acting on a
falling object are gravity and air resistance
Forces Acting on Falling Objects
• Gravity causes object to accelerate downward
• Air resistance acts in the opposite direction of
the motion
– Which means it reduces acceleration
• Recall…what happens to the amount of fluid
friction as an object speeds up?
Watch video clip: The Physics of Skydiving
• On the video notes page, describe what is meant by
terminal velocity & describe the forces on the
skydiver at this point.
Falling Objects
• If an object in freefall falls for long enough,
the upward force of air resistance will become
equal to the downward force of gravity.
– At this point, the two forces are BALANCED
– Acceleration is zero when forces are balanced
– The object will continue to fall at a constant
velocity
– This is called…
Terminal Velocity
• Terminal Velocity: the constant velocity of a
falling object when the force of air resistance
equals the force of gravity
Projectile Motion
• Projectile Motion: The motion of a falling
object (projectile) after it is given an initial
forward velocity. This causes the object to
have a curved path.
• If I shoot a bullet horizontally and at the same
time drop a bullet from the same height as the
gun…which will hit the ground first?
• The two bullets WILL hit the ground at the
same time!
Gravity doesn’t take a holiday on an object just because it’s
moving ~ P. Hewitt
Remember the
“Investigating Freefall
Lab” with the marbles?
How did that lab compare
with the previous
statement???
12.2 Newton’s First & Second
Laws of Motion
It’s not ALL about Newton…
• Aristotle incorrectly proposed that force is required
to keep an object moving at a constant speed
• Galileo studied how gravity produces constant
acceleration. He concluded that objects not
subjected friction or any other force would continue
to move indefinitely
• Newton built off the work of Galileo and later
published his work in a book entitled Principia
• Newton summarized his study of force and motion in
several laws of motion
1st Law of Motion – Law of Inertia
1st Law of Motion – Law of Inertia
• 1st Law: The state of motion of an object does
not change as long as the net force acting on
the object is zero
– In other words: Unless an unbalanced force acts,
an object at rest remains at rest, and an object in
motion remains in motion with the same speed
and direction
1st Law – Law of Inertia
• Example: A soccer ball resting on the grass
remains motionless until a force is applied to
it.
And…
• Example: A soccer ball in motion remains in
motion unless a force acts on it.
Why is it called the “Law of Inertia”
• INERTIA: the tendency of an object to resist a
change in its motion so…
an object at rest tends to remain at rest (resist
moving), and an object in motion tends to
remain in motion (resist stopping)
– Remember…soccer ball sat motionless (forces
were balanced) until an unbalanced force acted
on it
– The ball has inertia
– Everything with mass has inertia
– The more mass, the more inertia
Inertia…Front-end collision
• Example: Front-end collision
• Collision makes car stop suddenly
• Since you have inertia you continue moving
forward
• Page 364/365 Figure 12
Newton’s Second Law of Motion
2nd Law of Motion
Relates the acceleration of an object to the force acting
on it & the object’s mass.
• 2nd Law: The acceleration of an object is equal
to the net force acting on it divided by the
object’s mass. A = F/M
• http://www.bbc.co.uk/schools/scienceclips/ag
es/6_7/forces_movement.shtml
• In other words, the amount of acceleration an
object has depends on:
– How hard it is pushed (force)
– How heavy it is (mass)
Acceleration…
Force & Mass Relationship
What is the acceleration?
ANSWER
• Acceleration is calculated by dividing the
FORCE by the MASS
Acceleration = 100 N / 50 kg
A = 2 m/s2
• More practice:
How much force would it take to accelerate a
200kg object to 3m/s/s?
• 200kg x 3m/s/s = 600N
nd
2
Law
– Example: You apply force to a ball when you
throw it
• The harder you throw, the more the ball accelerates
• If you double the force, the acceleration of the ball
doubles as well
• If you double the mass of the ball the acceleration is
cut in half
• More Examples:
Important Notes…
Regarding Newton’s 2nd Law of Motion
• The acceleration of an object is always in the same
direction as the net force.
• In using Newton’s second law, it is helpful to realize
that the units N/kg and m/s2 are equivalent
• Newton’s second law also applies when a net force
acts in the direction opposite to the object’s motion
– producing deceleration (See figure 13, page 368)
– This is the principle used by automobile seat belts
• See Math Skills page 367 (in textbook) for extra
help
Weight & Mass…
What really is the difference?
• We often talk about weight and mass as if
they were the same thing
• Weight: is the force of gravity acting on an
object.
Mass Versus Weight
• Mass is a measure of the inertia of an object,
the amount of matter in an object
– Measure with a balance
• Weight is a measure of the force of gravity
acting on an object
– Measure with a spring scale
• Weight = Mass x Acceleration due to gravity
– W = mg
– Value of g = 9.8 m/s2 (Earth)
Practice Problem
• If an astronaut has a mass of 112 kilograms,
what is his weight on Earth where the
acceleration due to gravity is 9.8 m/s2?
– W = 112 kg x 9.8 m/s2
– W = 1097 kgm/s2 = 1097 N
• What would his weight be on the moon
(g=1.62 m/s2)?
– W = 112 kg x 1.62 m/s2
– W = 181 kgm/s2 = 181 N
Mass & Weight are Related
• Doubling the mass of an object also doubles
the object’s weight (if gravity remains the
same)
• If Gravity changes then…
– Example: Think about the astronaut
– On the moon, the acceleration due to gravity is
only about 1/6 that on Earth
– Thus, the astronaut weighs only about 1/6 as
much on the moon as on Earth
• *However, in both locations, the mass of the
astronaut is the same!
12.3 Newton’s 3rd Law of Motion
Forces CAN’T Exist Alone…
• Forces always exist in pairs.
• According to Newton’s 3rd law of motion,
whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object.
• These two forces are called action and
reaction forces
Action – Reaction Forces
• Example: The book lying on the table is
exerting a downward force on the table, while
the table is exerting an upward reaction force
on the book.
Newton’s Third Law Ex: A rocket
• More Examples:
Jumping off of a raft
Action force = you push on the raft
Reaction force = raft pushes back
A swimmer in a pool
Action force = swimmer’s arm pushes on water
Reaction force = water pushes back on swimmer
Action – Reaction Forces
• Question: Since these action/reaction forces
are equal in size and opposite in direction
then why don’t they cancel each other out
and produce a net force of zero?
Action – Reaction Forces
• Because the action/reaction forces do not act
on the same object!
• Swimmer Example:
– The action force acts on the water
– The reaction force acts on the swimmer
Momentum
• Momentum = the product of an object’s mass
and its velocity
• An object with a large momentum is hard to
stop
• Momentum = mass X velocity
• Practice Problem: What is the momentum of a
rock with a mass of 0.5kg that is moving at a
velocity of 5m/s?
• Momentum = 0.5kg X 5m/s = 2.5kg x m/s
– The units do NOT cancel!
• Momentum can be transferred from one
object to another during a collision
12.4 – Universal Forces
Universal Forces
1. Electromagnetic forces
2. Nuclear forces
3. Gravitational forces
Electromagnetic Forces
• only forces that both attract and repel.
• Ex: protons & electrons
Nuclear Forces
• Holds neutrons & protons together
• Works against the EM repeling forces
Gravitational Forces
• An attractive force between any two objects
GRAVITY
• Gravity is the weakest universal force
• On a daily basis, you don’t notice the force of
gravity that you exert on objects—this is
because your mass is sooooooo small.
• It takes a huge mass like the Earth’s to exert a
large gravitational force.
GRAVITY
• Gravitational force depends on two things: the
mass of an object and the distance between
two objects.
– A greater mass will exert a greater gravitational
force on an object.
– The greater the distance between two masses
significantly decreases the gravitational force.
Attractive Force of Gravity
GRAVITY…how can it be so weak?
• Even though gravity is the weakest universal force, it
is the most effective over large distances.
–
–
–
–
Gravity holds you on the ground
It holds the moon in orbit around the Earth
It holds the planets in orbit around the Sun
It holds the stars in orbit around their galaxies
Centripetal Force
• The force of gravity from the Earth
continuously pulls the moon in a nearly
circular orbit around the Earth.
• Centripetal force: a center-directed force that
continuously changes the direction of an
object to make it move in a circle.
– As an object moves in a continuous (constant)
circular motion, it is accelerating.
– How is it accelerating if it does not change speed?
• Change in direction!!!
Centripetal Force
• It’s a lot like a string tied to a ball…
• The force from the center of the string allows
the ball to twirl in a circle over your head
• As you twirl the ball, the string exerts a
centripetal force on the ball.
Centripetal force & Orbital Motion
• Objects need only a centripetal force and their
own inertia(velocity) to maintain an orbit.
• Orbital motion is a balance between the
centripetal force and inertia.
• If Centripetal force is stronger it will crash, if
inertia is stronger it will fly off into space
• Examples of objects in orbit:
– Satellites
– Our moon is a natural satellite of our planet
Orbital Motion
http://upload.wikimedia.org/wikipedia/commons/4/4e/Orbital_motion.gif
How is this possible?!?!
Same Place…Same Day…??? HOW!!!???
How is that possible?
• Ever been to the seashore? What do you
notice about the level of the water throughout
the day? Why does it change?
– The gravitational pull from the Moon produces
two tides in the Earth’s oceans as it moves around
the Earth.
• One bulge where the moon is closest to the Earth
• One bulge where the moon is farthest from the Earth
Tides
• Since the Earth rotates once per day, it results
in two high tides and two low tides per day on
Earth!
http://home.hiwaay.net/~krcool/Astro/moon/moontides/
Rotation versus Revolution
• The Earth moves in two major ways:
– Rotation- the spinning of the Earth on its axis
• This rotation causes day and night
– Revolution- the movement of one body in space
around another
• When does the Earth show revolution? Revolves
around the Sun
– How long does it take for the Earth to revolve?
• 365 Days
Ch 13 Forces in Fluids
13.1 Fluid Pressure
• Why is an Aeron chair so much more
comfortable than a bicycle seat?
• Pressure = the result of a force distributed
over an area
• Ex: the tip of a pencil easily pokes through a
sheet of paper while the eraser end does not
• Ex: finger poke vs needle poke
• Pressure = Force/Area
• Units of Pressure = N/m2 = Pascal (Pa)
• Practice problem: A box weighing 2700 N is
resting on the ground. If the box is covering
1.5 square meters of area, what is the
pressure?
• Pressure = 2700 N/1.5 m2 = 1800 Pa = 1.8 kPa
Pressure in Fluids
• Recall: a fluid is a substance that takes the
shape of its container, both liquids and gases
are fluids
• What happens to the pressure exerted on
your body as you dive deeper in a pool?
• Water pressure increases as depth increases
• The relationship between pressure and the
depth of a fluid can be shown with this
formula: P = d x h x g
• d = density
• h = depth
• g = accel due to gravity
• Fluid pressure is determined by the type of
fluid and its depth
• Which would exert more fluid pressure at a
depth of 20 m, Lake MI or the Gulf of Mexico?
Air Pressure & the Atmosphere
• We live at the bottom of a vast ocean of air.
• The weight of the Earth’s atmosphere exerts
pressure. At sea level = 101 kPa
• We refer to a certain altitude instead of a
depth
• Air pressure decreases as the altitude
increases
• Why do your ears pop while traveling up a
mountain?
• The difference in pressure between the inside
of your ear and the air outside causes air to
pass through a small tube within your ear to
equalize the pressure.
• This is the popping sensation you feel
• The atmosphere is exerting more than 1000 N
on the top of your head. Why aren’t we
crushed by this?
• The inside of your body exerts pressure
against the air pressure outside which
balances and results in a net force of zero.
Bernoulli’s Principle
• Bernoulli’s Principle = as the speed of a fluid
increases, the pressure within the fluid
decreases
• Demo: take a piece of paper and blow across
the top of the paper
Wings & Lift
• The ability of birds and airplanes to fly is
explained by Bernoulli’s principle
• The air traveling over the top of the curved
wing travels faster so the pressure is less
which create the lift