Transcript Ch 12 PowerPoint Notes

Chapter 12
Forces and Motion
12.1 Forces
What Is a Force?
A force is a push or a pull that
acts on an object.
It can cause a resting object to
move, or it can accelerate a
moving object by changing the
object’s speed or direction.
Units of Force
One newton (N) is the force
that causes a 1-kilogram
mass to accelerate at a rate
of 1 meter per second each
second (1 m/s2).
1 N = 1 kg•m/s2
Combining Forces
When the forces on an object are
balanced, the net force is zero
 There is no change in the
object’s motion.
When an unbalanced force acts on
an object, the object accelerates.
The net force is the overall force
acting on an object after all the
forces are combined.
• Forces in the same direction
add together.
• Forces in opposite directions
subtract from one another.
Balanced Forces
Balanced forces are forces that
combine to produce a net force of
zero.
An unlimited number of individual
forces can act on an object to
produce a net force of zero.
Unbalanced Forces
An unbalanced force is a
force that results when
the net force acting on an
object is not equal to
zero.
Activity Calculating
 Tugboat
A exerts a force of 4000 N on
a barge. Tugboat B exerts a force of
8000 N on the barge in the same
direction. What is the combined
force? Using arrows draw the
combined forces. Then draw the
forces involved if the tugboats were
pulling in opposite directions.
ANSWER:
In the same direction:
4000N + 8000N = 12000N
In opposite directions:
4000N – 8000N = 4000N toward the
direction of tugboat B
Friction
Friction is a force that opposes the
motion of objects that touch as they
move past each other.
four main types of friction:
static friction
sliding friction
rolling friction
fluid friction.
Static Friction
Static friction is the friction
force that acts on objects that
are not moving.
Static friction always acts in the
direction opposite to that of the
applied force.
Rolling Friction
Rolling friction is the friction
force that acts on rolling
objects.
• about 100 to 1000 times less
than the force of static or
sliding friction.
Sliding Friction
Sliding friction is a force that
opposes the direction of motion
of an object as it slides over a
surface.
Sliding friction is less than static
friction.
Fluid Friction
• Fluid friction opposes the motion of
an object through a fluid.
• increases as the speed of the
object moving through the fluid
increases.
• Fluid friction acting on an object
moving through the air is known as
air resistance.
Gravity
Earth’s gravity acts
downward toward the
center of Earth.
How do gravity and air resistance
affect a falling object?
Gravity causes objects to
accelerate downward, whereas
air resistance acts in the
direction opposite to the motion
and reduces acceleration.
Gravity is a force that acts
between any two masses.
• an attractive force.
• can act over large
distances.
Free Fall
fall – motion of an
object when gravity is the
only force acting on it
 All objects near earth
accelerate at 9.8 m/s2 in
the absence of air
resistance
 Free
Terminal Velocity
 Terminal Velocity – when
air resistance balances
weight and the object
stops accelerating and
reaches its maximum
velocity
320 km/h (200 mi/h)
Projectile Motion
A thrown ball follows a curved path.
Projectile motion is the motion of a
falling object (projectile) after it is
given an initial forward velocity.
Air resistance and gravity are
the only forces acting on a
projectile.
REVIEW
1.
If an object is at rest, which of the
following statements must be true?
a. There are no forces acting on the
object.
b. There is no friction acting on the
object.
c. The forces acting on the object are
unbalanced.
d. The net force acting on the object is
zero.
REVIEW
1.
If an object is at rest, which of the
following statements must be true?
a. There are no forces acting on the
object.
b. There is no friction acting on the
object.
c. The forces acting on the object are
unbalanced.
d. The net force acting on the object is
zero.
REVIEW
2.
Which of the following is not
a type of friction?
a. static friction
b. sliding friction
c. fluid friction
d. pull friction
REVIEW
2.
Which of the following is not
a type of friction?
a. static friction
b. sliding friction
c. fluid friction
d. pull friction
REVIEW
3.
In which direction does Earth’s
gravitational force act?
a. opposite the direction of motion
b. downward toward the center of
Earth
c. upward away from the center of
Earth
d. in the direction of motion
REVIEW
3.
In which direction does Earth’s
gravitational force act?
a. opposite the direction of motion
b. downward toward the center of
Earth
c. upward away from the center of
Earth
d. in the direction of motion
REVIEW
4.
A ball thrown into the air follows a
projectile course due to the initial
velocity and the
a. force of gravity.
b. effect of air resistance.
c. motion of Earth beneath it.
d. mass of the ball.
REVIEW
4.
A ball thrown into the air follows a
projectile course due to the initial
velocity and the
a. force of gravity.
b. effect of air resistance.
c. motion of Earth beneath it.
d. mass of the ball.
REVIEW
5. The SI unit for force is 1
kg•m/s2, also called one
kepler.
True
False
REVIEW
5.
The SI unit for force is 1
kg•m/s2, also called one
kepler.
False: The SI unit for force is
2
1 kg•m/s , also called one
Newton.
Chapter 12
Forces and Motion
12.2 Newton’s first and
second laws of motion
Newton
Newton built on the work of
scientists such as Galileo.
• Newton first defined mass
and force.
• He then introduced his
laws of motion.
Newton’s First Law of Motion
According to Newton’s first
law of motion, the state of
motion of an object does
not change as long as the
net force acting on the
object is zero.
Unless acted upon by an
unbalanced force, an object at rest
remains at rest and an object in
motion remains in motion with the
same speed and direction.
Inertia is the tendency of an object
to resist a change in its motion.
Newton’s Second Law of Motion
Newton’s second law of
motion: the acceleration of
an object is equal to the net
force acting on it divided by
the object’s mass.
F = ma
Newton’s Second Law of Motion
The acceleration of an object is directly
proportional to the net force acting on
it.
 also depends upon its mass.
 Mass is a measure of the inertia of
an object.
2nd Law Example
 How
much force is needed to
accelerate a 1400 kg car 2 m/s2?
2nd Law Example
 How
much force is needed to
accelerate a 1400 kg car 2 m/s2?
F = ma
F = 1400kg x 2 m/s2
F = 2800N
(Remember: 1 N = 1 kg•m/s2 )
Newton’s Second Law
An automobile with a mass of
1000 kilograms accelerates
when the traffic light turns
green. If the net force on the car
is 4000 newtons, what is the
car’s acceleration?
Newton’s Second Law
F = ma
4000N = 1000kg x
2
m/s
2
m/s
4000 / 1000 = 4
Acceleration = 4 m/s2
Newton’s Second Law of Motion
1. A boy pushes forward a cart
of groceries with a total mass
of 40.0 kg. What is the
acceleration of the cart if the
net force on the cart is 60.0 N?
Newton’s Second Law of Motion
1.
A boy pushes forward a cart of
groceries with a total mass of 40.0 kg.
What is the acceleration of the cart if
the net force on the cart is 60.0 N?
F = ma
2
60 = 40kg x m/s
60 / 40 = 4.5 m/s2
acceleration = 4.5 m/s2
2nd Law Practice
1.
2.
3.
How much force is needed to
accelerate a 66 kg skier 1 m/s2?
What is the force on a 1000 kg
elevator that is falling freely at 9.8
m/s2?
A baseball accelerates down at 9.8
m/s2. If the force is 1.4 N, what is
the baseball’s mass?
2nd Law Practice
1.
How much force is needed to accelerate a 66 kg
skier 1 m/s2?
66N
2. What is the force on a 1000 kg elevator that is falling
freely at 9.8 m/s2?
9800 N
3. A baseball accelerates down at 9.8 m/s2. If the force
is 1.4 N, what is the baseball’s mass?
0.143kg
Mass and weight are related but are
not the same.
• Mass is the measure of the
amount of material an object
contains.
• Weight is the force of gravity
acting on an object.
• Weight is the product mass and
acceleration due to gravity.
Weight and Mass
W = mg is a different form of
Newton’s Second Law, F = ma.
The value of g in the formula is
9.8 m/s2.
Weight and Mass
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?
Weight and Mass
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?
Weight = 112 kg x 9.8 m/s2
Weight = 1097.6 N
Weight Calculations
1.
2.
3.
A 10 kg mass would weigh __
A 50 kg mass would weigh __
A 100 kg mass would weigh __
Weight Calculations
1.
2.
3.
A 10 kg mass would weigh _98 N_
A 50 kg mass would weigh _490 N_
A 100 kg mass would weigh _980 N_
REVIEW
1.
What is inertia?
a. the force of gravity acting on an object
b. forces of friction slowing an object’s
motion
c. the mass of an object
d. the tendency of an object to resist
change in its motion
REVIEW
1.
What is inertia?
a. the force of gravity acting on an object
b. forces of friction slowing an object’s
motion
c. the mass of an object
d. the tendency of an object to resist
change in its motion
REVIEW
2.
A 3600-N force causes a car to
accelerate at a rate of 4 m/s2. What
is the mass of the car?
a. 600 kg
b. 900 kg
c. 14,400 kg
d. 1200 kg
REVIEW
2.
A 3600-N force causes a car to
accelerate at a rate of 4 m/s2. What
is the mass of the car?
a. 600 kg
b. 900 kg = 3600 / 4
c. 14,400 kg
d. 1200 kg
REVIEW
3.
How would your mass and weight change if
you were on the moon’s surface?
a. They wouldn’t change.
b. Your mass would remain constant, and your
weight would increase.
c. Your mass and weight would decrease.
d. Your mass would remain constant, and your
weight would decrease.
REVIEW
3.
How would your mass and weight change if
you were on the moon’s surface?
a. They wouldn’t change.
b. Your mass would remain constant, and your
weight would increase.
c. Your mass and weight would decrease.
d. Your mass would remain constant, and your
weight would decrease. (Acceleration due to
gravity would be less on the moon)
Chapter 12
Forces and Motion
12.3 Newton’s Third Law of
Motion and Momentum
Newton’s Third Law
According to Newton’s third
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.
Momentum
Momentum is the product of
an object’s mass and its
velocity.
momentum is large if the product
of its mass and velocity is large.
An object with large
momentum is harder to
stop than an object with
small momentum.
The momentum for any
object at rest is zero.
Momentum
p = mv
Mass is measured in kilograms.
Velocity is measured in meters per
second.
Momentum is measured kilogrammeters per second.
Momentum
Unit: kg·m/s
Ex: Calculate the momentum of a
6.00 kg bowling ball moving at
10.0 m/s down the alley
Momentum
p = mv
Unit: kg·m/s
Ex: Calculate the momentum of a
6.00 kg bowling ball moving at
10.0 m/s down the alley
p = mv
p = 6 kg x 10 m/s
60 kg·m/s = momentum
Momentum
Which has more momentum,
a 0.046-kilogram golf ball
with a speed of 60.0 meters
per second, or a 7.0-kilogram
bowling ball with a speed of
6.0 meters per second?
Momentum
Which has more momentum,
a 0.046-kilogram golf ball
with a speed of 60.0 meters
per second (2.76 kg·m/s) , or
a 7.0-kilogram bowling ball
with a speed of 6.0 meters
per second (42 kg·m/s)?
Momentum Practice
1.
2.
3.
4.
75 kg speed skater moving
forward at 16 m/s
135 kg ostrich running north at
16.2 m/s
A 5.0 kg baby on a train moving
east at 72 m/s
48.5 kg person on a stopped train
Momentum Practice
1. 75 kg speed skater moving forward at 16 m/s
75kg x 16m/s = 1200 kg·m/s
2. 135 kg ostrich running north at 16.2 m/s
135kg x 16.2m/s = 2187 kg·m/s
3. A 5.0 kg baby on a train moving east at 72
m/s
5kg x 72m/s = 360 kg·m/s
4. 48.5 kg person on a stopped train
48.5kg x 0m/s = 0 kg·m/s
Conservation of Momentum
A closed system means other
objects and forces cannot enter or
leave a system.
In a closed system, the loss of
momentum of one object equals
the gain in momentum of another
object— momentum is conserved.
Conservation of Momentum
Objects within a closed system can
exert forces on one another.
Law of conservation of
momentum:
if no net force acts on a system,
then the total momentum of the
system does not change.
REVIEW
1.
A stationary figure skater pushes off the
boards around an ice skating rink and
begins gliding backward, away from the
boards. Which law explains why the
figure skater moves backward?
a. the law of conservation of energy
b. the law of inertia
c. Newton’s second law
d. Newton’s third law
REVIEW
1.
A stationary figure skater pushes off the
boards around an ice skating rink and
begins gliding backward, away from the
boards. Which law explains why the
figure skater moves backward?
a. the law of conservation of energy
b. the law of inertia
c. Newton’s second law
d. Newton’s third law
Chapter 12
12.4 Universal Forces
Four universal forces exist
throughout the universe.
• electromagnetic
• strong nuclear
• weak nuclear
• gravitational
Electromagnetic Forces
Electric force and magnetic
force are the only forces
that can both attract and
repel.
Electromagnetic Forces
Electric and magnetic force
are two different aspects of
the electromagnetic force.
Electromagnetic force is
associated with charged
particles.
Electric Forces
Electric forces act between
charged objects or particles.
• Objects with opposite
charges attract one another.
• Objects with like charges
repel one another.
Magnetic Forces
Magnetic forces act on
• certain metals,
• the poles of magnets,
and
• moving charges.
Magnets have two poles—
north and south.
• Two poles that are
opposite attract each
other.
• Two poles that are alike
repel each other.
Nuclear Forces
Two forces, the strong
nuclear force and the weak
nuclear force, act within the
nucleus to hold it together.
Nuclear Forces
The strong nuclear force
overcomes the electric force of
repulsion that acts among the
protons in the nucleus.
The weak nuclear force is
involved in certain types of
radioactive processes.
Gravitational Forces
Newton’s law of universal
gravitation states that every
object in the universe
attracts every other object.
Gravitational force
is an attractive
force that acts
between any two
masses.
-depends upon
mass and
distance.
Gravitational Forces
Gravity Acts Over Long Distances
The gravitational force between two
objects is proportional to their masses.
 decreases with the square of the
distance between the objects.
Gravity is the weakest universal force,
but it is the most effective force over
long distances.
Gravitational Forces
The Earth, Moon, and Tides
Earth’s gravitational attraction keeps
the moon in a nearly circular orbit
around Earth.
A centripetal force is a center-directed
force that continuously changes the
direction of an object to make it move
in a circle.
REVIEW
1.
What are the only forces that can
both attract and repel?
a. electromagnetic forces
b. centripetal forces
c. strong nuclear forces
d. gravitational forces
REVIEW
1.
What are the only forces that can
both attract and repel?
a. electromagnetic forces
b. centripetal forces
c. strong nuclear forces
d. gravitational forces
REVIEW
2.
The nucleus of an atom is held
together primarily by the
a. strong force and weak force.
b. strong force and gravity.
c. weak force and electromagnetic
force.
d. electromagnetic force and strong
force.
REVIEW
2.
The nucleus of an atom is held
together primarily by the
a. strong force and weak force.
b. strong force and gravity.
c. weak force and electromagnetic
force.
d. electromagnetic force and strong
force.
REVIEW
3.
Which of the following statements about
gravitational forces is false?
a. They are the weakest universal forces.
b. They act between any two objects.
c. They become stronger as the distance
between two objects increases.
d. They become weaker as the mass of
either two objects decreases.
REVIEW
3.
Which of the following statements about
gravitational forces is false?
a. They are the weakest universal forces.
b. They act between any two objects.
c. They become stronger as the distance
between two objects increases.
d. They become weaker as the mass of
either two objects decreases.
REVIEW
4.
A center-directed force that
continuously changes the direction of
an object’s motion, making it move in a
circle, is called the radial force.
True
False
REVIEW
4.
A center-directed force that
continuously changes the direction of
an object’s motion, making it move in a
circle, is called the radial force.
True
False