Download CLASSICAL_PHYSICS_edit

CLASSICAL PHYSICS
Describing Motion
• In science, motion is the change of
position of an object relative to a reference
point.
• A reference point is an object that appears
to stay in place.
*Read pgs 157-159 in Science Matters book for
more on reference frames.
Describing Motion continued
• Position To accurately describe the
position of an object, you must use a
reference point, a distance, and a direction.
• When you describe the motion of an object,
you would describe how the object’s
distance or direction or both changed
relative to the reference point.
Describing Motion continued
• Speed is the distance traveled divided by
the time taken to travel that distance.
• Speed is important in describing motion
because it tells how fast an object is
moving away from its beginning position.
• The units for speed are often m/s, but can
be any distance unit divided by a time unit.
Calculating Average Speed
• An Olympic Athlete runs a 400 m race in 50 s.
What is her average speed?
• Step 1: Write the equation for average speed.
average speed= total distance
total time
• Step 2: Replace the total distance traveled and
the total time with the values given and solve.
average speed= 400 m = 8 m/s
50 s
Practice Problems
1. A hurricane in the Gulf of Mexico travels 360
km in 15 h. What is the average speed of this
hurricane?
2. Bishop walked 420 m in 17.5 min. What is his
average speed in m/min? What is his average
speed in m/s?
Answers to Practice Problems
1. A hurricane in the Gulf of Mexico travels 360
km in 15 h. What is the average speed of this
hurricane?
Answer: 24 km/h (360km/15h)
2. Bishop walked 420 m in 17.5 min. What is his
average speed in m/min? What is his average
speed in m/s?
Answer: 24 m/min; 0.4 m/s (420m/17.5min; 420m/1,050s)
Describing Motion continued
• Direction of Motion Speed and direction
of motion are combined when describing
an object’s velocity.
• Velocity is a quantity that tells both how
fast an object is moving (its speed) and
which way it is going (its direction of
motion).
Describing Motion continued
• Acceleration Sometimes the velocity of an
object changes. The change in velocity
over time is called acceleration.
• Acceleration can be a change in speed, a
change in direction, or both.
• The most common units of acceleration are
meters per second per second, or (m/s)/s.
Forces Changing Motion
• A force is any push or pull on an object.
• The SI unit for force is the newton (N).
• When a force is applied on an object, the
object’s motion can change.
Forces Changing Motion continued
• Finding Net Force A net force is the
combination of all of the forces acting on an
object.
• If two forces are in the same direction, you
add the forces to calculate the net force.
• If the forces are in opposite directions, you
must subtract the forces to find the net force.
Net Force and Motion
• If the net force on an object is 0 N, the forces
on the object are said to be balanced.
• But if the net force is not equal to 0 N, the
forces on the objects are unbalanced.
• Whether the forces are balanced or
unbalanced determines whether the motion of
an object changes.
Net Force and Motion continued
• Balanced Forces: No Change in Motion
If the forces on an object are balanced, the
motion of the object will not change.
• The object will not increase or decrease in
speed, or change direction.
• If an object is standing still, it will remain
so.
Net Force and Motion continued
• Increasing Speed with Net Force If the
forces on an object are unbalanced, the
motion of the object will change.
• A net force on a nonmoving object will cause
the object to move in the direction of the net
force.
• Decreasing Speed with Net Force A net
force can also slow down an object that is
already moving.
Friction Opposes Motion
• Friction is a contact force that opposes
motion when two surfaces are touching.
• There are two kinds of friction: static and
kinetic.
Friction Opposes Motion continued
• Static Friction When a force is applied to
an object but does not cause the object to
move, static friction occurs.
• Kinetic Friction Kinetic friction is friction
between moving surfaces.
Gravity is a Universal Force
• Gravity is a result of mass and all matter is
affected by gravity. Because gravity affects
all matter, it is a universal force.
• All objects experience an attraction toward
all other objects.
Weight as a Measure of Gravitational
Force
• Weight is a measure of the gravitational force
on an object.
• When you see or hear the word weight, it
usually refers to Earth’s gravitational force on
an object.
• Weight can also be a measure of the
gravitational force exerted on objects by the
moon or other planets.
Weight as a Measure of Gravitational
Force continued
• Units of Weight and Mass Gravity is a
force, and weight is a measure of gravity.
So, weight is also measured in newtons.
• The SI unit of mass is the kilogram (kg).
Mass is often measured in grams (g) and
milligrams (mg) as well.
Weight as a Measure of Gravitational
Force continued
• The Differences Between Weight and Mass
Weight changes when gravitational force
changes.
• Mass is the amount of matter in an object. An
object’s mass does not change.
• Because mass and weight are constant on
Earth, the terms weight and mass are often
used to mean the same thing.
Gravity and Falling Objects
• Gravity and Acceleration Objects fall to
the ground at the same rate because the
acceleration due to gravity is the same for
all objects.
• Acceleration Due to Gravity As shown
on the next slide, for every second that an
object falls, the object’s downward velocity
increases by 9.8 m/s.
Gravity and Falling Objects continued
• Velocity of Falling Objects You can calculate the
change in velocity with the following equation:
• ∆v  g  t
• v= velocity
• g= acceleration due to gravity on Earth (9.8 m/s2)
• t= time that the object takes to fall (in seconds)
• If an object starts at rest, this equation yields the
velocity of the object after a certain time period.
Calculating the Velocity of Falling Objects
• A stone at rest is dropped from a cliff, and the stone
hits the ground after a time of 3 s. What is the
stone’s velocity when it hits the ground?
• Step 1: Write the equation for change in velocity.
– ∆v  g  t
• Step 2: Replace g with its value and t with the time
given in the problem, and solve.
∆v  9.8 m/s X 3 s = 29.4 m/s
s
*To rearrange the equation to fine time, divide by the
acceleration due to gravity.
t= ∆v
g
Practice Problems
1. A penny at rest is dropped from the top of a tall
stairwell. What is the penny’s velocity after it
has fallen for 2 s?
2. The same penny hits the ground in 4.5 s. What is
the penny’s velocity as it hits the ground?
3. A marble at rest is dropped from a tall building.
The marble hits the ground with a velocity of 98
m/s. How long was the marble in the air?
4. An acorn at rest falls from an oak tree. The
acorn hits the ground with a velocity of 14.7
m/s. How long did it take the acorn to land?
Answers to Practice Problems
1. 9.8 m/s2 X 2 s = 19.6 m/s downward
2. 9.8 m/s2 X 4.5 s = 44.1 m/s downward
3. 98 m/s2
4. 14.7 m/s
9.8m/s2 = 10 s
9.8 m/s2 = 1.5 s
Air Resistance and Falling Objects
• Air resistance is the force that opposes the
motion of objects through air.
• The amount of air resistance acting on an
object depends on the size, shape, and
speed of the object.
• The image on the next slide shows the
effects of air resistance on a falling object.
Air Resistance and Falling Objects
continued
• Acceleration Stops at the Terminal
Velocity As the speed of a falling object
increases, air resistance increases.
• The upward force of air resistance
continues to increase until it is equal to the
downward force of gravity. The object then
falls at a constant velocity called the
terminal velocity.
Projectile Motion and Gravity
• Projectile motion is the curved path an
object follows when it is thrown or
propelled near the surface of the Earth.
• Projectile motion has two components—
horizontal motion and vertical motion.
These components are independent, so
they have no effect on each other.
Projectile Motion and Gravity
continued
• Horizontal Motion is a motion that is
parallel to the ground.
• When you throw a ball, your hand exerts a
force on the ball that makes the ball move
forward. This force gives the ball its
horizontal motion.
Projectile Motion and Gravity
continued
• Vertical Motion is motion that is
perpendicular to the ground.
• A ball in your hand is prevented from falling
by your hand. After you throw the ball,
gravity pulls it downward and gives the ball
vertical motion.
Newton’s First Law of Motion
• An object at rest remains at rest, and an
object in motion remains in motion at a
constant speed and in a straight line
unless acted on by an unbalanced
force.
• Newton’s first law of motion describes the
motion of an object that has a net force of 0
N acting on it.
Newton’s First Law of Motion continued
• Part 1: Objects at Rest Objects at rest will
stay at rest unless they are acted on by an
unbalanced force.
• Part 2: Objects in Motion Objects will
continue to move with the same velocity
unless an unbalanced force acts on them.
• The image on the next slide shows how you
can have fun with Newton’s first law.
Newton’s First Law of Motion continued
• Friction and Newton’s First Law Friction
between an object and the surface it is
moving over is an example of an
unbalanced force that stops motion.
• Inertia and Newton’s First Law Newton’s
first law is sometimes called the law of
inertia. Inertia is the tendency of all objects
to resist any change in motion.
Newton’s First Law of Motion continued
• Mass and Inertia Mass is a measure of
inertia. An object that has a small mass has
less inertia than an object that has a large
mass.
• So, changing the motion of an object that
has a small mass is easier than changing
the motion of an object that has a large
mass.
Newton’s Second Law of Motion
• The acceleration of an object depends
on the mass of the object and the
amount of force applied.
• Newton’s second law describes the motion
of an object when an unbalanced force
acts on the object.
Newton’s Second Law of Motion continued
• Part 1: Acceleration Depends on Mass The
acceleration of an object decreases as its
mass increases. Its acceleration increases as
its mass decreases.
• Part 2: Acceleration Depends on Force An
object’s acceleration increases as the force on
the object increases. The acceleration of an
object is always in the same direction as the
force applied
Newton’s Second Law of Motion continued
• Expressing Newton’s Second Law
Mathematically The relationship of
acceleration (a) to mass (m) and force (F)
can be expressed mathematically with the
following equation:
a = F , or F= m X a
m
Second Law Calculations
What is the acceleration of a 3 kg mass if a force
of 14.4 N is used to move the mass? (Note: 1N
is = 1 kg m/s2).
Step 1: Write the equation for acceleration
Step 2: Replace F and m with the values given in
the problem, and solve.
a = 14.4 kg m/s2 = 4.8 m/s2
3 kg
Practice Problems
1. What is the acceleration of a 7 kg mass if a force
of 68.6 N is used to move it toward Earth?
2. What force is necessary to accelerate a 1,250 kg
car at a rate of 40 m/s2?
3. Zookeepers carry a stretcher that holds a
sleeping lion. The total mass of the lion and the
stretcher is 175 kg. The lion’s forward
acceleration is 2 m/s2. What is the force
necessary to produce this acceleration?
Answers to Practice Problems
1. 68.6 N 7 kg = 9.8 m/s2
*This is acceleration due to gravity.
2. 1,250 X 40 m/s2 = 50,000 N
3. 175 X 2 m/s2 = 350 N
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.
• Newton’s third law of motion can be simply
stated as follows: All forces act in pairs.
Newton’s Third Law of Motion continued
• Force Pairs Do Not Act on the Same Object
A force is always exerted by one object on
another object. This rule is true for all forces,
including action and reaction forces.
• Action and reaction forces in a pair do not act
on the same object. If they did, the net force
would always be 0 N and nothing would ever
move!
Newton’s Third Law of Motion continued
• All Forces Act in Pairs—Action and
Reaction Newton’s third law says that all
forces act in pairs. When a force is
exerted, there is always a reaction force.
Newton’s Third Law of Motion continued
• The Effect of a Reaction Can Be Difficult to
See When an object falls, gravity pulls the
object toward Earth and pulls Earth toward the
object.
• You don’t notice Earth being pulled upward
because the mass of Earth is much larger
than the mass of the object. Thus, the
acceleration of Earth is much smaller than the
acceleration of the object.
Momentum, Mass, and Velocity
• The momentum of an object depends on
the object’s mass and velocity.
• Calculating Momentum The relationship
of momentum (p), mass (m), and velocity
(v) is shown in the equation below:
•p  m  v
Momentum Calculations
What is the momentum of an ostrich with a mass of
120 kg that runs with a velocity of 16 m/s north?
Step 1: Write the equation for momentum.
p=mXv
Step 2: Replace m and v with the values given in the
problem, and solve.
p = 120 kg X 16 m/s north
p = 19,200 kg m/s north
Practice Problems
1. What is the momentum of a 6 kg bowling ball
that is moving at 10 m/s down the alley
toward the pins?
2. An 85 kg person is jogging with a velocity of
2.6 m/s to the north. Nearby, a 65 kg teen is
skateboarding and is traveling with a velocity
of 2 m/s north. Which person has a greater
momentum? Show your calculations.
Answers to Practice Problems
1. p = 6 kg x 10 m/s down the alley = 60 kg m/s
down the alley.
2. The man jogging has greater momentum.
p = 85 kg X 2.6 m/s north = 221 kg m/s north
(man jogging)
p = 65 kg X 3 m/s north = 195 kg m/s north
(teen skateboarding)
The Law of Conservation of Momentum
• The law of conservation of momentum
states that any time objects collide, the
total amount of momentum stays the same.
• Objects Sticking Together After two
objects stick together, they move as one
object. The mass of the combined objects
is equal to the masses of the two objects
added together.
The Law of Conservation of
Momentum continued
• Objects Bouncing Off Each Other When
two objects bounce off each other,
momentum is usually transferred from one
object to the other.
• The transfer of momentum causes the
objects to move in different directions at
different speeds.
The Law of Conservation of
Momentum continued
• Conservation of Momentum and
Newton’s Third Law Conservation of
momentum can be explained by Newton’s
third law.
• Because action and reaction forces are
equal and opposite, momentum is neither
gained or lost in a collision.
Scalars and Vectors
Imagine I tell you that right now there is $1
million dollars located within 50 meters of
where you are sitting. Would you be the first
to find it? How would you go about your
search?
Scalars and Vectors
• Scalar: a quantity possessing only magnitude.
– example: 50 meters
• Vector: a quantity possessing both magnitude
and direction, represented by an arrow the
direction of which indicates the direction of
the quantity and the length of which is
proportional to the magnitude.
– Example: 50 meters northeast
Examples of Scalars
• Scalars
– Area
– Time
– Volume
– Speed
– Distance
Examples of Vectors
• Vectors
– Force
– Velocity
– Displacement
– Acceleration
Any Questions?
• See Science Matters book.
• Review Introduction to Physics on Beyond
Books
• Utilize materials in Curriculum Library.
• Search for interactive physics websites.
• 