Transcript chp. 8
How
do you know an object is
moving?
An
object is moving if
its position changes
against some
background that stays
the same.
The
stationary
background is called the
reference frame.
The
change in position is
measured by the distance
traveled by an object
from a fixed point.
Speed
describes how fast an
object moves.
Speed
is measured by calculating
the distance traveled by the time.
SI
units meters per second
(m/s)
Speed
can also be
measured in Kilometers per
hour (km/h) and miles per
hour (mi/h)
Constant
speed is the
simplest type of motion.
An
object is moving at a
constant speed when it
covers equal distances in
equal amounts of time
Speed
can be determined from
a distance-time graph
Time
is given on the X axis
Distance is given on the Y axis
Speed
can be determined by
calculating the slope of the line
Speed
= Distance
Time
V = d/t
http://www.youtube.com/watch?v=DRb5PSxJerM
Velocity
describes both
speed & direction of
the motion
Example:
The car was
traveling at 50 mi/h
headed north
The
velocity of an
object changes if its
speed or direction
changes.
Does
a car’s velocity change when it turns a
corner?
Which
is harder to stop…
A
speeding train or a speeding bicycle?
A
grown man or a small child
Stampede
mice?
Why?
of elephants or a stampede of
Momentum
is a quantity
defined as the product of
an object’s mass and its
velocity
Moving
objects have
momentum
If
an object is moving in a
straight line, momentum is
calculated by multiplying
an object’s mass by the
velocity
An
object’s momentum is in
the same direction as its
velocity.
Measured
in kilograms X
meters per second
kg
X m/s
Momentum
= mass X velocity
P =mv
The
total amount of
momentum in a system is
conserved
Practice Problems
Calculate the momentum for..
A
135 kg speed racer moving
forward at 16m/s
A
25 kg ostrich running north at
16.2 m/s
A
48.5kg passenger on a train
stopped on the tracks
Section 2 Acceleration & Force
Acceleration
is the
change in velocity
divided by the time
Acceleration
= Final velocity – Initial velocity
Time
If the acceleration is small, the speed
in increasing gradually.
If the acceleration is large, the speed is
increasing rapidly.
Positive
acceleration means
the object’s velocity will
__________.(speed up)
Negative
acceleration means
the object’s velocity will
decrease (slow down)
Increase
Practice Problem
A flowerpot falls off a
second-story windowsill.
The flowerpot starts
from rest & hits the
sidewalk 1.5s later with
a velocity of 14.7 m/s.
What is the average
acceleration?
Velocity-time graph Yay!
Acceleration
can be determined from a
velocity-time graph.
What
information is on the X axis?
What
information is on the Y axis?
The
Acceleration of an object is
O if its velocity is constant
Why?
Force
Force is the cause of
acceleration, or change in
an object’s velocity.
Net Force
Many
forces can act on an object
at a given time.
Net
Force is the combination of
all the forces acting on an object
They
determine whether the
velocity of an object will change
An
object’s acceleration is in
the direction of the net force.
Objects
will not accelerate if
the net force is zero.
Balanced
forces do not
change motion
Tug- of –war
Balanced
forces are forces
acting on an object that
combine to produce a net
force equal to zero
The
forces “cancel” each
other out.
Unbalanced Forces Do Not Cancel
Completely
Will
produce a net nonzero
force.
If
there are unbalanced forces
in different directions they
will act like a single force on
the object.
The net force will cause the object to
accelerate
Friction & Air Resistance
Friction
is the force between
2 objects in contact that
opposes the motion of either
object.
Example:
The forces to move a
car forward must be unbalanced.
The
force moving forward must be greater
than the friction opposing the car’s motion.
Frictional Force varies
depending on the
surface in contact.
Example:
Tennis
shoes on a waxed floor versus
shoes with very little tread.
(Someone is going to fall)
Air Resistance is a form of friction
Air
resistance opposes the motion of
an object.
Air
Resistance is caused by the
interaction between the surface of
the object & the surrounding air
molecules
Depends
shape
on the object’s size &
Key Vocabulary:
Speed
Velocity
•Unbalanced Forces
•Balanced Forces
Momentum
•Inertia
Acceleration
•Gravity
Force
•Friction
Gravity
Gravity is the attraction between
2 particles of matter due to their
mass.
The
force of gravity will act on
objects that do not touch.
Mass
& distance affect
gravitational force
The greater the mass of an
object, the larger the
gravitational force it is exerts on
other objects.
Earth’s
gravitational force is very
large due to Earth’s massive size.
What would happen if the force wasn’t as strong?
The
force of gravity
changes as the distance
between two objects
changes.
Gravitational
force between two
object will ____________as the
distance between 2 object
increases.
Chapter 8 Section 3
Newton’s Laws of
Motion!
Sir Isaac Newton
described the
relationship between
motion & force in 3
laws
Newton’s First Law
An
object at rest
remains at rest & an
object in motion
maintains its velocity
unless it experiences
an unbalanced force.
Example
Compare
sliding your book across
carpet & ice…
On
which surface will it slide longer?
Why?
Smooth
surfaces provide less
friction to oppose motion.
Inertia
Inertia
is the tendency of an
object to remain at rest or in
motion with a constant velocity.
All
object have inertia because
they resist changes in motion.
The
greater the mass of an object, the greater
the force that has to be applied to cause the
object to move.
Newton’s 2nd Law
The
unbalanced
force acting on an
object equals the
object’s mass times
its acceleration.
F = ma
Force = mass x acceleration
Example:
Pushing
an empty shopping cart vs.
pushing a really full one.
Which one would need more
force?
Acceleration
will occur in
the direction of the net
force.
Force
is measured in
Newtons
1N
= 1kg X m/s2
Practice Problem
Mass
= 175 kg
Acceleration = .657m/s2
What
115N
is the force?
Free Fall & Weight
When
the force of gravity is the
only force acting on an object it is
said to be in free-fall.
The
free-fall acceleration of an
object is directed toward the
center of the Earth.
g=
9.8 m/s2
Free-fall acceleration near
Earth’s surface is constant.
In
the absence of air
resistance, all objects near
Earth’s surface accelerate at
the same rate regardless of
their mass.
9.8m/s2
The
force on an object due to
gravity is called Weight.
Weight
= mass x free-fall acceleration
w=mg
(g
= free-fall acceleration)
The SI unit for weight is the
Newton
Weight
is the gravitational
force on an object due to its
mass.
Weight
influences shape
(structure)
Example:
Skeletons in elephants
Velocity
is constant when air
resistance balances weight
For
a falling object, when the
force of air resistance becomes
equal to the gravitational force
on the object it stops
accelerating & reaches Terminal
Velocity
For every action force, there is
an equal and opposite reaction
force.
“The
law of action & reaction”
Forces
always occur in pairs