Download Forces Notes

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.
SI
units meters per second
(m/s)
Constant
Simplest
speed
type of speed
When an object covers equal
distances in equal amounts of
time (Speed Stays The SAME)
Speed
can be determined from
a distance-time graph
Time
is on X axis
Distance is on Y axis
Speed
can be determined by
calculating the slope of the line
 Most
objects don’t move with constant speed
 Instead we calculate average speed
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?
 Time
 What
information is on the Y axis?
 Velocity
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 &
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 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
objects will ____________ as the
distance between 2 object
increases.
Key Vocabulary:
Speed
Velocity
•Unbalanced Forces
•Balanced Forces
Momentum
•Inertia
Acceleration
•Gravity
Force
•Friction
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