Forces and Motions
Forces
What is a Force?
 Anything that changes
the state of rest or motion
of an object
 It’s what causes
ACCELERATION

has magnitude and direction

Therefore force is a vector
Which of the following lines shows
acceleration?
67%
14%
10%
10%
Ne
ith
er
4.
Bo
th
3.
B
2.
A
B
Both
Neither
A
1.
Force cont.
A force can cause a
resting object to
move, or it can
accelerate a moving
object by changing
the object’s speed or
direction.
Units = Newtons (N)
B) Combining Forces
1) Net Force: the combination of all of the forces
acting on the object
2) Balanced Forces
a) When the forces on an object are balanced,
the net force is zero and there is no change in
the object’s motion.
b) Example: Tug of war,
Pushing piano
3) Unbalanced Forces
a) Results when net force acting on an object
is NOT equal to zero
b) When an unbalanced force acts on an
object, the object accelerates in the direction
of the net force
*moves in direction of net
force
c) Fig 14, p.332
http://www.darvill.clara.net/enforcemot/friction.h
tm
C) Friction a force that opposes motion
between two surfaces that are in contact
There are two main types of friction:
1. Static- keeps things “static”
Ex- the force that is keeping this block from sliding
downhill
Kinetic- friction of movement
2.
three types
1.
1.
Sliding,Rolling, Fluid
Complete the concept map for the 4 types of friction! Use your
textbook (pages 332-334)
FRICTION
STATIC
SLIDING
is defined as
ROLLING
is defined as
FLUID
is defined as
is defined as
Thre friction between
surfaces that are
stationary
force that exists
when objects slide
past each other
force that exists
when a round object
rolls over a flat
surface (usually less
than sliding friction)
& an example is
& an example is
& an example is
a book sitting on a
table
-hockey puck on ice
-child going down a
slide
-a sled down hill
force that exists
when an object
moves through a fluid
(air, water)
& an example is
-a roller blade on a
sidewalk
-bowling ball on
bowling alley
-a car driving down
the road
-swimmer swimming
through pool
How can we decrease friction?
Watch this demo and see…
http://www.scottso.net/examples_encfri.ht
m
Can you think of a situation in which you would want to increase friction?
D) Gravity: natural phenomenon in which objects
that have mass are attracted to one another
1) Gravity is an attractive force  pulls objects
together
2) Earth’s gravity acts downward toward the
center of the Earth.
3) There is an upward force that balances
gravity
4) Falling Objects
a) Gravity causes objects to accelerate
downward, whereas air resistance acts in the
direction opposite to the motion & reduces
acceleration.
b) If something falls for a long time, upward
force of air resistance becomes equal to
downward force of gravity.
c) Terminal velocity: the constant velocity of a
falling object when the force of air resistance is
equal in magnitude & opposite in direction to the
force of gravity
1) Example: Sky diving (Fig 10, p.356)
E) Free Fall: the motion of a body when only the
force of gravity is acting on the body
1) Free fall acceleration of an object is directed
toward the center of Earth
a) Because free fall acceleration results from
gravity, its symbol is g
b) Acceleration due to gravity on Earth = 9.8
m/s2
Formula for objects in free fall:
d = ½ at2
c) In a vacuum, two objects would accelerate at
the same rate because both are in free fall
(Fig 8, p.354)
1) Question: What other force is not
present in vacuum that would affect
acceleration?
Answer = air resistance
II.) Newton’s First Law of Motion
A) Historical Development
1) Aristotle (384 BC322 BC): Incorrectly
proposed that force is
required to keep an
object moving
2) Galileo (1564 – 1642):
Concluded that moving
objects not subjected to
friction or other force,
would continue to move
indefinitely; Disproved
Aristotle
3) Newton (1643 – 1727):
Defined mass and
force; Introduced 3
Laws of Motion
C) Newton’s 1st Law of Motion
1) According to Newton’s 1st Law, the state of
motion of an object does not change as long
as the net force is zero.
a) Basically saying that an object at rest will
stay at rest unless a force acts on it
1) Example: Soccer ball will remain (at
rest) on the grass unless a force is acted
on it
2) Sometimes called the “Law of Inertia”
a) Inertia: the tendency of an object to resist
being moved or, if the object is moving, to
resist a change in speed or direction until an
outside force acts on the object
1) Car crash: You continue
forward because of inertia
“Science and the
Consumer”
p.348
How is inertia related to mass?
P 347

Mass is a measure of inertia.