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Observing Motion
Motion:
object’s change in position relative to a
reference point.
Displacement:
change in position of an object.
• must always indicate direction.
• Distance measures path taken.
Speed and Velocity
Speed:
 distance traveled divided by time interval during
which motion occurred.
 describes how fast an object moves.
 measurements involve distance and time.
SI units:
meters per second (m/s).
constant speed :
• when an object covers equal distances in
equal amounts of time
Speed and Velocity
 can be studied with
graphs and equations.
 When an object’s
motion is graphed by
plotting distance on yaxis and time on xaxis, slope of graph is
speed.
Speed and Velocity
Average speed:
 calculated as distance divided by time.
distance
speed =
time
Instantaneous speed:
 speed at a given point in time.
d
v=
t
d
v t
VELOCITY
Metal stakes are sometimes placed in glaciers to help
measure a glacier’s movement. For several days in
1936, Alaska’s Black Rapids glacier surged as swiftly
as 89 meters per day down the valley. Find the glacier’s
velocity in m/s. Remember to include direction.
GIVEN:
WORK:
v=?
d= 89 meters
t = 1 day
v = d/t
d
v t
v = 89 m / 8.6 x 104 s
v = 1.0 x 10-3 m/s down
the valley
VELOCITY
Find the velocity in m/s of a swimmer who
swims 110 m toward the shore in 72 s.
GIVEN:
WORK:
v=?
d= 110 meters
t = 72 s
v = d/t
d
v t
v = 110 m / 72 s
v = 1.5 m/s toward the
shore
Chapter 10
Speed and Velocity
Velocity:
 speed of an object in a particular
direction.
Resultant velocity
 Combination different velocities
Acceleration and Motion
Acceleration:
 rate at which velocity changes over time.
 An object accelerates if its speed, direction, or both
change.
Uniform circular motion
 is constant acceleration.
Chapter 10
Calculating Acceleration
a = final velocity – initial velocity
t
a = acceleration
Vf = final velocity
Vi = initial velocity
t = time
SI units:
 meters per second per second (m/s/s) or m/s2.
ACCELERATION
A flowerpot falls off a second-story windowsill. The
flowerpot starts from rest and hits the sidewalk 1.5 s
later with a velocity of 14.7 m/s. Find the average
acceleration of the flowerpot.
GIVEN:
a=?
vf= 14.7 m/s
vi = 0 m/s
t = 1.5 s
WORK:
a = final velocity – initial velocity
t
a = 14.7 m/s – 0 m/s
1.5 s
a = 9.8 m/s2 down
ACCELERATION
A turtle swimming in a straight line toward shore has a
speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s.
What is the turtle’s average acceleration.
GIVEN:
a=?
vf= 0.80 m/s
vi = 0.50 m/s
t = 4.0 s
WORK:
a = final velocity – initial velocity
t
a = 0.80 m/s – 0.50 m/s
4.0 s
a = 0.075 m/s2 toward the
shore
Balanced and Unbalanced
Forces
Force:
 action exerted on a body in order to change body’s state of rest
or motion.
 has magnitude and direction.
net force
• combination of all forces acting on an object.
balanced forces:
 Objects either do not move or move at constant velocity.
unbalanced force
 any change in an object’s state of motion or rest.
Chapter 10
The Force of Friction
Friction:
 force that opposes motion between two
surfaces that are in contact.
 opposes applied force.
Chapter 10
The Force of Friction
Static friction:
 resists initiation of sliding motion
between two surfaces that are in
contact and at rest.
 is greater than kinetic friction.
Kinetic friction:
 opposes movement of two surfaces that
are in contact and are sliding over each
other.
TYPES OF KINETIC FRICTION
Sliding friction
•
When objects slide past each other
Fluid friction
•
When an object moves through a fluid
(air, water…) and surface of object
Rolling friction
•
When a round object rolls over a flat
surface
Chapter 10
Friction and Motion





Friction can be helpful or harmful.
necessary to roll a vehicle or hold an object.
can also cause excessive heating or wear of
moving parts.
Harmful friction can be reduced.
Helpful friction can be increased.