Transcript PowerPoint Lecture Chapter 3

UNIT 2: Physics
Chapter 3: Describing Motion (pages 68-95)
I. Describing Motion
A. Motion
1. Motion occurs when an object
changes position
2. To measure the position of an object, a
reference frame must be chosen
a. Reference frame- a group of objects that
are not moving relative to each other
b. One point in reference frame is chosen as
the reference point
3. Motion is relative- motion of object
depends on the reference frame that is
chosen
4. Distance and Displacement
a. Displacement- distance and direction of
an object’s final position from its initial
position
1). Includes both a size and direction
2). Vectors describe something with
both size and direction
b. Distance alone is not a vector
What is your displacement if you walked 100
meters North, then 10 meters West, then 100
meters South, and finally 10 meters East?
Your total distance
10 m West
traveled is 220 meters,
but your displacement is
zero.
100 m South
100 m North
Starting point
10 m East
B. Speed- distance an object travels in a given
amount of time
a. SI unit of speed is meters per second
(m/s)
b. Instantaneous speed- speed at any given
instant. Car speedometer measures
instantaneous speed
m/s
c. Average speed- total distance traveled divided
by the total time traveled
d
v 
t
C. Velocity- speed of the object and its direction
1. is a vector quantity
2. Size of objects velocity is speed
3. Object will have
different velocities if
have different
speeds or move in
different directions
4. The velocity of an object can change even if the
speed of the object remains constant (e.g. driving
in a circle)
D. Graphing Motion
1. Motion of object over a period of time can
be shown on a distance-time graph
a. Time is plotted on
horizontal axis (x-axis)
b. Distance plotted
along vertical axis
(y-axis)
2. The slope (steepness) of a line graph equals the
speed
a. When slope is zero
(line is horizontal) the
speed is zero
b. Steeper the slope,
the faster the speed
Remember- slope
equals rise over run
What is the relationship between the steepness
of the line and the objects speed?
What does a flat line represent?
Which runner started out fastest?
Which runner finished the 100 meters first?
What was Charlie doing from 8-13 seconds?
II. Acceleration
A. Acceleration, Speed, and Velociy
to
1. Acceleration- change in velocity
divided by the time for the change
occur
2.Velocity is a vector, so change in velocity can be
either change in speed or direction of the motion
3. Acceleration is a vector (has size and
direction)
4. Any time an object changes direction, its velocity
changes and it is accelerating
B. Calculating Acceleration
1. Can calculate an objects average
acceleration
v
a 
t
f
f
 vi 
 ti 
vf = final velocity, vi = initial velocity
tf = final velocity, ti = initial velocity
2. Speed-Time Graphs- for object moving in one
direction, the acceleration can be found from a
speed-time graph
a. vertical axis is the objects speed
b. horizontal axis is the time
Describe what is occurring by looking at the
speed vs. time graph between the start and
point A. A to B, B to C, C to D, D to E, and from
E to F.
3. Positive acceleration- positive slope (speed is
increasing)
4. Negative acceleration- negative slope (speed
is decreasing)
C. Amusement Park Acceleration- roller coasters
can give riders large accelerations
II. Motion and Forces
A. What is force?
1. force- is a push or pull that one
object exerts on another
2. Force is a vector (size and
direction)
3. SI unit of force is newtons (N)
4. A force can cause the motion of an object to
change
5. Unbalanced forces change velocity
a. If net force is zero no change
in velocity
b. If net force is not zero
(unbalanced forces) velocity will
change
B. Friction- force that opposes the sliding motion of
two surfaces that are in contact
1. The size of frictional
force depends on the
materials the surfaces
are made from and the
roughness of the
surfaces
2. Frictional force between two surfaces
increases when the force pushing the surfaces
together increases
Where would you find
the greatest friction
occurring on a typical
roller coaster?
3. Static friction- friction that prevents two surfaces
in contact from sliding past each other
Friction
force
Pulling
force
If pulling force exceeds friction you produce
movement
Friction
force
Pulling
force
4. Sliding friction- the force that acts in the
opposite direction to the motion of a surface sliding
on another surface.
C. Air Resistance- frictional force opposes motion
of objects that move through air
1. Air resistance acts in direction opposite
to velocity of an object moving in air
2. Size of air resistance depends on the
size and shape of an object
3. Terminal velocity- the maximum velocity a
falling object will reach
a. Depends on objects size, shape, and
mass of falling object
b. Object reaches terminal velocity when net
force equals zero