Transcript Speed PPT Review

CHAPTER 2
Get ready to take notes!
Ch. 2
Motion and Speed
Motion – when an object
changes position relative to a
reference point
Distance - how far an object
moves
Displacement - distance and
direction relative to an object’s
starting position
12 m
Distance = _____
0
Displacement = _____ m
Speed - distance an object
travels per unit of time
speed = distance
time
Speed that doesn’t change over
time is constant speed.
The red and green car are
traveling at a constant speed.
In real life, speed is usually not
constant; most objects have
changing speed.
Average speed - speed of
motion when speed changes
Average speed is used to
describe everyday motion.
Avg. speed = total distance
total travel time
5 m = ____
25 m/hr
= ____
0.2 hr
(Hint: Each burst of motion happens in 1 second. Count the number of movements.)
red car
120 m = ___
12 m/s
= ____
10 s
blue car
60 m = ___
6 m/s
= ____
10 s
Velocity - speed and direction
relative to starting position.
Determine the average speed
and the average velocity of
the skier during the 3 minutes.
average speed =
420 m = 140 m/min
3 min
average velocity =
140 m, right = 46.7 m , right
3 min
min
Instantaneous speed - speed at
any given point in time.
Your
speedometer
shows
instantaneous
speed.
Acceleration – change in speed
or direction or both.
Roller coasters show that
changes in
speed and/or
direction cause
acceleration.
Positive acceleration =
increasing speed.
Negative acceleration =
decreasing speed.
Acceleration =
change in velocity
time
Change in velocity =
final velocity – initial velocity
Unit - meters per second
2
squared (m/s )
Graphing Motion
A distance-time graph displays
motion of an object over time.
Plot:
• distance
on the
y-axis
• time
on the
x-axis.
• Green: Straight line, pos. slope
= constant speed to right.
• Red: Straight line, lower pos.
slope = slower constant speed.
A flat line = no motion.
Straight line, neg. slope =
constant speed, going left.
• Blue: Curved line = changing
speed or acceleration.
Albert runs faster and faster
(accelerates) steadily.
Bob is also accelerating but at
slower speeds than Albert.
Charlie travels:
• at a constant speed of about
6 m/s for 8 s,
• stops for 5 s,
• accelerates for 5 s more.
Who won
the race?
Albert, because he ran the
distance in the least time.
Slow, Constant, Right
Slow, Constant, Left
Fast, Constant, Left
Fast, Constant, Right
constant velocity in the
positive direction (right)
constant velocity in the
negative direction (left)
The blue car starts ahead of
the red car (to the left of the
screen). Since the red car is
moving faster, it catches up
with and passes the blue car.
Observe the velocity-time
graphs. Each car's motion is
represented by a horizontal
line (constant velocity).
Observe that
even though
the cars pass
each other, the lines on the
velocity-time graphs do not
intersect. Since the cars never
have the same velocity, the
lines on the velocity-time
graph never cross.
The lines would
intersect for a
position vs. time graph; the
red car passing the blue car
means that there is an instant
when they occupy the same
position (at 7 s); yet they
never have the same velocity
at any instant in time.
A velocity-time graph displays
speed of an object over time.
Plot:
• velocity
on the
y-axis
• time
on the
x-axis.
Positive Velocity,
Positive
Acceleration
Positive Velocity,
Zero Acceleration
Negative Velocity,
Negative
Acceleration
Negative Velocity,
Zero Acceleration
• dist.-time graph - up curve
• vel.-time graph - pos. slope
• acc.-time graph - above zero
Positive acceleration
• dist.-time graph - down curve
• vel.-time graph - neg. slope
• acc.-time graph - below zero
Negative Acceleration
Motion and Forces
Force - a push or pull
one body applies to
another. A force can cause
an object’s motion to change.
When two or more forces
combine at the same time,
they create a net force.
Balanced forces are equal in
size and opposite in direction.
Unbalanced forces are unequal
in size and/or not in the same
direction.
Inertia and Mass
Inertia - an object’s resistance
to any change in motion.
Objects with greater mass have
greater inertia.
st
1
Newton’s
law
of motion - an
object moving at a
constant velocity
keeps moving at
that velocity
unless a net force
acts on it; an
object at rest will stay at rest
unless a net force acts on it.
Auto crash - inertia at work
Passengers not wearing a seat
belt continue forward at that
speed even after the car stops.
Passengers
wearing a
seat belt
slow
down as
the car
slows
down.
THE END