1
What are velocity and
acceleration?
obje*itru
Differentiate between speed, velocity, and
acceleration.
Key Tut+u
accelerating. When a car goes around a curve or
turns a corner, it is changing direction. A change in
direction is a change in velocity. The car is
accelerating. The initial velocity of the runners in
Figure 13-6 is 0. So, as soon as they move down the
track, they are accelerating.
velocity (vuh-LAHS-uh-tee): speed and direction
acceleration (ak-sehl-uh-RAY-shuhn): rate of
change in velocity over time
Speed, Velocity, and Direction When you
move from place to place, you travei at different
speeds. However, to describe your motion, you
need to know more than just your average speed.
You must also know the direction in which you are
moving. Speed and direction describe velocity. An
example of velocity is 90 km/h west. Your speed is
90 km/h and your direction is west. Together, these
two values describe your velocity.
Figure 13-6 From
the minute they leave
the starting blocks,
these runners are
accelerating.
INFER:
Why is a car accelerating when it turns
a corner?
Measuring Acceleration Acceleration
describes
how fast the velocity of a moving object
is
changing. To find average acceleration, you must
know the change in velocity and the time it takes
for the change to occur. The equation for finding
acceleration in a straight line is shown here.
Figure 13-5 The cars are traveling at the same speed but
their velocities are different.
In Figure 13-5, what is the velocity
of the red car? The blue car?
INTERPRET:
Acceleration When a car changes speed or
direction, it is accelerating. Acceleration is a
change in velocity. When a car speeds up, its
velocity is changing. The car is accelerating. When
a car slows down, it is also changing velocity. It is
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acceleration
acceleration
:
:
change in velocity +time, or
(final velocity - initial velocity) + time
Suppose a car is stopped at a red light. When
the light turns green, the car accelerates to a speed
of 150 m/sec. The car takes 10 seconds to reach this
speed. What is its acceleration?
acceleration
acceleration
acceleration
:
:
:
(150 m/s - 0 m/s)
150 m/s + 10 s
15 mlsls
+
10
s
The acceleration is 15 meters Per second per
second, or 15 m/s2. This means that the car's
velocity increases 15 m/s every second.
INFER:
What is the initial velocity of the car?
lnterpreting Tables When an object falls through
the air, it accelerates as it falls. When it is released,
its speed is zero. As it falls, its speed increases.
Figure 13-7 shows how the speed of a falling
object changes.
CHECKING CONCEPTS
1.
BUILDING MATH SKILLS
What is velocity?
ACCELERATION OF
) What
does velocity tell you about a moving
object?
3. What is a change in velocity called?
4. What is the formula for finding acceleration?
5. How do you find a change in velocity?
THINKING CRITICALLY
COMPARE:A
t+arnls
- O tt, A
(seconds)
OBJECT
Velocity (misec downward)
lo
,1
Qr
9B
:
1
L
r3
19.6
29.4
4
39.7
)
0
E
truck is traveling east on a
highway at 80 km/h. What is its velocity?
7. ANALYZE:A highway speed limit is 90 km/h. Is
this average speed or instantaneous speed?
Explain.
6.
Time
A FALLING
Figure 13-7
What is the object's acceleration from 0 to 1s?
From 0 to 3s? From 2s to 4s? Based on your
calculations, what can you say about the
acceleration of a falling object?
cti,rity
'"HIi CHANGINGACCELERATION
need a large rubber band, a toy car, a metric
pencil,
and paper.
ruler, a
1. Place the car on the floor. Hold one end of the
rubber band on one side of the car while a partner
holds the other end.
2. Pull the rubber band and the car a distance of 5 cm,
as shown in the picture. Release the rubber band.
STEP 2 Making the car accelerate
Measure and record the distance the car travels.
3. Repeat Steps 1 and 2 two more times. The first time
pull the rubber band and car back 10 cm. The second time
pull them back '15 cm.
You
will
Practicing Your Skills
4. ANALYZE: What supplied the force to make the car accelerate?
5. OBSERVE: How did increasing the force affect the acceleration of the car?
CHAPTER 13:
Motion
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