S ECTION 11–3:
A CCELERATION
O BJECTIVES

Identify changes in motion that produce acceleration.

Describe examples of constant acceleration.

Calculate the acceleration of an object.

Interpret speed-time and distance-time graphs.

Classify acceleration as positive or negative.

Describe instantaneous acceleration.
A CCELERATION

The rate at which velocity changes is called
acceleration.

Acceleration can be described as changes in speed,
changes in direction, or changes in both. Acceleration is
a vector.
C HANGES
IN
S PEED

We often use the word acceleration to describe
situations in which the speed of an object is increasing.
This can be slightly misleading

ANY change in speed (speeding up or slowing down) is
what scientists would call “acceleration.”

We can call a negative acceleration, or a slowing down
effect, a “deceleration.”

Can an object have zero acceleration and still be
moving?
C HANGES
IN
S PEED

If you are in a car that is stopped at a traffic light, you
will feel a positive acceleration (speeding up) when the
light turns green. When you reach a cruising speed, you
will have zero acceleration. As you approach the next
red light, you will experience a negative acceleration, or
deceleration (slowing down).

An example of acceleration due to change in speed is
free fall, the movement of an object toward Earth solely
because of gravity.

The unit for acceleration is meters per second per
second, or meters per second squared (m/s2).
C HANGES
IN
S PEED

Objects falling near Earth’s surface accelerate
downward at a rate of 9.8 m/s2. Each second an object
is in free fall, is velocity increases downward by 9.8 m/s.

After one second, an object falling would be moving at
9.8 m/s. At two seconds, its speed would be 19.6 m/s.
At three seconds, its speed would be 29.4 m/s.

The change in the object’s speed is due directly to
gravity.
C HANGES
IN
D IRECTION

Acceleration isn’t always the result of changes in speed.
You can accelerate even if your speed is constant.

You experience this type of acceleration as you ride
around a curve at the same speed.

You feel acceleration/deceleration, not speed or
velocity.
C HANGES IN S PEED AND D IRECTION

You experience this type of acceleration when you ride
a roller coaster. Typically your speed and direction are
always changing.

You will also experience this type of acceleration when
you slow down to enter a curve in your vehicle. In this
case, both the speed and direction change.
C ONSTANT A CCELERATION

The velocity of an object moving in a straight line
changes at a constant rate when the object is
experiencing constant acceleration.

Constant acceleration is a steady change in velocity.
That is, the velocity of the object changes by the same
amount each second.

An example of constant acceleration is illustrated by a
jet airplane during takeoff.
C ALCULATING A CCELERATION

Acceleration is the rate at which velocity changes.

You calculate acceleration for straight-line motion by
dividing the change in velocity by the total time. If a is
the acceleration, vi is the initial velocity, vf is the final
velocity, and t is total time, this equation can be written
as follows:
a = (vf – vi)/t

Think about how a change in either your final velocity
or initial velocity would have an effect on the
acceleration. Acceleration is a vector quantity.
C ALCULATING A CCELERATION

A ball rolls down a ramp, starting from rest. After 2
seconds, its velocity is 6 m/s. What is the acceleration
of the ball?

Initial velocity (vi) = 0 m/s (starting from rest)

Final velocity (vf) = 6 m/s

Total time (t) = 2 s

Acceleration (a) = (6 m/s – 0 m/s)/2 s = 6 m/s/2s = 3 m/s2
G RAPHS
OF
A CCELERATED M OTION

You can use a graph to calculate acceleration.

When using a speed-time graph, the slope of the line is
acceleration.

This slope is change in speed divided by change in time.
S PEED -T IME G RAPHS

Constant acceleration is represented on a speed-time
graph by a straight line.

A linear graph is a graph in which the displayed data
forms straight-line parts.

When the slope of a line is positive, the acceleration is
positive. When the slope of a line is negative, the
acceleration is negative.
D ISTANCE -T IME G RAPHS

Accelerated motion is represented by a curved line on a
distance-time graph.

In a nonlinear graph, a curve connects the data points
that are plotted.
I NSTANTANEOUS A CCELERATION

Acceleration is rarely constant, and motion is rarely in a
straight line.

Instantaneous acceleration is how fast a velocity is
changing at a specific instant.

An object has an instantaneous acceleration at all times,
but sometimes the instantaneous acceleration is zero.
V OCABULARY

Acceleration

Free fall

Constant acceleration

Linear graph

Nonlinear graph