Acceleration
The rate at which velocity changes
Acceleration
• Acceleration can be described as changes in
speed, changes in direction, or changes in
both
• Acceleration is a vector
• Shows how velocity changes
Acceleration
• Acceleration can be caused by an positive
change in speed (increase)
• Acceleration can also be caused by a negative
change in speed (decreasing)
• Negative acceleration is known as
deceleration
Acceleration example
• You get on a bus and feel the bus “accelerate”
as you go down the street. When the bus is
keeping a constant speed, its acceleration is
zero.
• After a while the bus stops, since the speed is
changing the bus is accelerating. (called
deceleration)
Free Fall
• The movement of an object toward Earth
solely because of gravity
• It is acceleration due to a change in speed
Unit for Acceleration
• Remember the unit for velocity is
meters/second
• The unit for acceleration is meters/second2 or
m/s2
• Objects falling near Earth’s surface accelerate
downward at 9.8 m/s2
• Each second an object is in a free fall, its
velocity increases downward by 9.8 meters
per second.
Free Fall
• Look at p. 343 and the rock being dropped
down the well
• After 1 second the stone will be falling at
9.8 m/s
• After 2 seconds the stone will be falling at
19.6 m/s (2 x 9.8)
• The change in the stones speed (acceleration)
is 9.8 m/s2
Changes in Direction
• Acceleration is not always due to a change in
speed
• You can accelerate even if you are going at a
constant speed
• For example, if you are riding a bicycle and go
around a curve, you are accelerating even though
you are keeping a constant speed but you are
changing direction
• A carousel is traveling at a constant speed but is
accelerating since its direction is changing
constantly
Constant Acceleration
• The velocity of an object moving in a straight
line shows constant acceleration
• This is when there is a steady change in
velocity
• The velocity of an object changes by the same
amount each second
• A plane’s acceleration may be constant at
take-off time
Calculating Acceleration
• You can calculate acceleration for a straightline motion by dividing the change in velocity
over the total time
• a = acceleration, vi = initial velocity and
vf = final velocity
• So acceleration = change in velocity/total time
a= (vf – vi)/t
Calculating Acceleration
• If velocity increases, then the acceleration is
positive
• For example, coasting down a hill on a bicycle
• If velocity decreases, then the acceleration is
negative
• For example, coasting on a bike at the bottom of
the hill, your velocity will decrease