Chapter 3
Motion, Acceleration, and Forces
Motion and Velocity
3-1
Motion
Motion occurs when an object changes
position.
The distance between an object must
change in relation to a reference point.
A reference frame or reference point can be
something stationary or moving.
Are You In Motion?
You are seated on an
airplane flying. Are you in
motion compared to the
ground?
Are you in motion compared
to the seat in front of you?
Distance
A runner jogs 50 m North, then runs 30 m
South. How far was the total distance run?
For a moving object, distance is the length
of the path that the object traveled, so:
50 m + 30 m = 80 m
However, even though she ran 80 m, she
is only 20 m North of the starting point. We
need a new term to describe this.
Displacement
Displacement: The distance AND
direction of an object’s change in
position from the starting point
What is the Displacement 1?
Jon runs 200 meters north, turns and
heads 150 meters south. Displacement?

Since Jon runs in opposite directions you
subtract: 200 m – 150 m = 50 m North
You walk 30 meters east; then you head
10 meters west. Displacement?

Since you walk in opposite directions you
subtract: 30 m – 10 m = 20 m East
What is the Displacement 2?
A squirrel runs 4.8 m South across a lawn,
then runs 2.3 m in the opposite direction.
What is the squirrel’s displacement from
its starting point?

4.8 m – 2.3 m = 2.5 m South
Vectors
Vector: a quantity that is both a size and
direction.
Displacement is a vector.
Distance is NOT a vector!
Speed
Speed: the distance an object travels per
unit of time, or distance/time.
SI unit for speed is usually measured in
meters per second (m/s)
EX: 5m/s, 20 km/hr, 9mm/min
Calculate the Speed!
A race car travels 7000 km in 10 hours
An ant travels 75 cm in 15 seconds
A snail moves 4 cm in 2 minutes
An airplane travels 20,000 km in 3 hours
You took 6.5 hours to drive 550 km.
Instantaneous Speed
Instantaneous speed: the
speed of an object at a
given point in time
Can be measured with a
speedometer or radar gun
Average Speed
Average Speed: the total distance an
object travels divided by the total amount
of time it takes to travel
Average speed (in meters/second) =
total distance (meters) / total time (second)
ѷ=d/t
Now try to solve this one-step equation:
What is the average speed of a car that
travels a distance of 750 m in 25 s?
Average Speed Problem 1
Identify the known values:


Travels a distance of 750 m, so d = 750 m
In 25 seconds, so t = 25 s
Identify the unknown value:

What is the average speed, ѷ = ? m / s
Insert known values and solve problem:

Ѷ = (d / t) = (750 m / 25 s) = 30 m / s
Check answer and multiply: 30 x 25 = 750
Average Speed Problem 2
A bus leaves at 9 am and travels 350 km.
After lunch it travels another 250 km until it
stops at 3 pm. What is its average speed?
Velocity
Velocity: an object’s speed and direction.
Ex: 5 m/s north, 20 km/hr southeast
Objects have different velocities if they are
moving at different speeds or directions.
Is velocity changing if you
are on a merry-go-round?
Vectors or Not?
Speed?
Velocity?
Distance?
Displacement?
Graphing Motion
A distance-time graph gives the speed of
an object in motion.
•Time: x-axis
•Distance: y-axis
Distance-Time Graphs
The steeper the
slope on these
graphs, the
faster the object
or person
traveled.
A horizontal line
means an object
was at rest.
Distance-Time Graphs
A straight line indicates the object traveled
at a constant speed.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Make Distance-Time Graphs
Title
Label both the x and y axis
Examine data and create the scale
Place a dot where the distance matches
the time
Connect the dots when you are finished
Key/color code lines
The Race
Anita:
 3 seconds
2.5 meters
 6 seconds
7 meters
 10 seconds
12 meters
 15 seconds
17 meters
Ashley
 3 seconds
3 meters
 6 seconds
8 meters
 10 seconds
15 meters
 15 seconds
20 meters
John is driving to the store. He travels 1
kilometer in the first 6 minutes. After an
additional 10 minutes, John travels
another 2 kilometers. John stays at the
store for 20 minutes. The trip back home
takes John 20 minutes.
Acceleration
3-2
Acceleration
Acceleration: rate of change in velocity.
This can be calculated by dividing the
change in velocity by the time it takes for
the change to occur!
Acceleration = (Vf - Vi)/(Tf-Ti)
Acceleration = Final velocity - Initial velocity
Final time - Initial time
The word initial means starting
Acceleration Units
The typical unit for acceleration is meters
per seconds squared or m/s2
Other examples: km/hr2, m/min2
You would read this as kilometers per hour
per hour and meters per minute per
minute
There will always be 2 units of time. Ex:
km/hr/min
Acceleration Cont.
Acceleration can be positive, negative, or
zero.
Positive acceleration: an object is
speeding up
Negative acceleration: an object is
slowing down
Zero acceleration: an object is traveling
at a constant speed
Is it accelerating?
Remember, an
object IS
accelerating if it:
 speeds up
 slows down
 changes direction
Calculate Acceleration
A car that was at rest suddenly increases
its speed to 65km/hr in 6 seconds traveling
west. Acceleration?
Calculate Acceleration
A car is traveling south on the freeway at
95 km/hr. The car takes 5 seconds to
slow to a speed of 45 km/hr.
Acceleration?
A bicyclist is riding at 15 meters per
minute northwest. While traveling up a
steep hill, the bicyclist slows to a speed of
6 meters per minute in 3 seconds.
A person waiting to run a race suddenly
sprints to a speed of 4 m/s in 2 seconds
while traveling east.
An airplane traveling north at 20,000 km/hr
takes 15 minutes to slow to a speed of
10,000 km/hr.
Graphing Acceleration
These are
called speedtime graphs.
Time is plotted
again on the xaxis and
speed/velocity
is on the y-axis.
Speed-time Graphs
A line with a positive slope means that
there is positive acceleration (speeding
up).
If the slope of the line is negative, the
object has negative acceleration (slowing
down).
If there is a horizontal line on this graph,
the object is traveling at a constant speed
and has zero acceleration.
Speed-Time Graph
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Forces
3-3
Forces
A force is a push or
a pull.
A force can be
from a living thing,
wind, moving
water, magnetism,
friction or gravity.
Net Force
Net Force: sum of forces acting on an
object.
If two or more forces are acting on an
object (in same direction), you can add the
total amount of forces up to get the net
force.
Force is measured in Newtons, which is
represented by a capital N.
Balanced Forces
Balanced forces are forces on an object
that combine to give a zero net force
Balanced forces do not change the
motion of the object
40 N
40 N
Unbalanced Forces
Unbalanced forces are forces that combine
to produce a net force that is not equal to
zero (opposite directions).
This will cause the velocity of an object to
change.
20N
3N
15N
Friction
Friction: a force that opposes motion
when two surfaces are in contact.
Friction is what slows objects down.
Friction will turn kinetic energy into thermal
energy (heat).
More about Friction
The rougher an object, the greater the
amount of friction that object has.
Gravity can increase the amount of
friction between two objects sliding past
each other.
Static Friction
Static friction: Frictional force that
prevents two surfaces from sliding past
each other.
When friction is greater than the forces
acting on it, the object does not move.
Sliding Friction
Sliding Friction: the frictional force that
opposes motion of two surfaces sliding
past each other.
Rolling Friction
Rolling Friction:
frictional force from
round surfaces
Fluid Friction
Fluid Friction: frictional force occurring
when a solid material passes through a
fluid.
A fluid is any substance that can change
its shape easily.
Liquids and gases are fluids.
Air resistance
Air resistance is a
type of fluid friction.
Air resistance
opposes motion in
the air. Over time,
enough air
resistance will
generate thermal
energy.
Video Clip
Air resistance
Air resistance is part
the reason why objects
appear to fall at
different accelerations.
In the absence of air, all
objects fall at the same
rate of acceleration
Acceleration due to
gravity: 9.8m/s2
Terminal Velocity
When air resistance and gravity’s forces
become equal, a falling object’s velocity
will stay constant.
This is terminal velocity.