MOTION:
Describing and Measuring
Motion
CHAPTER 10
Measuring Motion
Motion is described as a change in
position
An object is considered in motion
when observed in relation to a
REFERENCE POINT.
Describe motions with reference
terms such as: north, south, east,
west, up, down, right, left, etc.
Frame of Reference
The object or
point from which
movement is
determined
Any object can
appear to be in
motion or at rest
depending on the
frame of reference
of the observer.
Motion can only
be determined
when there is a
frame of
reference
Earth is the
most commonly
used frame of
reference
Frame of Reference
and Relative Motion
Relative Motion is based on the person
describing the motion based on their
frame of reference.
Example: When you are standing on
the ground, that is your frame of
reference. Anything you see, watch,
or measure will be compared to the
reference point of the ground.
Frame of Reference
Frame of reference for 2 girls…
Red Dress Girl uses the moving vehicle
As her frame of reference and is not
Moving
Yellow Dress girl uses her frame of
Reference as a stationary position to
See the other girl in motion.
Frame of Reference for the
Skydivers
a. Plane – in motion away from them
b. Themselves – not in motion
c. The ground – in motion towards them
Example 1:
Sitting at your desk, how fast are
you moving?
– Relative to the ground: ZERO
– Relative to the sun: 2.97 X104 m/s! An
observer standing on the sun would say
you are moving at 2.97 X104 m/s
Example 2:
While sitting at a red light a bus
stops in the lane next to you…as you
are daydreaming looking at the side
of the bus…you all of a sudden feel
like you are rolling backwards
Then you realize it is the bus
moving forwards when you take into
account a stationary frame of
reference.
This is your brain “confused”
Did you know?
Motion Sickness is caused by your
brain getting two different sets of
information about your body’s
motion based on its frame of
reference, the information from your
eyes and the information from your
inner ear
To help with motion sickness, try to
look forward at a point far in the
distance and stay focused on that.
Distance vs. Displacement
Distance
• Scalar (amount only)
• Direction does NOT matter.
• The actual path
matters!
• The total distance traveled.
• In the Daytona 500, the
cars travel a distance of
500 miles.
Displacement
• Vector (amount and
direction)
• Direction DOES matter!
• The path does NOT
matter!
• The straight-line distance
(and direction) the object
has travelled from its
starting point.
• In the Daytona 500, the
cars experience zero
displacement (they
start where they end).
Distance vs. Displacement
What is the overall distance traveled
in the picture below?
What is the overall displacement in
Leg 1 Distance = ______
the picture below?
Leg 1 Displacement = _____
Leg 2
Leg 1
Leg 2 Distance = ______
Leg 2 Displacement = _____
Leg 3 Distance = ______
Leg 3 Displacement = _____
START
Leg 3
End
Final Distance = ______
Final Displacement = _____
Distance vs. Displacement
Vector Diagrams
The objects
motion is
represented
with an arrow
in the
direction
traveling.
The size of
the arrow
indicates the
relative speed
of that
object.
Practice with Frames of
Reference / Distance and
Displacement
Practice
Worksheet as a class
fun with mapping … check out
Santa’s route to your house!
Speed and Velocity
A little lesson through music
SPEED
SPEED – the rate of change in position or
rate of motion
INSTANTANEOUS SPEED – Rate of motion
at any given instant
– (ex: speedometer in a car)
CONSTANT SPEED – A speed that does
not vary
– (ex: cruise control)
– Usain Bolt races at ESPN video clip
– Usain Bolt Olympics
Constant Speed vs. Instantaneous
Speed….Let’s graph
CALCULATING SPEED
When we calculate speed, we are
calculating the average speed
traveled.
AVERAGE SPEED – a measure of
total distance traveled divided by
total time of travel
SPEED (v) = DISTANCE (d)
TIME (t)
UNITS FOR SPEED ---m/s
DISTANCE-TIME GRAPH
The distance covered by an object is
noted at regular intervals of time.
Measuring Speed
Sport Science - John Wall
Sport Science - Lebron James
Sport Science - Ndamukong Suh
VELOCITY
VELOCITY – Describes both speed
AND direction.
Velocity can change even if the
speed of the object does not change.
Calculating Velocity:
– Velocity = Displacement / time
– Answers will include direction
Velocity – Time Graph
A velocity – time graph can show
acceleration.
Distance-Time Graph
vs.
Velocity Time Graph
Shows velocity of object,
time.
Can calculate Acceleration
Shows Motion, distance
traveled, and time.
Can calculate speed.
ACCELERATION
ACCELERATION – The rate of change
of velocity.
Acceleration can be a change in
speed OR direction
Circular motion is a constant
acceleration
– Example: blades of a fan
Acceleration…Let’s graph
Constant Velocity =
Zero Acceleration
Positive Velocity =
Positive Acceleration
CALCULATING
ACCELERATION
Divide the change in velocity by the
time interval
Acceleration (a) = (final velocity – initial velocity)
time interval
a = (vf – vi)
t
* Unit for Acceleration: m/s2 (plus
direction)
Acceleration Practice
Natalie accelerates her skateboard
along a straight path from 0 m/s
to 4.0 m/s in 2.5 seconds. Find her
average acceleration.
a=
4.0 m/s – 0 m/s
2.5 s
a = 1.6 m/s2 along her path
* Practice: p. 328 # 2-5
Example: Positive
Acceleration
Acceleration = (30 m/s – 0 m/s)/10
s
a = 3 m/s/s
The car’s acceleration is increasing 3
m/s2
Examples: Negative
Acceleration
a = (0m/s – 20 m/s)/10 s
a = - 2 m/s/s
The cars acceleration is decreasing
at 2 m/s2
MOTION AND FORCE
FORCE – A push or a pull one
object puts on another.
BALANCED FORCE – Forces that
are equal in size and opposite in
direction.
UNBALANCED FORCE (NET FORCE)
– Forces that are unequal in
size…Cause a change in motion.
– Unbalanced forces always change the
velocity of the object.
Balanced vs. Unbalanced Forces
Balanced force pushing on each
other
Balanced Force opposite in direction pulling
Unbalanced forces….showing displacement of the object
Unblalanced Forces – adding up to displace the object in one direction
Forces that Act on Objects
1. Normal Force –
A support force when
one object is in contact with a stable object (ex:
Book on a table)
2. Tension Force –
A force transmitted
through a string, rope, or cable
3. Gravitational Force –
The force of
attraction to a very large object in the universe
4. Friction Force –
The force that
opposes motion between two surfaces.
Normal Force vs. Gravity Force
Tension Force
Friction Force
FRICTION
FRICTION – The force that opposes
motion between two surfaces that
are touching each other.
2 TYPES OF FRICTION:
– STATIC FRICTION – The friction
between surfaces that are stationary.
– KINETIC FRICTION – The friction
between moving surfaces.
Kinetic Friction can be SLIDING or ROLLING
Increasing or Decreasing Friction
Increase Friction by using rough
surfaces
– EX: Cleaning surfaces with a rough
sponge
Decrease Friction by adding a liquid
– Ex: Oil, WD40
Gravity and Force
Gravity is a Field Force
All objects in the universe have a
gravitational force. Larger objects
exert more force than smaller
objects.
Calculating Gravitational Force
Gravitational force (weight) of an
objects is proportional to its mass
Force (gravity) = (mass) (gravity)
– Acceleration due to Gravity (g) is equal
to 9.8 m/s2
Newton’s Second Law
An unbalanced
force acting on
an object causes
that object to
change speed or
direction
Acceleration =
Force / Mass
Action - Reaction
Every action has an equal and
opposite reaction.
Example: The truck hits the sign
therefore the sign hits the truck with
an equal force in the opposite
direction.