Transcript Usain Bolt is the world’s fastest man!!!

Usain Bolt is the world’s fastest man!!!
Physics of Motion
We will look at:
– Distance
– Displacement
– Speed
First you need to realize that motion
is relative…
Motion is relative
What is meant by saying that motion
is relative? For everyday motion,
what is motion usually relative to?
Motion is Relative
Motion is relative to the observer’s
position and their reference point
– Sometimes called a “reference frame”
Consider the picture…
• If this man is driving at 15 mph,
how fast is his coffee cup moving?
•Does the man feel like the cup is
moving?
•Why?
Direction
We use N, S, E, W
to describe the
direction of
movement.
Distance and Displacement
Distance measures the
actual path an object takes
Displacement measures
your overall distance from
the starting point in a
STRAIGHT LINE.
DISPLACEMENT values must
include a DIRECTION!
Which color line represents
distance?
Displacement?
Distance vs. Displacement
http://www.physics.gatech.edu/acad
emics/classes/2211/main/demos/dis
placement/DDis.html
Speed
Describes how fast an object moves.
We know some things move faster
than others…but how do we measure
it?
What two quantities must you know
to determine speed?
What two quantities must you
know to determine speed?
Choose from: displacement,
distance, time, velocity
– Hint…what is speed measured in???
– Speed= distance/time
Ex- miles/hour,
m/s, etc.
There are three types of speed
you must know…
Constant speed
Average speed
Instantaneous speed
But first let’s look at some
graphs…
Distance
If I wanted to graph speed, what
should I label my axes???
Time
So the slope of the line=SPEED
Constant Speed
When an object covers equal
distances in equal amounts of time
Ex- if a race car travels at a
CONSTANT SPEED of 96m/s, it will
travel a DISTANCE of 96 meters
EVERY SECOND.
Constant Speed
What would a d-t graph look like for
a constant speed?
But most objects do not travel at
a constant speed.
The speed of an object can change from
one minute to another.
So we can use AVERAGE SPEED to
describe its motion.
Use this equation…
Average Speed = total Distance / total Time
Let’s try it
A runner finished a 3 mile race in 22
minutes. He may not have run at
the same pace the whole time, but
you can still calculate his AVERAGE
SPEED
22 minutes / 3 miles = 7.33
min/mile
Instantaneous vs. Average speed
Average speed- overall distance
over time the object traveled
Instantaneous speed- measures
speed over small time interval (at an
instant)
Does a speedometer of a car read
instantaneous or average speed?
What 2 controls on a car enable a
change in speed?
What if I want to describe speed
AND direction?
For example…what if you wanted to
find a plane. Knowing the speed
would only tell you how far away to
look but not in what direction. For
that we need…
VELOCITY- the speed and direction
of motion.
Let’s get back to the car
example…
Name another control that enables a
change in velocity.
In order to analyze such situations, we
need to understand what are known as
vectors…
All measured quantities can be
classified as being either a
scalar or a vector.
Scalar
Vector
Magnitude only
_________
_________
Magnitude and
(size of the quantity _________
Direction
….a number)
Speed is Velocity…kind of…
• Velocity is the vector for the scalar of
speed.
• The magnitude is the same!!!
• Therefore in calculations, you can use speed or
velocity interchangeably
• Speed = distance / time
• Velocity = distance / time in a direction
Let’s use our math skills
Page 323…
Read through “MATH SKILLS”
DO problems 1-3 “Practice Problems”
Use the diagram to determine the resulting
displacement and the distance traveled by
the skier during these three minutes.
Answer
The skier covers a distance of
(180 m + 140 m + 100 m) = 420 m
and has a displacement of 140 m,
rightward.
What is the coach's resulting
displacement and distance of
travel?
Answer
The coach covers a distance of
(35 yds + 20 yds + 40 yds) = 95
yards and has a displacement of 55
yards, left.
Consider a car moving with a constant, rightward (+) velocity
of 10 m/s.
How many meters will he travel in 5
seconds?
Constant velocity
Now consider a car moving with a rightward (+), changing velocity that is, a car that is moving rightward but speeding up or accelerating.
Acceleration
Distance time graphs
Draw 2 graphs
– One showing a slow constant speed
– One showing a faster constant speed
Slow, Rightward(+)
Constant Velocity
Fast, Rightward(+)
Constant Velocity
Slow, Leftward(-)
Constant Velocity
Fast, Leftward(-)
Constant Velocity
Consider a car moving with a constant, rightward (+) velocity - say of
+10 m/s. A car moving with a constant velocity is a car with zero
acceleration.
Draw a graph!
Constant velocity = zero
acceleration
Now consider a car moving with a rightward (+), changing velocity - that is,
a car that is moving rightward but speeding up or accelerating. Since the car is
moving in the positive direction and speeding up, the car is said to have a
positive acceleration.
Positive acceleration
Describe this graph!
Does the velocity of the wind affect
such things as a sprinter’s speed or
an airplane’s flight time?
Resultant Velocity animation
http://www.glenbrook.k12.il.us/GBS
SCI/PHYS/mmedia/vectors/plane.ht
ml
“Adding Vectors” Example:
A small airplane heads east with a speed of
200 mph with respect to the air (the “air speed”).
This would be the plane’s speed if
the air was NOT moving – no wind)
• If the wind/jet stream is moving east at 50 mph,
what is the plane’s resulting velocity with respect
to the ground (the “ground speed”)?
200
50
250 mph, east
with the wind
• If, later, the airplane is flying west into
the 50 mph wind with an “air speed” of
200 mph, now what is the plane’s
resulting velocity with respect to the
ground (the “ground speed”)?
200
50 150 mph, west
against the wind
1. Find the velocity in m/s of a
swimmer who swims 110 m toward
the shore in 72 s.
1.5 m/s toward the shore
1. Imagine that you could ride a
baseball that is hit high enough and
far enough for a home run. Using
the baseball as a reference frame,
what does the Earth appear to do?
1. Calculate the displacement in
meters a cyclist would travel in
5.00 h at an average velocity of
12.0 km/h to the southwest.