2/11/2016
MOTION
(Chapter 2)
https://www.youtube.com/watch?v=Oxc-hhQldBE
Student Learning Objectives
• Compare and contrast terms used to
describe motion
• Analyze circular and parabolic motion
PHYSICS:THE MOST
FUNDAMENTAL PHYSICAL SCIENCE
Physics
is concerned with the basic
principles that describe how the
universe works.
Physics
deals with matter, motion,
force, and energy.
Intro
PHYSICS – AREAS OF STUDY
Classical
Waves
mechanics
and sounds
Thermodynamics
Electromagnetism
Quantum
Atomic
mechanics
and nuclear physics
Relativity
Intro
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MOTION
Motion
is everywhere – walking, driving,
flying, etc.
This
chapter focuses on
definition/discussion of: speed,
velocity, and acceleration.
There
are two basic kinds of motion:
 Straight
line
 Circular
Intro
DEFINING MOTION

Position – the location of an object

A reference point must be given in order to define the position of
an object

Motion – an object is undergoing a continuous change in
position

Description of Motion – the time rate of change of position

A combination of length and time describes motion
Section 2.1
HOW DO SPEED AND VELOCITY COMPARE?

Average speed is the amount
of distance covered in some
amount of time.

Time can only go forward


https://www.youtube.com/w
atch?v=-LZq_w-tIl8
Velocity is speed in some
direction.

Up, Down

Left, Right

North, South, East, West

–, +
s=d
t
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VECTORS
Note that vectors may be both positive and negative.
Section 2.2
INSTANTANEOUS SPEED
Section 2.2
PRACTICE: IF AN OLYMPIC RUNNER
COMPLETES THE 100 METER DASH IN 11
SECONDS, WHAT IS THE RUNNER'S AVERAGE
SPEED? HOW FAST IS THIS IN MPH?
 Scalars
are quantities that indicate size, indicate a
numerical value.
 Vectors
are quantities that indicate the size and
direction.
 The
magnitude of a vector is a scalar; it is the size of
the vector.
Example: 65 mph North
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 Acceleration
is the
change of velocity
in some amount of
time.
 Deceleration
is
negative acceleration.
(opposite direction of
motion)
a = v
t
HOW IS ACCELERATION DEFINED?
http://www.physicsclassroom.com/mmedia/kinema/pvpa.cfm
http://www.physicsclassroom.com/mmedia/kinema/pvna.cfm
http://www.physicsclassroom.com/mmedia/kinema/avd.cfm

A car that maintains a
constant speed in a straight
line has zero acceleration.

The distance traveled
depends on how long the
object has been
accelerating.
d = 1/2)at2
PRACTICE
1) When driving through a curve at a constant rate of
speed, is there acceleration?
On the freeway, it takes 3 seconds to increase your
speed from 15 mph (6.7 m/s) to 65 mph (29 m/s).
2)What is the average acceleration?
3)If the car continues to accelerate, at the same rate,
for another 1.5 seconds, what will the final velocity be?
4) How far will the car have traveled during the 4.5
seconds of acceleration?
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WHAT CAUSES VERTICAL
ACCELERATION?
 Objects
gravity.
are accelerated downward because of
 The
gravitational pull from the Earth is the same on
all objects, so the downward acceleration is the
same on all objects, 9.81 m/s2.
 Any
difference in the net acceleration is due to air
resistance.
https://www.youtube.com/watch?v=6f6lH
6FH5ho
PRACTICE
1) I toss a ball upward and it returns to my hand.
What is the ball’s rate of acceleration if there is
no air resistance?
a)
Just after the ball leaves my hand?
b)
At the top of the trajectory?
c)
Just before the ball hits the ground?
2) Professional cliff divers fall from a cliff into the
sea. If the dive takes 2.7 seconds, what is the
distance they fall? What would be the speed of
the diver as the diver enters the water?

An object in free fall is only affected by gravity (no air
resistance).

Air resistance provides acceleration upward.


Surface area increases air resistance.

Speed increases air resistance.
Terminal velocity occurs when air resistance balances
gravity.IS TERMINAL VELOCITY?
WHAT
http://www.physicsclassroom.com/mmedia/newtlaws/sd.cfm
https://www.youtube.com/watch?v=p0IZsfzDS4s
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WHAT CAUSES CIRCULAR MOTION?
 In
 Objects
move in a
straight line, unless an
unbalanced force
changes the motion.
circular motion, a
centripetal force
(“center-seeking”
force) causes a
constant change in
motion, a constant
change in direction.
ac = v2
r
Fc = mac
http://www.physicsclassroom.com/mmedia/circmot/ucm.cfm
ACCELERATION IN UNIFORM
CIRCULAR MOTION

Although an object in uniform circular motion has a constant
speed, it is constantly changing directions and therefore its
velocity is constantly changing directions.

Since there is a change in direction there is a change in
acceleration.

What is the direction of this acceleration?

It is at right angles to the velocity, and generally points toward
the center of the circle.

Supplied by friction of the tires of a car

The car remains in a circular path as long as there is enough
centripetal acceleration.
CENTRIPETAL (“CENTERSEEKING”) ACCELERATION
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PRACTICE
1) What are some examples of centripetal
motion?
2) An object is circling two times every second at
the end of an 8 cm long string.
a)
Determine how to calculate the velocity,
and calculate it.
b)
Calculate the centripetal acceleration.
c)
Calculate the amount of centripetal force
on a 0.5 kg object.
PROJECTILE MOTION

An object thrown horizontally combines both
straight-line and vertical motion each of which
act independently.

Neglecting air resistance, a horizontally
projected object travels in a horizontal direction
with a constant velocity while falling vertically
due to gravity.

An object thrown
horizontally will fall at
the same rate as an
object that is dropped.
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
The velocity in the horizontal
direction does not affect the velocity
and acceleration in the vertical direction.

Shoot the Monkey
WHY DOES A PROJECTILE HAVE A PARABOLIC
TRAJECTORY?
 The
addition of the horizontal velocity and the
vertical acceleration due to gravity results in a
curved path.
http://science360.gov/obj/tkn-video/fc729ef0-22ee4f61-bb2a-b6c07685fb02/science-nfl-football-projectilemotion-parabolas
PROJECTED AT AN
ANGLE (NOT HORIZONTAL)
Combined Horz/Vert.
Vertical
Horizontal
=Component + Component
Components
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A
projectile's motion in x does not affect its
motion in y! (and vice versa)

Velocity in x is constant if there is no air resistance.
Why?

Velocity in y changes. Why?
https://www.youtube.com/watch?v=zMF4CD7i3hg
http://www.physicsclassroom.com/mmedia/vectors/hlp.cfm
http://www.physicsclassroom.com/mmedia/vectors/pap.cfm
 The

A projectile will fall a vertical
distance (y) each second
after reaching maximum
height.
y = 1/2)gt2
range (x) of a
projectile relative to
the horizontal plane
depends on the
object’s horizontal
velocity and time of
flight.
x = vt
https://www.youtube.com/watch?v=D9wQVIEdKh8
https://www.youtube.com/watch?v=0jGZnMf3rPo

Under real-world conditions, air resistance causes the
paths to be non-symmetric. Air resistance reduces the
horizontal velocity.
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PRACTICE
1) At what angle relative to the ground should a football be
thrown to have the greatest horizontal range?
2) What are some examples of projectile motion that you have
observed?
MORE PRACTICE
3) A player throws a baseball parallel to the ground
with a horizontal velocity of 90 mph (40 m/s).
Assume no air resistance.
a)
What is the direction of the acceleration?
b)
Does the horizontal speed change?
c)
Does the vertical speed change?
d)
What is the horizontal speed the instant before the ball
hits the ground?
e)
If there was no gravity, what would be the path of the
ball? Why?
f)
What is the maximum horizontal range of the baseball if
it is released from a height of 1.5 meters?
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