Motion: Distance and

doutfanaticalMechanics

Nov 14, 2013 (3 years and 11 months ago)

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Motion: Distance and
Displacement

Chapter 11 Section 1

What is motion?


Can you
see

motion?


Do you have to be able to see it
for it to be happening?


Are you moving now? Am I?


Close your eyes.

Motion occurs when:


An object changes its
position.

Motion is relative.



You
are

moving relative
to the Sun and other
planets even though
maybe you
aren’t

moving
in relation to your desk.

Frame of Reference


Necessary to describe an
object’s position


Movement in relation to a
frame of reference is relative
motion


Distance


How far an object has
moved.


Length of a path
between two points.

Distance


Measured in meter,
kilometer, centimeter,
millimeter

Scalar Quantities


A scalar quantity can be
described by magnitude
(amount) only and has no
direction.


Time, distance, and volume
are scalar quantities.


Displacement


Distance and direction of an
object’s change in position
from the starting point.


If distance is 20 meters,
displacement might be 20
meters south (distance and
direction).

Vector Quantity


Displacement is a vector
quantity.


It has both magnitude and
direction



The bell rings. Jenny gets up
from her desk and walks 10
meters down the hall to her next
class. She then realizes that her
next class is in the same
classroom she just left. So she
turns around and walks 10
meters back to the same desk.

Jenny’s trip:


Distance she traveled is?


20 meters


Her displacement is?


ZERO! She’s right back
where she started from!

Practice on paper! Draw this:


Exercise 1: John walks east 5
meters, then turns north and
walks 2 meters, then turns west
and walks 5 meters, and finally
turns south and walks 2 meters.
What is John’s distance
traveled and displacement?


Remember:


Distance

is how far
something has moved.


Displacement
is how far
and what direction
something is from its
starting point!

Motion: Speed and
Velocity

Chapter 11 Section 2

Then what is Speed?


Speed is the distance an
object travels
per unit of
time. (per min. or per hour)


Speed is the rate at which
distance is traveled or the
rate of change in position.

Think about your parents’
car:


What is a standard speed on
a city street?


What about on I
-
40?


You give speed as miles per
hour, don’t you?


Distance/time

The formula for speed is:


speed =
distance


time


or

s =
d


t

When you set your car on
cruise control,


It can move at a constant
rate of speed.

But think about riding your
bike:


What is about
to happen to
this girl’s
speed on her
bike?

What about NOW?


Speed is not
always
constant!


One of these
guys is now
WALKING his
bike!

How do you talk about the
speed of that bicycle?

Two ways:


Average speed


Instantaneous speed

Average Speed


Total distance traveled
divided by the total time of
travel


S =
d
=
2 km
= 4 km/h


t


.5 h

Instantaneous speed


Speed at any given point in
time


If you look at the
speedometer in your car and
it say 55 m/h, that is your
instantaneous speed at that
exact moment.

We can GRAPH changing
speed:


What kind of graph would you
use?


Bar graph,


Line graph, or


Pie (circle) graph?


You can graph a change in distance:

time (s) 0 1 2 3 4 5

distance (m) 0 20 50 130 150 200

So, now you know what
speed is




S =
d


t

And how to graph it.

(with a line graph)

But what about
“velocity”?


The best example of velocity is
learning how to track a
hurricane.

Velocity


Velocity must include both
speed and direction

of motion.




Ex: 20 mph east


Is given in meters/second





or m/s

Motion: Acceleration

Chapter 11 Section 3

Section 3: Acceleration



Acceleration is the rate of
change of velocity.


Can be a change in how
fast something is going
OR a change in direction.

Acceleration has one twist!


Getting fast = positive
acceleration


Slowing down = negative
acceleration!

Calculating Acceleration


Acceleration =
change in velocity


time


Change in velocity =

final velocity minus initial
velocity

Acceleration


a = (v
f
-

v
i
)


____________________________
_____________________________








t


Acceleration is given
in m/s/s or m/s
2

Forces and Motion

Chapter 12 Section 1

Sect. 3

Motion and Forces


Force

a push or pull that
one body exerts on another


What are some examples?


Force can cause change in
motion


Net Force


A force is a push or a pull.


A force of some kind is
always required to change
the state of motion of an
object.

Measuring Force


The scientific unit of force is
the
newton
, abbreviated N.


Force is measured most often
using spring scale.


Net Force


The combination of all forces
acting on an object is called
the net force.


Balanced and Unbalanced Force


When two or more forces are
acting on an object, we look at
the NET FORCE


Balanced forces cancel each other
out
--

Sum F = 0


Balanced forces = NO
change

in
movement

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

Net Force

The net force
depends on the
magnitudes
and directions
of the applied
forces.

If forces are balanced…


Net force is zero.


Forces are equal and
opposite in direction.


No change in motion.


Net Force

When the girl
holds the rock
with as much
force upward as
gravity pulls
downward, what
is the net force
on the rock?

Unbalanced forces


If one force is larger than
the other


Movement occurs in the
direction of the larger force



Force Vectors


A vector can be represented by
an arrow that shows the
magnitude and direction of a
quantity.


Force is an example of a vector
quantity.

Force Vectors

This vector represents a force of 60
N to the right.

N

Force Vectors


This vector shows two forces at
work:







NET FORCE

Newton’s First Law of Motion


Inertia

the tendency of an
object that is at rest to stay
at rest, or if in motion to
stay in motion UNLESS
some force acts on it

Inertia

Newton’s 1st


The object will keep moving
in the
same direction and at the
same speed

unless something
makes it change


The
velocity

won’t change

Have you ever felt inertia???


In a car?


During a quick turn?


During a fast stop?