Physics 1 Lecture 2 Notes

8/2/2019 Physics 1 Lecture 2 Notes

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Announcements Day 2 Read Website for announcements (i.e. online hw trouble, etc.)

Website troubles

No Cell Phones

Give me details

Find a computer that works

Online HW due by 8:20am

Long HW due Monday

Always write: Legibly, name, quiz/hw #, lab section

OK to reuse Quiz Paper, but write new Quiz #, date, lab section

Extra Credit-survey on website Slides covered fast, because I expect you to have read the material-

most of time spend expanding on it.


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3.1 From reality to model

As a first step in studying classical mechanics,we will describe motion in terms of space andtime (ignoring, for now, the causes of this

motion) Kinematics is a quantitative study of this

motion.

First we will consider only motion in onedimension (motion along a straight line).


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We first will define position, displacement, velocity, and acceleration.

Then using these concepts We study the motion of objects traveling in one

dimension with a constant acceleration.

Our everyday experience tells us that motionrepresents a continuous change in theposition of an object.


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We will see as we move forward through thebook that motion can be categorized intothree types:

Translational (car moving down the highway)

Rotational (Earth rotating on its axis)

Vibrational (back and forth movement of apendulum)

For now, we concern ourselves withtranslational motion.


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In order to study translational motion We must create a simplified model

The models can be Graphs

Tables

Equations


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To start our study of motion

We will use what is called the particle model We describe the moving object as a particle

regardless of size.

Cars, people, balls, etc. will all be treated as point-like objects. We need to know when this is a goodapproximation!

For example if we wish to describe the motion of the Earth

around the Sun, we can treat the Earth as aparticle (as long as we care about years but not

days).


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3.2 Calibrating the data

Building a model, taking data, are all fine; butscientists must be able to communicate thesedata to one another.

The laws of physics are expressed asmathematical relationships among physicalquantities.


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Physical quantities

Basic quantities Length (meters) Mass (kilograms)

Time (seconds)

Derived quantities Can be expressed as combinations of the basic

quantities


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Standards of Measurements-dont memorize!

Length The meter (m) is defined as the distance traveled

by light in a vacuum during a time of 1/299,792,458second

Mass The kilogram (kg), is defined as the mass of a

specific platinum-iridium alloy cylinder.

Time The second (s) is defined as 9,192,631,770 times

the period of vibration of radiation from the cesium

atom.


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Are Units Important?Not so fun fact. Mars Orbiting Satellite



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3.3 Position and Displacement

The motion of aparticle is completelyknown if the particlesposition in space isknown at all times.

In lab, you may havenoticed Position depends on

origin

Displacement does not


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A particles position is thelocation of the particle withrespect to a chosen

reference point. This reference point can be

considered to be the originof the coordinate system.

We will consider that the

side mirrors on the car arethe center point for ourparticle


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Consider the Diagram, whenwe start collecting positiondata

Take the road sign to be yourorigin (x = 0)

Start your clock (t = 0) andrecord the data once every 10seconds

At position A (t = 0s) - the car

is 30m to the right of the roadsign (x = 30m ).

At position B (t = 10s) - the caris 52m to the right of the roadsign (x = 52m ).


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Position data At position C (t = 20s) - the car

is 38m to the right of the roadsign (x = 38m).

At position D (t = 30s) - the caris at the origin (x = 0m).

At position E (t = 40s) - the caris 37m to the left of the roadsign (x = -37m).

At position F (t = 50s) - the caris 53m to the left of the roadsign (x = -53m).


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One way to study thismodel

Make a table


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With the position data intabular form, we can easilydetermine the change in

positionof the car forvarious time intervals.


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The displacement of aparticle (car) is defined as itschange inpositionin some

interval of time. Note that the displacement isindependentof the choice oforigin.

For example, if I move the car

from A to B, it doesnt matterwhere the origin is, thechange in the cars position isstill the same.

Displacement can be negative


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The distance traveled is the length of a pathfollowed by a particle.

For example

Lets say a basketball player runs from her ownbasket, down to the other teams basket, and then

back to her own.

Displacement of the player during this time interval will be

zero, because she ended up where she started. However, during this time interval the distance traveled by

the player was twice the length of the basketball court.


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3.4 Position versus Time Curves

Another way to studythis model

Make a graph


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The position-timegraph shows themotion of the particle(car) Use a standard

Cartesian coordinategrid.

Position is graphed on

the y-axis Time is on the x-axis


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Each data point, A - F isindividually plotted onthe grid.

The smooth curve is aguess as to whathappened between thedata points. It is our

model. From the datagiven, we dont really

know what happened.


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3.5 Average Speed and Average Velocity

Average Speed The ratio of the

distance traveled tothe time it takes to

travel that distance. Speed= _distance_

travel time

Speed has no

direction Speed is never

negative


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3.5 Average Speed and Average Velocity

Average Velocity The ratio of the

displacement to thetime it takes to

achieve thisdisplacement

Velocity has direction Velocity can be

negative

Velocity= displacementtravel time

A 30m


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Example 1-Section 3.6

Find the Displacement

Average velocity

Average speed

Of the particle (car)between positions A

and F

A 30m

B 52m

C 38m

D 0m

E -37m

F -53m


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Example 2 The position versus

time for a certainparticle movingalong the x axis isshown in figure.Find the average

velocity in the timeintervals (a) 0 to 2 s,(b) 2 s to 4 s, (c) 4 sto 7 s, (d) 0 to 8 s, (e)average speed 0 to8s

Is this motionrealistic?


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Example 3

A person walks first at a constantspeed of 5.00 m/s along a straight linefrom point A to point B and then

back along the line from B to A at aconstant speed of 3.00 m/s. What is(a) her average speed over the entiretrip? (b) her average velocity over the

entire trip?Hint on (a): went equal distances, not times. So cannot average-have to find

times for each part, t1=x/v1, t2=x/v2, xtot=2x. Can only take an average iftime is the same, since it is the denominator!

Physics 1 Lecture 2 Notes

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