chapter 2a physics71

8/13/2019 Chapter 2a Physics71

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Announcements

Syllabus is still in press.

Form 5s please

Consultation hours:Mondays9AM to 12NN and 1PM to 4PM;

Tuesdays/Thursdays3PM to 5PM;

Fridays10AM to 12NN and 1PM to 3PM EMAIL: [email protected]


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Chapter 2: Motion along a

Straight Line

Second Semester AY 2012-2013


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On unit vectors (cross products)


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Coordinate system

These are by

convention!


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Vector quantities

Mathematical entities that has both

magnitude and direction.

They can be added, subtracted, multiplied,

and transformed according to well-defined

mathematical rules.

A position vector can be

represented by anarrow pointing from the

origin of a chosen

coordinate system to

the location of the

object.


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Scalar quantities

Values that are NOT dependent on the

orientation of any coordinate system and

has no direction associated with it.

Mass, distance, length, area, temperature,

and volume are some examples.


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Introduction to Vectors

Vectors have magnitudeand direction

Consider a vector Magnitude is

Components:


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Definitions

Speedhow fast a particle is moving;

scalar quantity

Velocityhow fast AND in what

direction its moving; vector quantity

Accelerationrate of change of velocity

with time; vector quantity


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Displacement, Time, and Average

Velocity

x

0x

1x

2x

Displacementchange in the coordinate

x

0x

2 1x x x

t1 t2

Direction is to the right


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Velocity

Average Velocitythe x-component of thedisplacement divided by the time interval during

which the displacement occurs.

2 1

2 1

av x

x x xv

t t t


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Velocity

x

0x

1x

2x

2 1x x x

t1 t2

2 1

2 1

av x

x x xv

t t t

Average velocity

is ALWAYS

POSITIVE


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Velocity

x

0x

2x

1x

2 1x x x

t2 t1

Direction is to the left

Average velocity

is ALWAYS

NEGATIVE

2 1

2 1

av x

x x xv

t t t


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Velocity

Whenever x is positive and increasingor

is negative and becomes less negative,

the particle is moving in the +x-direction

and vav xis positive

Whenever x is positive and decreasing

or is negative and becomes more

negative, the particle is moving in thex-directionand vav xis negative


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Graphical Picture

x-t Graph of the

Dragster

Average velocity depends ONLY on the totaldisplacementx that occurs during the timeintervalt and not on the details of whathappens during the time interval


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Finding Velocity on an x-t Graph

The velocity of a particle can be foundfrom the graph of the particles positionas a function of time.

Instantaneous

velocity


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Average velocity of a particle during a

time interval cant tell us how fast, or in

what direction, the particle was moving

at any given time during the interval


Instantaneous Velocityvelocity at any specific

instant of time or specific point along the path.


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Motion diagram showing the position and velocity of the particle


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Average and Instantaneous

Acceleration

x

0x

1x

2x

2 1x x x

t1 t2

2 1x x xv v v

2 1

2 1

x x x

av x

v v va

t t t


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Graphical Picture

Note:

Instantaneous Acceleration

same procedure that was used in

defining instantaneous velocity


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Finding Velocity a vx-t Graph or

an x-t Graph


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Finding Velocity a vx-t Graph or

an x-t Graph


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Test you understanding

At which points P, Q, R, and S isthe velocity vxpositive? At which

points is the velocity negative? At

which points is it zero? At which

point is the speed greatest?


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Thank you very much!See you all next meeting.

chapter 2a physics71

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