Chapter 2 Mini-lecture

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Chapter 2 mini-lecture:

Motion along a Straight LineTopics:

Displacement

Velocity Acceleration

Key Idea:

Motion is described in terms of a

coordinate system that defines

position x

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PHSX 213, Chapter 2 mini-lecture

Kinematics

The subject of describing motion is called

kinematics

Starting with Chapter 5 well explore the

causes of motion. This subject is called

dynamics

2

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Terminology

This chapter presents important terminology

that well use all semester

Physics uses words from everyday Englishin precise technical ways

Ill refer to these technical terms as

physics-speak Its important to be able to use these terms

in their technical sense

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Coordinate axis

In this chapter the motion will be limited to a

straight line (like I-70)

Call the positionx (units of meters)

Define an origin and a direction that is positiverelative to the origin

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Stationary object

An object is sitting

still 2 m to the left

of the origin

Thenx=-2 m is theposition in our

coordinate system

The graph of

position vs. time is

just a horizontal

line

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Displacement

Its more interesting when an object changes

position

Assume an object starts at positionx1and at a later

time is at position x2

The displacement is the change in position, x

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x>0, if the object moves to the right

x

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Distance traveled

Displacement can differ from distance

travelled

Initial and final position is all that matters

for displacement, not what happens in

between

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x path

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Average velocity

Displacement divided by the time interval is the

average velocity

tis always taken to be positive

The direction of the average velocity is

determined by the direction of the displacement

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Instantaneous velocity

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Given the position as

a function of time,

x(t), take thederivative to get v(t)

The magnitude of the

instantaneous velocity

is the speed.

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Example

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35)( 2 ttx

ttv 10)(

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Acceleration

Acceleration is the rate of change of the velocity

The sign of the acceleration comes from the sign

ofv a>0does not necessarily mean your speed is

increasing (see checkpoint 3 in the chapter)

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Constant acceleration

Important special case

Ex: an object falling under gravity near the

Earths surface

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constatdtatv )(

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Position for constant acceleration

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constattvdtatvdttvtx2

21

00 )()()(

Other equations can be derived from these two.

Ex: eliminate tand get )(2 02

0

2xxavv

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Constant a graphs

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