Concept Summary

Batesville High School Physics

Motion of What?To simplify things as much as possible,

we will first consider one-dimensional motion (motion along a straight line) of particles (points that can’t spin, rotate, flip, flop, or wiggle around).

Describing MotionsWe will concern ourselves (for now)

with describing motion - kinematics.We will worry about explaining motion

(dynamics) later.

Two Simple Motions In our course, we will be primarily

concerned describing with 2 simple motions: Motion with constant velocity Motion with constant acceleration

PositionMark a zero point on the line, pick a

direction to be positive, and measure from there.

Positions can be positive or negative.Units of position: centimeters, meters,

kilometers, inches, feet, miles, etc.Common symbol: x

Operational DefinitionPosition, like other physical quantities,

is defined by telling how you go about measuring it - not by giving synonyms or descriptive phrases. This is called an operational definition.

Positions are RelativeDifferent people can mark the line

differently, so they can get different numbers for position.

The position number (and unit) really don’t mean anything until you specify where you marked “0”, and which way you made positive - your frame of reference.

DisplacementDisplacement = net distance moved or

net change in positionCommon symbol, d or ∆x If you move from xo to x,

displacement, d = ∆x = x - xo

RatesA rate measures how fast something

changes. In physics, a rate is almost always

calculated as a quantity divided by time.Rate Q changes = change in Q

time for Q to change

SpeedSpeed is the rate position changes, or

the rate distance is covered.There are two kinds of speed:

Average speed Instantaneous speed

Average Speed Average speed = distance traveled

Or, average speed = displacement

In symbols, v = d or ∆x

Units of speed: m/s, km/h, mi/h, etc.

time it takes

time

t t

Instantaneous Speed Instantaneous speed is what the

speedometer says. It is not measured over a time interval,

like average speed.

Constant Speed If an object’s instantaneous speed is

always the same value, the object has a constant speed.

In this case, average speed = instantaneous speed

VelocityVelocity = speed + direction2 kinds of velocity

Average velocity = average speed + direction

Instantaneous velocity = instantaneous speed + direction

How Velocity ChangesThe velocity of an object changes if:

It speeds up, or It slows down, or It changes direction.

What Velocity MeansAn object’s velocity tells you how fast its

position is changing. 5 m/s means the object’s position changes

by 5 meters each second. 60 mi/hr means that the object’s position

changes by 60 mi each hour.

Velocities are RelativeSpeed and velocity are relative

quantities. Different observers, in different frames of reference, can measure different velocities.

You measure speed and velocity by comparing two motions.

Acceleration

Acceleration is the rate velocity (not speed) changes.

2 kinds: Average acceleration Instantaneous acceleration

Average Acceleration Ave. Accel. = change in velocity

in symbols, a = ∆v

Accelerations are not relative quantities.

time it takes

t

Units of AccelerationSince acceleration is a velocity divided

by a time, its units are a distance unit divided by 2 time units.

This is commonly written 2 ways: m/s/s = m/s2

km/hr/s = km/hr.s

Constant Acceleration In many common situations, an object’s

acceleration is constant, or at least approximately constant.

In this case: Average accel. = instantaneous accel.

Free FallFree fall is motion under the influence of

gravity only - no friction or air resistance.

Acceleration in Free FallThe acceleration of an object in free fall

is constant.At the surface of Earth, the free-fall

acceleration is about 10 m/s2, or 9.8 m/s2 if you have a calculator (or 32 ft/s2 or 22 mi/hr/s in “English” units).

Air ResistanceThe effect of air resistance is to slow an

object down and/or decrease its acceleration.

The End