Transcript Document
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 Motions
We
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
Position
Mark
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 Definition
Position,
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 Relative
Different
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.
Displacement
Displacement
= net distance moved or
net change in position
Common symbol, d or ∆x
If you move from xo to x,
displacement, d = ∆x = x - xo
Rates
A 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
Speed
Speed
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
time it takes
Or, average speed = displacement
time
In symbols, v = d or ∆x
Units of speed: m/s, km/h, mi/h, etc.
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
Velocity
Velocity
= speed + direction
2 kinds of velocity
Average
velocity = average speed +
direction
Instantaneous velocity = instantaneous
speed + direction
How Velocity Changes
The
It
velocity of an object changes if:
speeds up, or
It slows down, or
It changes direction.
What Velocity Means
An
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 Relative
Speed
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
is the rate velocity (not
speed) changes.
2 kinds:
Acceleration
Average
acceleration
Instantaneous acceleration
Average Acceleration
Ave. Accel. = change in velocity
time it takes
in symbols, a = ∆v
t
Accelerations are not relative quantities.
Units of Acceleration
Since
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/s2
km/hr/s = km/hr.s
m/s/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 Fall
Free
fall is motion under the influence of
gravity only - no friction or air
resistance.
Acceleration in Free Fall
The
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 Resistance
The
effect of air resistance is to slow an
object down and/or decrease its
acceleration.
The End