# Free-fall Acceleration - Batesville Community Schools

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
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
