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Motion and
Force
Frame of Reference
An object’s movement is judged An astronaut in space might
by comparing the position of the
use his spacecraft, or the
object to another object.
moon, the sun, or a star as
The object you choose for
his frame of reference to
comparison appears to be still.
judge his motion.
We call this comparison object
your “frame of reference.”
The most common frame of
reference is the earth, but other
things may also be used.
While inside a vehicle, you might
use the inside of a moving car or a
plane as your frame of reference.
Vectors vs. Scalars
Measures that have direction Measures that have no
associated with them are
direction associated with
called vectors.
them are called scalars.
Examples include velocity,
Examples include,
acceleration, and force
temperature, time, mass,
speed.
Measuring Motion
Speed is a measure of how fast If an object is traveling at
constant speed, then its speed
you are moving. Speed is a
does not change.
scalar quantity.
A “distance/time” graph of an
Speed = distance/time
object moving at constant
Velocity is a measure of both
speed and direction. Therefore speed will be a straight line.
velocity is a vector quantity.
Velocity can change even if
your speed doesn’t. (change of
direction!)
Graphing Speed
0
10
20
30
40
50
60
Distance as a Function of Time
Distance (kilometers)
Speed is graphed using a “distance
as a function of time” graph.
The graph should always have a
proper title. Both axes must be
labeled and units must be shown.
The distances should be plotted on
the y-axis, and time should be
plotted on the x-axis.
Speed equals the slope of the line
graphed.
The steeper the slope, the faster the
speed (rise/run=d/t).
Average speed = total distance /
total time
0
1
2
3
Time (hours)
4
5
More Graphs
Many times, the line on the distance-time graph is not straight.
These lines also give you information about the object’s motion.
Object is
accelerating
(speeding up).
Distance
Distance
Object is moving
at constant speed.
Time
Time
Time
Object is
decelerating
(slowing down).
Distance
Distance
Object moves at
constant speed,
then stops, then
moves again at
a different
constant speed.
Time
Acceleration
Acceleration is a change in
velocity. This means that your
speed can change, your
direction can change, or they
both can change.
The formula for calculating
velocity is: a = vf - vi
t
Positive acceleration means
“speeding up.”
Negative acceleration means
“slowing down.”
Changing direction without
changing speed is still
acceleration.
Centripetal acceleration causes
things to move in a circular
path..
Energy of Motion
Any change in motion
requires a change in energy.
If energy is not put in or
taken away from the object,
no change in motion will
happen.
Energy = the ability to do
work or to cause a change.
Types of Energy
Potential energy is energy Kinetic energy is energy of
stored for later use.
motion. All moving things have
kinetic energy
Gravitational potential
energy is also called energy Kinetic energy equals the mass of
of position.
the object times the square of its
velocity all divided by 2.
Gravitational potential
energy equals the mass of K.E.= mv2
the object times its height
2
times the acceleration of
gravity.
G.P.E.= mgh
Conservation of Energy
There are many forms of energy.
Chemical energy is the energy in
the bonds that join atoms together
to make molecules.
Electrical energy makes electrons
move from place to place.
Mechanical energy is the energy
of moving parts. It is similar to
kinetic energy.
Light energy is what we get from
the sun or a light bulb.
Nuclear energy is released when
an atomic bomb explodes.
Heat energy is the energy of
moving molecules.
The Law of Conservation of
Energy says: Energy is
neither lost nor gained. It
only is converted from one
form to another.
Chemical energy can be
converted to electrical or
mechanical energy.
Potential energy can be
converted to kinetic energy.
Mechanical energy can be
converted to heat energy.