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When you see an object that appears to
be in motion, you must realize that it is changing its
position over a period of time with respect to you, the
observer. In other words, motion is relative to the
observer. The distance the object goes in a period of
time is its speed or velocity. The change in velocity
over time is its acceleration.
Some questions you may have include:
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Why must motion be with respect to
the observer?
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What is the difference between speed
and velocity?
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Where is acceleration used?
This lesson will help satisfy your
curiosity about motion. There is a mini-quiz near the
end of this lesson.
Motion is relative
All motion is relative to the observer
or to some fixed object. When you see a car drive by, it
is moving with respect to you. If you are in a car going
at the same speed, the other car will not by moving with
respect to you. But both cars are moving with respect to
the ground.
In talking about motion, it is important
to indicate your point of reference. In the case of
moving automobiles, it is usually assumed the speed is
with respect to the ground. But there are situations
where the speed or velocity may be with respect to
another object or an observer.
For example, a car was traveling at 60
miles per hour (mph) and hit another car, but there was
hardly a dent. The reason is that the second car was
traveling in the same direction at 59 mph, so the car
was going only 1 mph with respect to the second car when
it hit it.
Another example of relative motion is
how the sun appears to move across the sky, when the
earth is actually spinning and causing that apparent
motion.
Usually, we consider motion with respect
to the ground or the Earth. Within the Universe there is
no real fixed point. The basis for Einstein's Theory of
Relativity is that all motion is relative to what you
define as a fixed point.
Speed and velocity
Speed is how fast an object is going
with respect to an object. Velocity is a measure of the
speed in a given direction. You can say the top speed of
an airplane is 300 kilometers per hour (kph). But its
velocity is 300 kph in a northeast direction.
We distinguish between speed and
velocity because if you add the speeds of objects, their
directions are important. For example, the velocity of
an airplane with respect to the ground would vary
according to the direction of the wind.
In order to determine how fast an object
is going, you measure the time it takes to cover a given
distance. Its velocity (v) or speed equals the distance
(d) traveled divided by the time (t) it takes to go that
distance:
v = d / t.
For example, if a car went 120 miles in
2 hours, its average speed would be the distance of 120
miles divided by the time of 2 hours equaling 60 miles
per hour (mph.
If it takes a car 2 minutes to travel 1
mile, its speed is 1 mile divided by 2 minutes, which
equals 1/2 mile per minute or 30 miles per hour.
If you travel from Milwaukee to Chicago
(90 miles) at an average velocity of 60 mph, it would
take you 90 mi. / 60 mph = 1.5 hours to travel the
distance.
Adding velocities at angles can be
difficult. Thus, scientists use a graphical method
called vectors to help calculate the sum of velocities
in a given direction.
The way this works is that you draw a
picture with lines or arrows representing different
velocities. The length of the arrow (or vector)
represents the speed. Combining the speed with the
direction of the arrow, you get its velocity.
Now to add two velocities�say the
velocity of an airplane and the velocity of the wind�the
sum of two vectors is the diagonal of the parallelogram
formed from the two sides.
In the illustration below, you could say
that if A was the velocity or an airplane and B was the
velocity of the wind, blowing at an angle to the
airplane, then the total velocity of the airplane would
be represented by the vector A + B.
This can be a very handy tool in such a
calculation.
Acceleration is the increase of velocity
over a period of time. Deceleration is the decrease of
velocity. When you start running, you accelerate
(increase your velocity) until you reach a constant
speed.
Mathematically, acceleration is the
change in velocity divided by the time for the change:
a = (v2 - v1)/(t22-t1).
For example, if an object speeds up from
a velocity of 240 feet/second to 560 feet/second in a
time period of 10 seconds, the acceleration is (560 -
240) / 10 = 320 / 10 = 32 feet/second/second or 32 feet/
second squared.
Changing direction can also cause
acceleration (or deceleration) because the velocity in
that direction has changed.
Motion is change in position. All motion
is relative to some fixed point or object. Speed is a
measurement of that change in position over time.
Velocity is speed in a given direction. Vectors help to
determine the sum of velocities.
Acceleration is the increase in speed or
velocity over a period of time. Deceleration is the
decrease of speed or velocity over time. |
