When there is a translational motion, the object moves some amount of distance in some amount of time. The motion can
be described by how fast its position changes. The change depends on the size
of the path namely distance covered in the motion and the time taken for the
object for this distance.
Let’s say the relation
between this distance and time is Speed. So, speed is the function of distance
and time.
But
can it be related with some mathematical operations?
Let’s say speed as S and
the distance covered in the motion as D and
the time taken for this distance as T.
the time taken for this distance as T.
It can be defined for unit
of time as follows,
To know what happen to the motion when one of these
factors of variables change in the function of Speed, we should make one
variable as a constant and change the other variable.
Making the time constant,
we changed the distance. For a fixed time if the distance is too high it means
the object moves very fast. If the distance is low, then it implies the object moves
at a slow rate. Thus we found that the function Speed is directly proportional
to the distance covered by the object in some amount of time.
Similarly, if you make the
distance constant and changed the time taken for this distance, we can know
that,
If the time taken for some distance is high then it was
understood that the motion happens at a slow rate and if the time taken for that
distance is low then the motion happens quickly.
Thus speed is indirectly proportional to the time taken for
the distance in any motion.
Hence, the Speed is directly proportional to the distance
covered and indirectly proportional to the time taken and it can be simply written
as
Speed α Distance
and
Speed
α 1/Time [where
“α” – “alpha” means “proportional to”].
Combining
the above two, we can rewrite it as,
Speed
α Distance / Time
and
let us take the proportionality constant as 1.
Finally we described the
first mathematical term in motion as
Speed
(s)= Distance (d) / Time (t) or s= d/t
We can get the meaning of
this term d/t as follows,
“d”
distance was covered in time “t”.
consequently,
time
“t” for distance
“d”
[Note that d, t are the number
values of distance and time]
value of t*unit time for
distance “d”
unit time for distance
equal to the (value of d)/(value of t)
unit time for
(d/t) distance
Then the length covered in
unit time is
Distance covered for Unit time = d/t
This is the same mathematical
equation that we defined for Speed.
As a result finally speed “s”
can be easily understood as the distance covered in unit time t and it is
denoted as s = d/t.
To check whether the definition
of speed fulfills our need to describe translational motions, we should try it with
some real life problems, so that we can find its validity.
Uses:
As we have already seen,
now we can compare the speeds of various runners of various ages using this
definition of speed. The shape of the track is not a problem. We can make them
run in a circular path and we can evaluate their speed by measuring the
perimeter of the path distance “d” and time taken “t”.
Using
this definition of speed, we calculated the various speeds of different animals,
people, vehicles, and many other natural things.
And
another major use of “d/t” was the measurement of distances.
Using
the relation “d/t” we can measure any distance between two places that lies
very far away from each other. And we can also predict the time that will take
for an object or animal or people having certain speed limits.
These
measurements are often used in travelling and to determine the borders of any kingdom
or country.
The
measurements were done using the values of Speed, Distance and Time by applying
it in simple mathematical operations like multiplication and division.
Let’s
start measuring,
We know that speed = d/t
Which implies that,
value of distance d = value of speed * value of time t
[*Provided that the value
of t is not equal to zero as per the conditions of Divisions in Mathematics]
If we know speed “s” and time
“t” then distance “d” can be measured.
But at each instant of
time, speed can be calculated only if we know length and time.
Then how it can be used the
above equation to find distance?
This
problem can be solved by making the speed constant over some time t.
[Note: Constant means nothing but remaining same without any change.
In
physics the changings are happening with respect to time. So if any quantity
doesn’t change its value with respect to time then it is called constant with
respect to time. In real, all the measurements are done with the help of
constancy only. The time of period of this constancy can change but the
constancy might happen.
For
example we will say the speed of the vehicle is 10 meters per second. From this
sentence we can know that the vehicle is achieving the constant speed of
reaching 10 meters for one second at some instant. In that instant of small
time the vehicle doesn’t change its value.
Though
the vehicle changes its speed in the following instants of time, we can be sure
that the speed is constant for that instant. Maybe the amount of time being
constant can be 1 millisecond or 1 Micro second or 1 Nano second but the
constancy happens.
Thus
the notion of “being constancy for small intervals or instants of time” is the initialization
for limits and it serves as the basic for differentiation and Integration.]
If speed remains constant for a long time then
we can find the distance “d” by using the above equation [d= s*t].
Here
if the speed of an object remains constant for 5 seconds with the speed of 5meters
per second then we can find the distance covered by that object in 5 seconds by
multiplying 5 with the value of speed that is 5 m/s.
It
equals to 5 seconds*5 meters/ seconds = 25 meters [using basic mathematics of
multiplication and division]
The
above calculation hold true only if the object moves in constant speed over
time because, in constant speed, the vehicle will reach same amount of length
in same amount of time. In the above case, in each second the object will move
5 meters. And for 5 seconds 25 meters.
Though
there are no any perfect vehicles or animals that move at constant speed, we
can estimate the near value.
Thus
constancy helps us to solve many problems.
The
simple real life of use of this constancy for a king on protecting his country
is…
First
we will calculate the speed of a horse or any other vehicle by making it run
for some distance. Horse was specified here as just because it is the vehicle
for ancient people.
Then
it was made to run that horse of known speed constantly over some time t until
it reaches the border of the country in the shortest path.
As
we have already calculated the value of the speed of the horse, the distance
covered by the horse in time t can be calculated as “speed s* time t”.
Thus
we calculated the nearly distances of the border of the country from the palace
or the capital of the kingdom using the animals like horses.
So,
when there happens a war, we can estimate the minimum value of the time that
takes for the enemy troops to reach the palace of the kingdom.
Here we know that “The minimum time to reach the palace is the maximum
time got by the king to ready his army”.
So
what is the minimum time?
Speed
= Distance / Time
Time
= Distance / Speed
If
we want Minimum time then
condition
1: the distance (numerator) should be minimum and
condition
2: the speed (denominator) should be maximum as we have studied in mathematics.
[Note: In mathematics,
when p/q=r , r is minimum only if p is minimum and q is maximum because p is
directly proportional to r and q is indirectly proportional to r]
As
because of we have done this experiment in the shortest path, the distance is
the minimum one. Accordingly, the first condition is satisfied.
The maximum speed can be calculated by taking the fastest
horse and making it to run at its maximum speed. According the second condition
is also satisfied.
And
so, now we can calculate the maximum time the King have to save his palace and
his kingdom.
In
the same way, most of the things in transportations can be predicted by calculating
the value of speed.