What is light? What are the sources of light and it's properties ?

For explanation of this topic follow the link given below.

https://youtu.be/2uQwGrBOOVA

Creative ways to use Ice Cream Sticks to make projects.

What are the rules for using Apostrophe ?

Rule 1a. Use the apostrophe to show possession. To show possession with a singular noun, add an apostrophe plus the letter s.

Examples:
a woman's hat
the boss's wife
Mrs. Chang's house

Rule 1b. Many common nouns end in the letter s (lens, cactus, bus, etc.). So do a lot of proper nouns (Mr. Jones, Texas, Christmas). There are conflicting policies and theories about how to show possession when writing such nouns. There is no right answer; the best advice is to choose a formula and stay consistent.

Rule 1c. Some writers and editors add only an apostrophe to all nouns ending in s. And some add an apostrophe + s to every proper noun, be it Hastings's or Jones's.

One method, common in newspapers and magazines, is to add an apostrophe + s ('s) to common nouns ending in s, but only a stand-alone apostrophe to proper nouns ending in s.

Examples:
the class's hours
Mr. Jones' golf clubs
the canvas's size
Texas' weather

Care must be taken to place the apostrophe outside the word in question. For instance, if talking about a pen belonging to Mr. Hastings, many people would wrongly write Mr. Hasting's pen (his name is not Mr. Hasting).

Correct: Mr. Hastings' pen

Another widely used technique is to write the word as we would speak it. For example, since most people saying "Mr. Hastings' pen" would not pronounce an added s, we would write Mr. Hastings' pen with no added s. But most people would pronounce an added s in "Jones's," so we'd write it as we say it: Mr. Jones's golf clubs. This method explains the punctuation of for goodness' sake.

Rule 2a. Regular nouns are nouns that form their plurals by adding either the letter s or es(guy, guys; letter, letters; actress, actresses; etc.). To show plural possession, simply put an apostrophe after the s.

Correct: guys' night out (guy + s + apostrophe)
Incorrect: guy's night out (implies only one guy)

Correct: two actresses' roles (actress + es+ apostrophe)
Incorrect: two actress's roles

Rule 2b. Do not use an apostrophe + s to make a regular noun plural.

Incorrect: Apostrophe's are confusing.
Correct: Apostrophes are confusing.

Incorrect: We've had many happy Christmas's.
Correct: We've had many happy Christmases.

In special cases, such as when forming a plural of a word that is not normally a noun, some writers add an apostrophe for clarity.

Example: Here are some do's and don'ts.

In that sentence, the verb do is used as a plural noun, and the apostrophe was added because the writer felt that dos was confusing. Not all writers agree; some see no problem with dosand don'ts.

However, with single lowercase letters, it is advisable to use apostrophes.

Example: My a's look like u's.

Imagine the confusion if you wrote that sentence without apostrophes. Readers would see as and us, and feel lost.

Rule 2c. English also has many irregular nouns(child, nucleus, tooth, etc.). These nouns become plural by changing their spelling, sometimes becoming quite different words. You may find it helpful to write out the entire irregular plural noun before adding an apostrophe or an apostrophe + s.

Incorrect: two childrens' hats
The plural is children, not childrens.
Correct: two children's hats (children + apostrophe + s)

Incorrect: the teeths' roots
Correct: the teeth's roots

Rule 2d. Things can get really confusing with the possessive plurals of proper names ending in s, such as Hastings and Jones.

If you're the guest of the Ford family—the Fords—you're the Fords' guest (Ford + s + apostrophe). But what if it's the Hastingsfamily?

Most would call them the "Hastings." But that would refer to a family named "Hasting." If someone's name ends in s, we must add -es for the plural. The plural of Hastings is Hastingses. The members of the Jones family are the Joneses.

To show possession, add an apostrophe.

Incorrect: the Hastings' dog
Correct: the Hastingses' dog (Hastings + es+ apostrophe)

Incorrect: the Jones' car
Correct: the Joneses' car

In serious writing, this rule must be followed no matter how strange or awkward the results.

Rule 2e. Never use an apostrophe to make a name plural.

Incorrect: The Wilson's are here.
Correct: The Wilsons are here.

Incorrect: We visited the Sanchez's.
Correct: We visited the Sanchezes.

Rule 3. With a singular compound noun (for example, mother-in-law), show possession with an apostrophe + s at the end of the word.

Example: my mother-in-law's hat

If the compound noun (e.g., brother-in-law) is to be made plural, form the plural first (brothers-in-law), and then use the apostrophe + s.

Example: my two brothers-in-law's hats

Rule 4a. If two people possess the same item, put the apostrophe + s after the second name only.

Example: Cesar and Maribel's home is constructed of redwood.

However, if one of the joint owners is written as a pronoun, use the possessive form for both.

Incorrect: Maribel and my home
Incorrect: Mine and Maribel's home
Correct: Maribel's and my home

Incorrect: he and Maribel's home
Incorrect: him and Maribel's home
Correct: his and Maribel's home

Incorrect: you and Maribel's home
Incorrect: yours and Maribel's home
Correct: Maribel's and your home

Note: As the above examples demonstrate, when one of the co-owners is written as a pronoun, use possessive adjectives (my, your, her, our, their). Avoid possessive pronouns(mine, yours, hers, ours, theirs) in such constructions.

It should be mentioned that compound possessives are often clunky as well as confusing. For instance, a picture of her and Cesar's house could refer to a photo of "her" in front of the house that Cesar owns or a photo of the house that she and Cesar co-own. Big difference. Such ambiguous sentences should just be rewritten.

Rule 4b. In cases of separate rather than joint possession, use the possessive form for both.

Examples:
Cesar's and Maribel's homes are both lovely.
They don't own the homes jointly.

Cesar and Maribel's homes are both lovely.
The homes belong to both of them.

Rule 5. Use an apostrophe with contractions. The apostrophe is placed where a letter or letters have been removed.

Examples: doesn't, it's, 'tis, can't, you'd, should've, rock 'n' roll, etc.
Incorrect: does'nt

Rule 6. There are various approaches to plurals for abbreviations, single letters, and numerals.

Many writers and editors prefer an apostrophe after single capitalized letters.

Example: I made straight A's.

With groups of two or more capital letters, apostrophes seem less necessary.

Examples:
There are two new MPs on the base.
He learned his ABCs.
She consulted with three M.D.s. OR She consulted with three M.D.'s.
Some write M.D.'s to give the s separation from the second period.

Single-digit numbers are usually spelled out, but when they aren't, you are just as likely to see 2s and 3s as 2's and 3's. With double digits and above, many (but not everyone) regard the apostrophe as superfluous: I scored in the high 90s.

There are different schools of thought about years and decades. The following examples are all in widespread use:

Examples:
the 1990s
the 1990's
the '90s
the 90's

Awkward: the '90's

Rule 7. Amounts of time or money are sometimes used as possessive adjectives that require apostrophes.

Incorrect: three days leave
Correct: three days' leave

Incorrect: my two cents worth
Correct: my two cents' worth

Rule 8. The personal pronouns hers, ours, yours, theirs, its, whose, and the pronoun oneself never take an apostrophe.

Examples:
Correct: Feed a horse grain. It's better for its health.

Incorrect: Who's glasses are these?
Correct: Whose glasses are these?

Incorrect: Talking to one's self in public is odd.
Correct: Talking to oneself in public is odd.

Rule 9. When an apostrophe comes before a word or number, take care that it's truly an apostrophe (’) rather than a single quotation mark (‘).

Incorrect: ‘Twas the night before Christmas.
Correct: ’Twas the night before Christmas.

Incorrect: I voted in ‘08.
Correct: I voted in ’08.

NOTE

Serious writers avoid the word 'til as an alternative to until. The correct word is till, which is many centuries older than until.

Rule 10. Beware of false possessives, which often occur with nouns ending in s. Don't add apostrophes to noun-derived adjectives ending in s. Close analysis is the best guide.

Incorrect: We enjoyed the New Orleans' cuisine.

In the preceding sentence, the word the makes no sense unless New Orleans is being used as an adjective to describe cuisine. In English, nouns frequently become adjectives. Adjectives rarely if ever take apostrophes.

Incorrect: I like that Beatles' song.
Correct: I like that Beatles song.

Again, Beatles is an adjective, modifying song.

Incorrect: He's a United States' citizen.
Correct: He's a United States citizen.

Rule 11. Beware of nouns ending in y; do not show possession by changing the y to ies.

Correct: the company's policy
Incorrect: the companies policy

To show possession when a noun ending in ybecomes plural, write ies'. Do not write y's.

Correct: three companies' policies
Incorrect: three company's policies

Exception: Names and other proper nouns ending in y become plural simply by adding an s. They do not form their plurals with an apostrophe, or by changing the y to ies.

Correct: The Flannerys are coming over.
Incorrect: The Flannery's are coming over.
Incorrect: The Flanneries are coming over.

Correct: The Flannerys' house was robbed.
Incorrect: The Flanneries' house was robbed.

What is Motion ?

1. Introduction

• In our daily life, we see lots of things moving around for example car passing through from one place to other, person riding on a bicycle and many more like this.
• In scientific terms an object is said to be in motion ,if it changes its position with the passage of time and if it does not change it position with the passage of time then it is said to be at rest
• Both the motion and rest are relative terms for example mobile kept on the table is resting at its position but it is moving in the sense as earth is rotating on its axis. So for a person seeing mobile from earth it is at rest and for person on moon earth seems to change its position with time and so mobile is moving.
• Simplest case of motion is rectilinear motion which is the motion of the object in a straight line
• In our description of object, we will treat the object as a point object.
• Object under consideration can be treated as point object if the size of the object is much smaller than the distance traveled by it in reasonable time duration for example length of a motor car traveling a distance of 500km can be neglected w.r.t. distance traveled by it.

2. Motion along a straight line

• The simplest type of motion is the motion along a straight line.
• Two different quantities Distance and Displacement are used to describe the overall motion of an object and to locate its final position with reference to its initial position at a given time.
• Distance in physics, is the length of the path (the line or curve) described by an object moving through space. Distance is independent of direction. Thus, such physical quantities that do not require direction for their complete description are called scalars.
• When a body moves from one position to another the shortest distance between the initial and final position of the body along with its direction is known as displacement. Displacement has both direction and magnitude for its complete description and hence such physical quantities are called a vectors.
• The distance travelled by a moving body cannot be zero but the final displacement of a moving body can be zero.
• If a body covers equal distances in equal intervals of time then it is said to be having uniform motion.
• If a body covers unequal distances in equal intervals or equal distances in unequal intervals then body is said be having non-uniform motion.

Worksheet 1 - 

Fill in the blanks
1) __________________ and ______ are used to describe the overall motion of an object and to locate its final position with reference to its initial position at a given time.
2) Physical quantities that do not require direction for their complete description are called ______
3) When a body moves from one position to another the shortest distance between the initial and final position of the body along with its direction is known as _______
4) Displacement has both direction and magnitude for its complete description and these physical quantities are called a _____
5) The distance travelled by a moving object cannot be zero but the final _______ of a moving object can be zero.
6) when the  object covers unequal distances in equal intervals or equal distances in unequal intervals then motion is said be having ____ motion.
7) when body covers equal distances in equal intervals of time then it is said to be having _____________________.

Answer

1)Distance and Displacement
2) Scalars
3) Displacement
4) Vectors
5) Displacement
6) Non Uniform
7) Uniform

4. Velocity

• The rate of change of displacement of a body with the passage of time is known as velocity of the body. Velocity of an object is measured in meter per second in SI units. So,
  

• Velocity is nothing but the speed of an object moving in a definite direction.
• The velocity of an object can be uniform or variable. It can be changed by changing the object's speed, direction of motion or both.
• So velocity of a body is a vector quantityinvolving both distance and displacement whereas speed of a body is a scalar quantity and it only has magnitude and does not have specific direction.
• Thus a body is said to be moving withuniform velocity if it covers equal distances in equal intervals of time in a specified direction.
• A body is said to be moving with non uniform velocity if it covers unequal distances in equal intervals of time and vice-versa in a specified direction or if it changes the direction of motion.
• The velocity of a body can be changed in two ways first by changing the speed of the body and second by changing the direction of motion of the body by keeping the speed constant. Also both speed and direction of the body can be varied in order to change the velocity of the body.
• When velocity of the object changes at a uniform rate, then average velocity is given by the arithmetic mean of initial velocity and final velocity for a given period of time. That is,
  
  Where u is the initial velocity of the object and v is the final velocity of the object.

5. acceleration

• Acceleration is a measure of the change in the velocity of an object per unit time and mathematically it is given as
  
• If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration a is given by,
  $a=\frac{v-u}{t}$
  and this kind of motion is called accelerated motion.
• A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amount in equal intervals of time for example freely falling bodies, motion of ball rolling down the inclined plane etc.
• A body has non uniform acceleration if its velocity increases or decreases by unequal amount in equal intervals of time.
• If acceleration is in the direction of the velocity then it is positive acceleration and if it is in the direction opposite to the direction of velocity then it is negative and the negative acceleration is termed retardation.
• SI unit of acceleration is ms^-2.

Worksheet 2 - 

Fill in the Blank Questions 
1)  __________________ is defined as the total distance travelled by the body in the time interval during which the motion takes place.
2) __________________ unit of speed is m/s
3) The ratio of total distance to total __________________ taken by the body gives its average speed.
4) The rate of change of displacement of a object with the passage of time is known as __________________ of the object.
5) velocity of a body is a vector quantity involving both distance and __________________ whereas speed of a body is a scalar quantity and it only has magnitude  and does not have specific direction.
6) A body is said to be moving with uniform velocity if it covers equal distances in equal intervals of time in a specified __________________.
7) A object  is said to be moving with non uniform velocity if it covers unequal distances in equal intervals of time and vice-versa in a specified direction or if it
 changes the direction of __________________.
8) If acceleration is in the __________________ of the velocity then it is positive acceleration and if it is in the __________________ opposite to the ___________
_______ of velocity then it is negative and the negative acceleration is termed retardation.
 
True and False Statement
1) Acceleration is a measure of the change in the speed of an object per unit time 
2)  A body has Non Uniform acceleration if its velocity increases or decreases by unequal amount in equal intervals of time.
3) When velocity of the object changes at a uniform rate, then average veloicty is given by the arithmetic mean of initial velocity and final velocity for a given period of time.
4) The velocity of a body can be changed in two ways first by changing the speed of the object  and second by changing the direction of motion of the body by keeping the speed constant.
5) The odometer in the car measures the velocity of the car
6) A object is said to have constant or uniform velocity  if it travels equal distance in equal intervals of time.
7) A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amount in equal intervals of time
 

Answers

Fill in the blanks
1)Speed
2) SI
3) time
4) Velocity
5) Displacement
6) direction
7) Motion
8) Direction
True and False Statement
1) False
2) True
3) True
4) True
5) false
6) false
7) false

6. Equations of uniformly accelerated motion

• There are three equations of bodies moving with uniform acceleration which we can use to solve problems of motion

First Equation of motion

• The first equation of motion is $v=u+at$ , where v is the final velocity and u is the initial velocity of the body.
• First equation of motion gives velocity acquired by body at any time $t$.

Now we know that acceleration

so, $a=\frac{v-u}{t}$
and, $at=v-u$
rearranging above equation we get first equation of motion that is
$v=\; u\; +\; at$

Second Equation of motion

• Second equation of motion is
  $s=ut+\frac{1}{2}a{t}^2$
  where $u$ is initial velocity, $a$ is uniform acceleration and $s$ is the distance travelled by body in time $t$.
• Second equation of motion gives distance travelled by a moving body in time $t$.
• To obtain second equation of motion consider a body with initial velocity $u$moving with acceleration a for time $t$ its final velocity at this time be $v$. If body covered distance $s$ in this time $t$ , then average velocity of the body would be
  
  Distance travelled by the body is
  
  From first equation of motion 
  $v=u+at$
  So putting first equation of motion in above equation we get ,
  $s=\frac{u+u+at}{2}\times t=\frac{\left(2u+at\right)t}{2}=\frac{2ut+a{t}^2}{2}$
  Rearranging it we get
  $s=ut+\frac{1}{2}a{t}^2$

Third equation of motion

• Third equation of motion is
  $v^2=u^2+2as$ where $u$ is initial velocity, $v$is the final velocity, $a$ is uniform acceleration and $s$ is the distance travelled by the body.
• This equation gives the velocity acquired by the body in travelling a distance $s$.
• Third equation of motion can be obtained by eliminating time t between first and second equations of motion.
  So, first and second equations of motion respectively are
  $v=u+at$ and $s=ut+\frac{1}{2}a{t}^2$
  Rearranging first equation of motion to find time t we get
  $t=\frac{v-u}{a}$
  Putting this value of t in second equation of motion we get
  $s=\frac{u\left(v-u\right)}{a}+\frac{1}{2}a{\left(\frac{v-u}{a}\right)}^2$
  
  $s=\frac{uv-u^2}{a}+\frac{a\left(v^2+u^2-2uv\right)}{2a^2}$
  
  $s=\frac{2uv-2u^2+v^2+u^2-2uv}{2a}$
  
  Rearranging it we get 
  $v^2=u^2+2as$
• These three equations of motion are used to solve uniformly accelerated motion problems and following three important points should be remembered while solving problems
1. if a body starts moving from rest its initial velocity $u=0$
2. if a body comes to rest i.e., it stops then its final velocity would be $v=0$
3. If a body moves with uniform velocity then its acceleration would be zero

7. Graphical representation of motion

• A graph is a pictorial representation of the relation between two sets of data of which one set is of dependent variables and the other set is of independent variables.
• To describe the motion of an object, we can use line graphs. In this case, line graphs show dependence of one physical quantity, such as distance or velocity, on another quantity, such as time.

Distance Time Graphs

• The change in the position of an object with time can be represented on the distance-time graph.
• In this graph, time is taken along the x-axis and distance is taken along the y-axis.
• Distance time graphs of a moving body can be used to calculate the speed of the body as they specifically represent velocity.
• The distance time graph for a body moving at uniform speed is always a straight line as distance travelled by the body is directly proportional to time as shown below in the figure 1.
• The distance time graph for a body moving with non uniform speed is a curve and is shown below in the figure 2.
  
• The distance time graph is parallel to time axis when the object is at rest and is shown below in figure 3.
  
• To calculate speed of the body from distance time graph say at point A first draw a perpendicular AB on time axis and a perpendicular $AC$ on distance axis so that $AB$ represents the distance travelled by the body in time interval OB and since we know that
  

Velocity time graphs

• The variation in velocity with time for an object moving in a straight line can be represented by a velocity-time graph.
• In this graph, time is represented along the x-axis and the velocity is represented along the y-axis.
• The product of velocity and time give displacement of an object moving with uniform velocity. The area enclosed by velocity-time graph and the time axis will be equal to the magnitude of the displacement.
• If a body moves with a constant velocity then velocity time graph for this body would be straight line parallel to time axis as shown below in the figure 5
  
• The velocity time graph of uniformly changing velocity is shown in figure 6 and is a straight line. We can find out the value of acceleration using the velocity time graph.
• For calculating acceleration at time corresponding to point R draw a perpendicular RP from point R as shown in figure 6 and we know that 
  
  Here change in velocity is represented by PR and time taken is equal to OR. So, 
  
  which is equal to the slope of velocity time graph. So we conclude that slope of velocity time graph of moving body gives its acceleration.
• The distance travelled by moving body in a given time will be equal to area of triangle OPR as shown in figure 6
  
  so,
  
• When the velocity of a body changes in an irregular manner then velocity time graph of the body is a curved line.

8. Equations of motion by graphical method

• We already know about equations of motion when an object moves along straight line with uniform acceleration. We already know how to derive them but these equations can also be derived by graphical method.

a. Equation for velocity time relation

• Consider the velocity-time graph of an object that moves under uniform acceleration as shown below in the figure 7.
• From this graph, you can see that initial velocity of the object is $u$ (at point A) and then it increases to $v$ (at point B) in time $t$. The velocity changes at a uniform rate $a$.
• Again from figure it is clear that time $t$ is represented by OC , initial velocity $u$ by OA and final velocity of object after time $t$ by BC.
  
• From graph as given in figure 7 it is clear that $BC=BD+DC=BD+OA$.
  So we have 
  v=BD+u               (1)
• We should now find out the value of BD. From the velocity-time graph (Fig. 7), the acceleration of the object is given by
  
  which gives, $BD=at$
  putting this value of BD in equation 1 we get
  $v=u+at$
  which is the equation for velocity time relation.

b. Equation for position time relation

• Let us consider that the object has travelled a distance s in time t under uniform acceleration a. In Fig. 7, the distance travelled by the object is obtained by the area enclosed within OABC under the velocity-time graph AB.
• Thus, the distance s travelled by the object is given by
  s = area OABC (which is a trapezium) 
  s= area of the rectangle OADC + area of the triangle ABD
  So, 
  $s=OA\times OC+\frac{1}{2}\left)AD\times BD\right)$
  Substituting $OA=u$, $OC=AD=t$ and $BD=at$, we get
  $s=\left(u\times t\right)+\frac{1}{2}\times \left(t\times at\right)$
  or,
  $s=ut+\frac{1}{2}a{t}^2$
  which is the equation of position time relation

c. Equation for position velocity relation

• Again consider graph in figure 7. We know that distance travelled s by a body in time t is given by the area under line AB which is area of trapezium OABC. So we have
  
  Since $OA+CB=u+v$ and $OC=t$, we thus have
  $s=\frac{\left(u+v\right)t}{2}$
  From velocity time relation
  $t=\frac{v-u}{a}$
  putting this t in equation for s we get
  $s=\frac{\left(u+v\right)}{2}\left(\frac{v-u}{a}\right)$
  or we have
  $v^2=u^2+2as$
  which is equation for position velocity relation.

Worksheet 

Fill in the blanks 1) The motion of an object moving  at uniform acceleration can be described with the help of three equations, the equations are _____, ______ and __________
2) The change in the position of an object with time can be represented on the _______ graph.
3) The distance time graph for a body moving with non uniform __________________ is a curve
4) The motion of an object could be uniform or non-uniform depending on whether its velocity is constant or __________________.
5)The product of velocity and time give __________________ of an object moving with uniform velocity.
6) The area enclosed by velocity-time graph and the time axis will be equal to the magnitude of the ________
7) If a body moves with a constant velocity then velocity time gragh  for this body would be _____________ parallel to time axis
8) The acceleration of an object is the change in velocity per __________________time.
9) The value of _____________can be determined using  the velocity time graph.________________ of moving body gives its acceleration.

Answers


1) v=u+at
s=ut+ (1/2)at²
v²=u²+2as
2) Distance-Time
3) Speed
4) Changing
5) Displacement
6) Displacement
7) Straight line
8) Unit
9) acceleration,Slope of velocity time graph 

9. Uniform circular motion

• When an object moves in a circular path at a constant speed then motion of the object is called uniform circular motion.
• In our everyday life ,we came across many examples of circular motion for example cars going round the circular track and many more .Also earth and other planets revolve around the sun in a roughly circular orbits
• If the speed of motion is constant for a particle moving in a circular motion still the particles accelerates because of constantly changing direction of the velocity.
• If an object moves in a circular path with uniform speed, its motion is called uniform circular motion
• Here in circular motion, we use angular velocity in place of velocity we used while studying linear motion.
• Force which is needed to make body travel in a circular path is called centripetal force.
• We know that the circumference of a circle of radius $r$ is given by $2\pi r$. If the body takes $t$ seconds to go once around the circular path of radius $r$, the velocity $v$ is given by
  $v=\frac{2\pi r}{t}$
• One thing we must keep in mind is that uniform linear motion is not accelerated but uniform circular motion is accelerated motion.

• Examples of uniform circular motion are 
  (a) Motion of artificial satellites around the earth
  (b) Moon, the natural satellite of earth, moves in uniform circular motion round the earth.
  (c) Cyclist moving on a circular track with a constant speed exhibits uniform circular motion.