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WORK AND ENERGY Class IX

WORK : When the force is applied to the body , it displaces in the direction of force applied than work is said to be done. The main condition of work done are  a) application of force  b) displacement in the body

Work done = Force x displacement  made   It is a scalar quantity  SI unit of work is Newtom – meter or Joule

If greater the force  more will be the work done or if more is the displacement more shall be the work done

One Joule is defined as when one newton of force is applied to the body such that  it  displaces by one meter in the direction of the application of force.

When work is done against the gravity  than the amount of work  done is equal to the product of weight of body and the vertical distance  through which the is lifted

weight of body = mass x acceleration due to gravity =Mg

vertical height to which body is lifted =h

WORK DONE AGANIST GRAVITY = Mgh  unit Joule or Newton meter

Work is done when a cyclist is moving/ peddling the cycle or  man is lifting a load up ward/ down wards but no work is done when a coolie carrying the load on his head stands stationary or man applied the force on big heavy rock .

Work Done is said to be  positive  if the displacement and the force is made in same direction example a child pulling the toy   Work Done is said to be negative if the displacement of body is in opposite direction  to the force of application example a boy throw a ball on the wall the ball bounces back or a boy kicks the football and it stops after moving some distance due frication Work Done is said to be zero when force acts at the right angle to the direction of motion of the body example circular motion the moon moves around the earth , the revolves around the sun.

Example A person lift the luggage of 16kg  from the ground  to the height of 5meter than work against gravity is Mgh = 16 x10x5 = 800 joule

ENERGY : As we know the sun is the main source of energy and most of the energy is derived from the sun , some from the nucleus of atoms as well as interior of the earth but  ENERGY IS DEFINED AS THE CAPACITY OF DOING WORK. The amount of energy present in body  is equal  to  the amount of work done by the body  there are case that in working it may loses /gain the energy .It is scalar quantity  SI unit is Joule

Form of Energy :  Kinetic , Potential, Heat OR sound etc

The Energy possessed  by the body due to motion or position of the body is called mechanical energy. Mechanical Energy = Kinetic Energy + Potential Energy

KINETIC ENERGY :  The energy possessed by the body due to motion of the body  example as moving of cricket ball , running water , moving bullet etc.

It is directly proportional to the mass of the body and square of the velocity .

Formula of KE  : Kinetic energy of the moving body is equal to the work  it can do before coming to rest. If an object of mass m is moving body with the uniform velocity u  it displaces by distance s with constant force F acts on it in direction of displacement velocity changes to v having an acceleration a

WORK DONE = Force x displacement   = F x s  = ma x s     (f= m.a)

V^2 -U^2 = 2 a. s     ==== .>      s =( V^2 -U^2) -;- 2a

W = m.a.s =.> W= ma. (v^2- u^2) /2a ===>  W= m.( v^2-u^2)/2

W = 1/2 .m (v^2- u^2)  if u=0  ====>  W= 1/2 m. v^2

K = 1/2 m. v^2

example : If the body with the mass of 15kg moving with the velocity of 4 m/s than

KE = 1/2 x 15×4^2 = 1/2x15x16= 15 x8 = 120 joule

If the mass of body become double/ half the KE  also become double / half . If the velocity becomes double than KE  becomes  square time ie 4 times or if the velocity  is made half than KE becomes 1/4 times

POTENTIAL ENERGY : The energy possessed by the body due to position of the body/ change in the shape of the body Example a) water store in dam b) wound of sting toy c) Bent string of bow.

Factors  : Mass of the body , greater the mass more is the potential energy  if height remain unchanged, It also depend s upon the height from the surface of earth if mass is constant .Change in shape greater the stretching/twisting/bending greater the potential energy.

Formula of Potential Energy : If the mass of the  object be m it is raised to the height H from the surface of earth acceleration due to gravity tending downwards is g  than the work done in taking the body to height  H is  equal to force x displacement made

PE = Weight x height = mg x H= mgH unit joule .

It is to be noted that if weight of body is constant than PE remain same if height is same.

Example : a) A stone on  a certain height has entire potential  energy. But when it starts moving downward.  PE of stone goes on decreasing as the height  goes on decreasing but KE increasing as the velocity increasing as it reaches on the ground the KE is maximum and PE is zero as total PE changes to KE b) In hydroelectric power  the water stored in dam  is transformed into KE  to produce into electricity . c) In thermal power  the chemical energy of coal is changed  into heat in energy which further into KE and  Electrical  energy.

LAW OF CONSERVATION OF ENERGY : Energy can neither be created  nor can be destroyed total energy of the system remain conserved .

CONSEVATION OF ENERGY DURING FREE FALL OF BODY : A ball  of mass m  at the height h has PE is mgh. As a ball falls downwards , the height h decreases  so PE changes to KE . In this case PE decreases continuously whereas KE increases .Total energy  remain conserved.

Total Energy = Potential Energy + Kinetic Energy  ==> TE = PE+KE

1/2 mv^2+ mgh = Constant

conservation of energy 1

The conservation of energy ins the pendulum shows the example the  a simple pendulum A swing shows the conservation of energy > A swinging of simple pendulum which consist  of pith ball / bob which is free to swing back and forth when displaced In this  energy is continuously transformed  from PE to KE and vice versa . The total energy remain conserved.conservation 2

 

POWER :  The rate of energy consumption is called power . Power = work done/ time taken

SI unit is Watt = Joule/sec  when one joule of work done in one sec than the power consumed is one watt

1 kilo watt =1000 watt

1 Horse power = 746 watt

Example : 20 joule of work done in 5 sec than power is =20/5 = 4 watt

I kilowatt hour = 3.6 x 10^6 joule = one unit of electrical consumption.

Example : A bulb of 60 watt is used for 6 hrs /day for 30 day than electricity  consumed

power x hours /1000 = unit

60 x30 x 6 /1000 = 10800/1000= 10.8 unit

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