Chapter-4Work and Energy

NOTES FOR THE

CHAPTER

What is Work ?In Ordinary Language the work is define

as Work is done when a force produce motion.

for example-when an engine moves a train along a railway line it is said to be doing .

What is Work ?In language of Physics it is define as Work

done in moving a body is equal to the product of force exerted on the body and the distance moved by the body in the direction of force .

Work=Force*distance moved in the direction of force Work done= Force*distance W= F*S

Unit of WorkThe unit of work is Newton Meter which is

written as NMWhen a force of 1 newton moves a body through

a distance of 1 meter in its own direction ,then the work done is known as 1 Joule. that is ,

1Joule = 1 newton* 1 meter or 1 J = 1NM The SI unit of work is Joule . The work is scalar quantity .The condition for a force to do work is that it

should produce motion in an object .

A person pushes with a 110 N force for a distance of 30 m. How much work was done?

Examples:

W = F d

W = ( 110 N ) ( 30 m )

W = 3300 N m

or

W = 3300 J 1 Joule (J) = 1 N m

Work done against GravityWhenever work is done against gravity ,the amount

of work done is equal to the weight of the body and the vertical distance through which the body lifted .

suppose a body of mass M is lifted vertically upward through a distance h in this case ,the force required to lift the body will be equal to the weight of the body ,m*g ,(where m is the mass and g is acceleration due to gravity ). Now,

Work done in lifting a body =weight of body *vertical

distance

What is M,G,H? w=work donem=mass of the body g=acceleration due to gravity h=height through which the body is lifted

Work done by a force acting obliquelyIn many cases ,the movement of the body is

at angle to the direction of the applied force

Positive ,Negative and Zero work Work done is positive when a force acts in

the direction of motion body .Work is negative when a force acts opposite

to the direction of motion body . work done is zero when a force acts at right

angle to the direction of motion of the body .

EnergyEnergy is something that an object possesses.The amount of energy that an object

contains, is a measure of how much work it is capable of doing.

Energy can be thought of as stored work (and it has the same units as work).

S I unit of energy . The S I unit of work is Joule whenever work is done ,an equal

amount of energy is consumed .keeping this thing in mind , we can define 1joule energy as follow : the energy required to do 1 joule of work is called1 joule energy .

Joule is small unit of energy ,sometime a bigger unit of energy called ‘Kilojoule’ the symbol of kilojoule is KJ .

1 Kilojoule = 1000 Joule 1KJ = 1000J the unit of energy called ‘joule’ is named after a British

Physicist JAMES PRESCOTT JOULE .

James JouleBritish physicist James Joule is best known for

his work in electricity and thermodynamics Together with the physicist William Thomson (later Baron Kelvin), Joule found that the temperature of a gas falls when it expands without doing any work. This principle, which became known as the Joule-Thomson effect, underlies the operation of common refrigeration and air conditioning systems.

The metric system unit of energy is the joule (J), after James Joule.

Different forms of energy 1. Radiant energy

2. Chemical energy

3. Electrical energy

4. Nuclear energy

5. Thermal energy

6. Sound energy

7. Mechanical energy

8. Kinetic energy

9. Potential energy

Radiant energy is also called electromagnetic energy. Radiant energy is the movement of photons. All life on earth is dependent on radiant energy from the sun. Examples of radiant energy include radio waves (AM, FM, TV), microwaves, X-rays, and plant growth. Active solar energy uses photovoltaic panels and light to turn radiant energy into chemical energy.

Chemical energy is the energy stored in the bonds of atoms and molecules. This a form of potential energy until the bonds are broken. Fossil fuels and biomass store chemical energy. Products that contain chemical energy include: TNT, baking soda, and a match. Biomass, petroleum, natural gas, propane and coal are examples of stored chemical energy.

Forms of Energy

Electrical energy is the movement of elections. Lightning and static electricity are examples of electrical energy that occur naturally. Science hasn't found a way to use natural forms of electrical energy, like lightning. Instead, we use different energy sources to create electrical energy by using generators and turbines.

Forms of Energy

Thermal energy is the internal energy in substances-the vibration and movement of atoms and molecules within substance. Thermal energy is created in the movement of atoms. Boiling water, burning wood, and rubbing your hands together really fast are all examples of heat energy. Geothermal and passive solar are sources of heat energy, but biomass (a type of chemical energy) can be burned to produce heat energy.

Forms of Energy

Sound energy is the movement molecules in the air that produces vibrations. Alarms, music, speech, ultrasound medical equipment all use sound energy. VCR tapes change sound energy into electrical energy. The electrical energy records the sound using magnetic tape. Speakers read the magnetic tape and change it back into sound.

Forms of Energy

Mechanical energy is the movement of machine parts. Mechanical energy is also the total amount of kinetic and potential energy in a system. Wind-up toys, grandfather clocks, and pogo sticks are examples of mechanical energy. Wind power uses mechanical energy to help create electricity.

Potential energy + Kinetic energy =

Mechanical energy

Next

Potential energy + Kinetic energy = Mechanical energy

Forms of Energy

Example of energy changes in a swing or pendulum.

Magnetic energy is the attraction of objects made of iron. Medical equipment, compass, refrigerator magnets are all examples of magnetic energy. Any type of energy source that uses a generator in the process to make electricity uses magnetic energy.

Forms of Energy

Kinetic energy exists whenever an object which has mass is in motion with some velocity. Everything you see moving about has kinetic energy.  The kinetic energy of an object in this case is given by the relation:

KE = (1/2)mv2

m=mass of the object

V=velocity of the object

The greater the mass or velocity of a moving object, the more kinetic energy it has.

Kinetic Energy Lab

Next

The greater the mass or velocity of a moving object, the more kinetic energy it has.

Kinetic Energy Lab

Potential energy exists whenever an object which has mass has a position within a force field. The most everyday example of this is the position of objects in the earth's gravitational field. The potential energy of an object in this case is given by the relation:

PE = mgh

 PE = Energy (in Joules)

m = mass (in kilograms)

g = gravitational acceleration of the earth (9.8 m/sec2)

h = height above earth's surface (in meters)

Potential Energy Lab

Law of Conservation of Energy- Energy can neither be created nor destroyed. Energy is always changing from one kind to another. The total energy of an object never changes.

Potential energy + Kinetic energy = Total energy and Total energy – Kinetic energy = Potential energy and Total energy - Potential energy = Kinetic energy

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