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If a candle is burnt the chemical energy is transformed into light and heat energy.

2. mechanical energy changes into heat energy

A wound up watch spring has the potential energy because of its wound up state. As the spring unwinds itself, the potential energy changes into the kinetic energy. This kinetic energy does work in moving the arms of the watch

(i) Kinetic energy. 

(ii) Potential energy. 

(iii) Potential energy. 

(iv) Kinetic energy. 

(v) Kinetic energy. 

(vi) Potential energy. 

(vii) Potential energy

Yes, a body not in motion can possess energy.

Example: Water stored in the dam through not in motion possess potential energy.

Or

A stone at rest on the top of a building possesses P.E.

A stretched rubber band, Mangoes in the tree, water stored in the dam, stretched bow pressed spring stretched sling of a catapult.

It possesses Potential energy.

The energy possessed by a body by virtue of its motion is defined as kinetic energy.

The factors on which it depends are

(i) mass of the body.

(ii) velocity of the body

The kinetic energy of a moving body depends on the following two factors : 

(a) The mass of the body — Greater the

(b) The speed of the body — More the speed of the body, higher is its kinetic energy.

When a cyclist doubles his speed. His kinetic energy increases four times. 

K.E. = 1 / 2 Mv² 

When v = doubles the K.E. quadriples

The toy car ‘A’ of mass ‘500 gm’ has the greater kinetic energy. This can be explained as : 

K.E. = 1 / 2 Mv² 

Here, both the cars are moving with same speed. So the car with greater mass will possess greater kinetic energy.

Unit of energy is joule.

Unit of power is watt.

Kinetic energy of a ball of mass m and moving with speed v is K.E. = 1/2 Mv²

The water stored in an overhead tank possesses potential energy energy.

Kinetic energy =1/2 Mv²

Where m is the mass of body v is the speed of body.

EXPRESSION FOR GRAVITATIONAL POTENTIAL ENERGY: P.E. = U = mgh

Where U is gravitational potential energy m is the mass of body.

g: is force of gravity on mass of 1 kg

mg : is the force acting on body

h : is the distance or height moved above the ground level.

(a) When a body of mass m at A on ground is raised above ground through height h at B force is applied. This force applied = weight of body

Force on mass m = F = mg

Where ‘g’ is the force of gravity on a mass of 1 kg.

(b) Work done in lifting the body = Force × displacement h W = mg × h

(c) This work done W is stored in body in the form of Potential energy P.E. mgh

m = 60 kg

Speed v = 50 m s^-1

K.E = 1/2 MV²

1/2 × 60 × 50 × 50 = 75000

K.E = 75/10 × 1000 × 10 = 7.5 × 10⁴ J

Mass m = 2.5 kg

Gravitational potential energy is the work done against force of gravity is stored in the body at a height h.

P.E = U = mgh

U = 2.5 × 10 × 15

U = 25/10 × 10 × 15 = 375 j

Potential energy = (m g) h

100 J = (0.5 × 10) N × h

h = (100/ 0.5) x 10 = 100/(5/10) x 10 = 20 m

(a) Potential energy. 

(b) Potential energy + Kinetic energy. 

(c) Kinetic energy.

During the vertical fall of ball, if friction due to air is neglected, the total sum of potential energy and kinetic energy at each point of its path remains same.

Mass = 20 kg h = 2.5 m

Force of gravity on a mass of 1 kg = 10 N

Force of gravityona mass of 20kg

F = mg = 20 × 10 = 200 N

Work done in lifting the mass to height h = 20 m is

W = F × h

= 200 N × 2.5 m

= 200 × 25/10 =500 J

A boy lifts a luggage from height 2 m to 4 m. The potential energy will become double.

F = 30 N

d= 5m

Work done = w = F × d

W = 30 × 5= 150 J

3. from electrical energy to heat and light energy

A wound up watch spring has P.E. stored ¡n ¡t due to it wound up state.

A spring UNWINDS itself, the potential energy changes into KINETIC ENERGY with which it moves the arms of the watch.

WORK “When a force is applied to a body and there is displacement of the body, work is said to be done.”

(a) An electric bulb — Electrical to light energy

(b) An electric oven — Electrical to heat energy

(c) A loud speaker — Electrical to sound energy

(d) A microphone — Electrical to sound energy

(e) An electric motor — Mechanical to’aHcaI energy

Work is said to be done only when the force applied on a body i. makes the body more (/. e. there is a displacement of the body).

(a) As wall does not move from its place distance moved is zero. Hence, no work is done.

(b) On lifting the Hammer, its potential energy is stored in the hammer on striking the nail with a hammer this energy is used in driving the nail into the wood.

(c) A horse has more mass than dog. As both are running at the same speed.

M₁ of horse is greater than M₂ of dog

∴ K.E. of horse is more than K.E. of dog

(d) A child reading a book while standing is not moving from its place i.e. displacement is zero. Hence product of force and displacement is zero

W = F × S

W = mg × 0 = 0

Hence, a child is not doing any work. Whereas the teacher is moving from its place is doing work.

Two conditions are :

(i) There should be no displacement i.e. S = 0

(ii) The displacement is NORMAL to the direction of FORCE i.e. – θ = 90°

(b) A boy climbing up the stairs

(d) A girl moving on the road.

No work is done since displacement is NORMAL to the direction | of force on the body.

The force is CENTRIPETAL.

A coolie carrying a luggage on his head moving on the ground does not work against the force of gravity as displacement is normal to the direction of the force of gravity.