9th Class Physics: Chapter 6 Work and Energy Short Questions Answers

Work: Work is said to be done when a force acting on a body displaces it in the direction of the force.

Work = Force x distance

SI unit of work: SI unit of work is joule (J)

Definition of Joule: The amount of work is one joule when a force of one newton displaces a body through one meter in the direction of force.

When force acting on a body displaces it through some distance S in the direction of force as shown in the given figure, it does work on a body.

Sometimes the force and displacement do not have the same direction such as shown in the figure.

In this case, force F is making an angle (θ) with the surface on which body is moved. Resolving F into its perpendicular components F_x and F_y as;

F_x = Fcosθ

F_y = Fsinθ

In case when force and displacement are not parallel then only the x-component F_x parallel to the surface causes the body to move on the surface and the y-component F_y.

Hence W = F x S

W = (Fcosθ)S

W = FS cosθ

Energy used by us to perform many activities of life.

Energy is necessary for running and walking for humans.

A body possesses energy if it is capable to do work.

Types of mechanical energy: Mechanical energy has two following types.

(i) Kinetic energy (ii) Potential energy

Kinetic energy: The energy possessed by a body due to its motion is called its kinetic energy.

Formula: K.E = ½(mv²)

Unit of kinetic energy is joule (J).

Potential energy: The ability of a body to do work due to its position is known as its potential energy.

Formula: P.E = mgh.

Unit of potential energy is joule (J).

Kinetic Energy: The energy possessed by a body due to its motion is called its kinetic energy.

Formula of K.E can be proved as: Consider a body of mass m moving with velocity v. The body stops after covering some distance S due to some opposing force such as force of friction acting on it.

The body possesses kinetic energy and is capable to do work against opposing force F until all of its kinetic energy is used up.

As we know, Kinetic energy of the body = Work done by it due to motion

1. E = Force x displacement

K.E = F x S —————- (1)

In this case

V_i = v

V_f = 0

According to Newton’s second law of motion:

F= ma

a = –

In the given case, the motion is opposed, hence (a) is negative.

According to 3^rd equation of motion:

2aS =

2(-F/m)S = (0)² – (v)²

2(-FS/m) = -(v)²

2(-FS/m) = -v²

Both sides of equation have minus sign, so these signs are cancelled out. So,

2 = v²

FS =1/2mv² ————- (2)

From eq (1) and (2) following relation can be obtained.

K.E = K.E = ½(mv²) ——— (3)

Eq (3) gives the kinetic energy possessed by a body of mass m moving with velocity v.

Potential Energy: The ability of a body to do work due to its position is known as its potential energy.

P.E =mgh this relation can be proved as: Consider a body of mass m. Let it be raised through height h from the ground. The body will acquire potential energy equal to the work done in lifting it to height h.

Mass of the body = m

Force lifting the body = F= w = mg

Height attained = S = h

Work done on the body = W = FS

As we know, Fore = Weight, Distance = height

F = w, S=h

So

Work done on the body on the body = W = w x h ——– (1)

According to the formula of weight

Weight = mass x gravitational acceleration

w= mg

By putting the value of weight in eq (1)

Work done on the body = W= mgh

The work done W in raising the body to height h appears as its potential energy.

Potential energy of the body = mgh

P.E = mgh.

Thus, the potential energy possessed by the body with respect to the ground is mgh and is equal to the work done in lifting it through height h.

The fossil fuels take million of years for their formation. So these are known as non-renewable resources.

Solar energy is most preferred energy because Sunlight does not pollute the environment in any way.

Solar energy reaching Earth is thousand times more than the energy consumption of mankind.

Energy cannot be destroyed however it can be converted from one form to another form. But total energy of the system at any time remains constant.

For Example: By rubbing hands quickly, these will be warmed. Its mean in rubbing hands muscular energy is used and as a result heat is produced.

In this process of rubbing hands, mechanical energy is converted into heat energy.

The given figure shows interconversion of kinetic energy to potential energy and potential energy into kinetic energy.

At the surface of the earth, the potential energy of the body will be zero while at this point its kinetic energy is maximum.

At the highest point, the kinetic energy of the body will be zero while its potential energy is maximum at this point.

1. Juicer, 2. Electric motor, 3. Electrical grinder, 4. Electric spinner, 5. Washing machine

Generator is a device that converts mechanical energy into electrical energy.

Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.

Efficiency of a system can be found out by following formula.

Efficiency = required form of output/ total input energy

%Efficiency = (required form of output/total input energy) x 100

Power is defined as the rate of doing work.

Power = work done/time taken

P = W/t

The unit of power is watt. Bigger units of power are kilowatt (kW), megawatt (MW) etc.

The power of body is one watt if it does work at the rate of 1 joule per second (1Js^-1).

SI unit of work: SI unit of work is joule (J).

Joule: The amount of work is one joule when a force of one Newton displaces a body through one meter in the direction of force.

Thus, 1J = 1N x 1m

Data: Distance = S = 35m

Force = F = 300N

Required: Work done =?

Formula: W = F x S

Work = Force x distance

Sol: By putting the values in the formula amount of work can be found out.

W = F xS

W = 35 x 300

W = 10500 J

The required work done by the man is 10500J.

Energy: A body possesses energy if it is capable to do work.

Types of energy: Energy exists in various types which are given below:

1. Mechanical energy, 2.Heat energy, 3. Light energy, 4. Sound energy, 5, Electrical energy, 6.Chemical energy, 7.Nuclear energy

Kinetic Energy: The energy possessed by a body due to its motion is called its kinetic energy.

Equation: K.E = ½(mv²)

Sol. Mass of stone = m = 500g =0.5kg

Velocity = v= 15ms^-1

K.E =?

We know that

K.E = ½(mv²)

= ½(0.5 x (15)²)

K.E = ½(0.5 x 225) = 56.25J

Geothermal well can be built by drilling deep near hot rocks at places, where magma is not very deep.

Sol. Mass =m= 2kg

K.E. = K.E. = 25J

Speed =V=?

We know that

K.E = ½(mv²)

25 = ½(2)v2

V² = 25

V =5ms^-1

Sol. Weight = w = 12kN

Speed = v = 20 ms^-1

Required: Kinetic energy = K.E=?

Formula: K.E = mv²

Solution: Value of K.E can be found out by using above formula

Weight = 12kN

As we know;

w = mg

w/g =m

12k/10 = m

m = 1.2kg

K.E = mv²

K.E = (1.2)(20)²

K.E = (1.2)(400)

K.E = 480

K.E = 240J

The required K.E of the car is 240kJ.

Sol. Mass = m = 0.5kg

Velocity = v= 20ms^-1

K.E =?

We know that

K.E = mv²

K.E = (0.5)(20)²

K.E = (0.5)(400) = 100J

Thus, the K.E. of the stone is 100J as it strike the ground.

Potential energy: The ability of body to do work due to its position is known as its potential energy.

Formula: P.E = mgh

A body of 50kg is raised to a height of 3m. Find its potential energy.

Sol. m = 50kg

h = 3m

g = 10ms^-2

We know that

P.E. = mgh

P.E. = 50 x 10 x 3 = 1500J

Sound Energy: Sound is a form of energy.

Sound is produced when a body vibrates.

Following instruments produce sound.

1. Vibrating diaphragm of a drum.
2. Vibrating strings of a sitar.
3. Vibrating air column of wind instruments such as flute pipe etc.
4. When a door is knocked, a sound is produced.

Mechanical energy: The energy possessed by a body both due to its motion or position is called mechanical energy.

Examples: Water running down a stream, wind, a moving car, a lifted hammer, a stretched bow, a catapult or a compressed spring etc. possesses mechanical energy.

Work: Work is said to be done when a force acting on a body displaces it in the direction of the force.

Work = Force x Distance

Major sources of energy: There are basically two major sources of energy.

1. Fossil Fuels 2. Nuclear Fuels

Chemical energy is present in food, fuels and in other substances. The energy is released from these substances during chemical reactions.

Energy cannot be destroyed however it can be converted from one form to another form. But total energy of the system at any time remains constant.

By rubbing hands quickly, these will be warmed. Its mean in rubbing hands muscular energy is used and as a result heat is produced.

In this process of rubbing hands, mechanical energy is converted into heat energy.

Mechanical energy: The energy possessed by a body both due to its motion or position is called mechanical energy.

Examples: Water running down a stream, wind, a moving car, a lifted hammer, a stretched bow, a catapult or a compressed spring etc. possesses mechanical energy.

Chemical Energy: Chemical energy is present in food, fuels and in other substances. The energy is released from these substances during chemical reactions.

Biomas: Biomass is plant or animal wastes that can be burnt as fuel.

When animal dung, dead plants and dead animals decompose, they give off a mixture of methane and carbon dioxide which is used to generate electricity.

Mechanical energy has two following types.

• Kinetic Energy
• Potential Energy

Electrical energy:

Electricity is one of the widely used form of energy.

Electrical energy can get form batteries and electric generators. Most of the electric generators are run by hydro power, thermal or nuclear power.

Light energy:

Light is an important form of energy.

Sun is the great source of light energy.

Light energy can be obtained from candles, electric bubs, fluorescent tubes and also by burning fuel.

Environmental problems such as pollution that consist of noise, air pollution and water pollution may arise by using different sources of energy such as fossil fuel and nuclear energy.

Uses of solar cells:

Solar cells provide power to telephone booths, light houses and scientific research centers.

Solar cells are also used to power satellites.

Sound Energy: Sound is a form of energy. Sound is produced when a body vibrates.

Electrical energy: Electricity is one of the widly used form of energy. Electrical energy can be supplied through wires, easily to any derired place.

Sound energy: Sound is a form of energy. Sound is produced when a body vibrates.

The process taking place in nuclear reactors is known as nuclear fission. During fission reaction, the heavy nucleus of an atom splits up into smaller parts releasing a large amount of energy. Nuclear power plants give out a lot of nuclear radiations and vast amount of heat.

Non-renewable Resources: 1. Fossil fuels 2. Nuclear fuels

Renewable Resources: 1. Solar energy 2. Water power

Solar energy is most preferred energy because Sunlight does not pollute the environment in any way.

Non-renewable source: The fossile fuels took millions of years for their formation, so, they are known as non-renewable resources.

For example: Oil, gas, coal.

A heating system consists of

1. A collecter 2. A storage device 3. A distribution system

1. Energy from water
2. Energy from sun

Energy from water: Energy from water power is very cheap. Water power is becoming more popular. For water power dams are being constructed at suitable location in different parts of the world.

The relation between mass and energy is given by Einstein mass energy equation:

E=mc²

Here c is the speed of light.

Its value is 3 x 10⁸ ms^-1

Every citizen can be helpful in controlling air pollution by sharing rides and using public transport, due to this less fuel and energy is used and less pollutant are released in the environment.

Uses of Wind energy:

1. Wind energy has powered sailing ships across the oceans.
2. Wind energy has been used by windmills to grind grain and pump water.
3. More recently, wind power is used to turn wind turbines.

• Thermal pollution upsets the balance of life.
• Thermal pollution endangers the survival of many species.

Nuclear energy: Nuclear energy is the energy released in the form of nuclear radiations in addition to heat and light during nuclear fission and fusion reactions.

Solar Cells: Solar energy can also be converted directly into electricity by solar cells.

By defining magma, describe geothermal energy.

There is hot molten part, deep in the Earth called magma. Water reaching close to the magma changes to steam due to the high temperature of magma. This energy is called geothermal energy.

When animal dung, dead plants and dead animals decompose, they give off a mixture of methane and carbon dioxide. Electricity can be generated by burning methane.

Mechanical Energy: The energy possessed by a body both due to its motion or position is called mechanical energy.

Examples: Water running down a stream, wind, a moving car, a lifted hammer, a stretched bow, a catapult or a compressed spring etc. possesses mechanical energy.

The fossil fuels take millions of years for their formation. So these are known as non-renewable resources.

Energy cannot be destroyed however it can be converted from one form to another form. But total energy of the system at any time remains constant.
By rubbing hands quickly, these will be warmed. Its mean in rubbing hands muscular energy is used and as a result heat is produced.
In this process of rubbing hands, mechanical energy is converted into heat energy.

• Juicer
• Electric motor
• Electrical grinder
• Electric spinner

Once the supply of fossil fuels is exhausted, the world would face serious energy crisis.

Fossil fuels would not be able to meet our future energy demands.

Exhausted supply of fossil fuels would cause serious social and economical problems for countries like us.

Efficiency of a system: Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.

Formula of efficiency: Efficiency can be obtained by following formula.

Efficiency = required form of output/Total input energy

Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.

Power is defined as the rate of doing work.

Power = work done/time taken

P = W/t

Watt: The SI unit of power is watt.

The power of body is one watt if it does work at the rate of 1 joule per second (1Js^-1).

Watt: The power of body is one watt if it does work at the rate of 1 joule per second (1Js^-1).

1 horse power = 1 hp =746 watt

Work = w = 11200 J

Time = t = 10 sec

Power = p = ?

We know that

P = w/t = 11200/10

P = 1120 watt

Work = W = 20 J

Time = t = 4 sec

Power = P =?

We know that

P = W/t

P = 20/4 = 5 watt