Different types of energy | ICSE Class 10 Physics Notes

This note covers forms of energy, mechanical energy, translational, rotational and vibrational motion, kinetic energy, gravitational potential energy, their derivations and calculations, and conversions between chemical, electrical, heat, nuclear, light and sound energy.

What is energy, and how is it related to work?

Definition: Energy is the capacity to do work. Work is done by a force when it produces displacement in its direction. A force is a push or pull; displacement describes a change in position with direction.

A moving cricket ball can knock over a wicket. A raised flowerpot can damage an object below if it falls. Both have a capacity to do work, although the ball demonstrates energy associated with motion and the raised pot demonstrates stored energy.

Energy transfer means energy passing from one object or system to another. A system is the object or group of objects being considered. Work is one way of transferring energy; heating, electric circuits and sound waves provide other ways.

How is energy measured?

The International System of Units, abbreviated SI, provides standard units for physical quantities. The SI unit of energy is the joule, symbol J. The SI unit of work is also the joule. The SI unit of force is the newton, symbol N.

The SI unit of displacement is the metre, symbol m. The SI unit of mass is the kilogram, symbol kg. A second, symbol s, is the SI unit of time. A unit symbol and a quantity symbol must be distinguished by their context.

1 J = 1 N × 1 m. One joule of work is done when a constant force of one newton displaces an object by one metre in the force's direction. Energy and work share a unit because work transfers energy.

For a constant force acting along the displacement, W = Fs. Here W denotes work, F denotes the force magnitude, and s denotes displacement along the force. In this formula, s is a quantity symbol, not the abbreviation for the second.

Having energy and transferring energy are related but different ideas. A raised object can possess stored energy while stationary. Its ability to do work becomes apparent when it is released and can move another object or deform the surface it strikes.

How do chemical and nuclear energy differ from mechanical energy?

Mechanical energy is energy associated with the motion or position of objects. Its two principal forms here are kinetic energy, energy due to motion, and potential energy, energy stored through deformation or the relative positions of interacting objects.

Chemical energy is energy stored in substances such as food and fuels through chemical bonds. Atoms are the constituent units of elements; chemical bonds are the interactions holding atoms together in substances. Chemical changes can transfer this energy into other forms.

Food provides energy for the muscles to perform mechanical work. Fuel supplies energy for a moving car. These examples connect a stored form of energy with motion without treating chemical energy and mechanical energy as identical.

Where is nuclear energy associated?

Nuclear energy is energy associated with the nuclei of atoms. A nucleus is the central part of an atom. Nuclear reactions involve changes in atomic nuclei and can release energy. Nuclear reactions power the Sun.

The distinction concerns where the energy is stored and what changes. Chemical energy concerns the bonds between atoms; nuclear energy concerns their nuclei. For this comparison, the names of the forms and their meanings are more useful than calculations of nuclear reactions.

An energy description should identify both the starting form and the resulting form. Saying that food supplies energy is correct but incomplete when explaining movement. The fuller description is that chemical energy supplied by food is converted into mechanical energy through muscular activity.

Energy conversion means a change from one form of energy to another. The existence of several names for energy does not mean that a separate unit is needed for every form. Each is an energy quantity and can be measured in joules.

What are electrical, heat, light and sound energy?

Electrical energy is associated with the positions or motion of electric charges. Electric charge is the property of matter responsible for electric interactions. An electric circuit provides a way to transfer energy to devices such as a bulb, heater or fan.

Thermal energy is associated with the microscopic motion of matter's particles. In simple descriptions it is the energy associated with things being warm or hot. Heat is energy transferred because of a temperature difference; temperature describes how hot or cold something is.

When objects at different temperatures are in contact, energy flows from the hotter object to the colder one. An electric water heater converts electrical energy into thermal energy of the water. This is a conversion and transfer of energy, not the creation of energy.

How do light and sound carry energy?

Light energy is energy carried by light, which enables us to see. The Sun's energy reaches the Earth by radiation, energy transfer that does not require direct contact between the Sun and Earth. An electric bulb converts electrical energy into light energy.

Sound energy is energy carried through vibrations of particles in a medium. A medium is the material through which sound travels. A vibration is repeated to-and-fro motion. A ringing bell provides an example of mechanical energy being converted into sound energy.

The illustration pairs mechanical energy with gears, thermal energy with a thermometer, light energy with a bulb, sound energy with a loudspeaker, electrical energy with a lightning symbol, nuclear energy with a marked container, and chemical energy with laboratory glassware.

See Fig. 7.10 in your NCERT textbook

These pictures represent categories of energy. The energy definition is the important part: electrical energy involves charges, chemical energy involves bonds, and nuclear energy involves nuclei. A picture used to represent a form is not itself a complete account of how that energy is transferred.

A device can produce more than one output form. Therefore, naming a useful conversion should not imply that all input energy becomes that single useful output. Identifying thermal or sound transfers can be necessary when describing what happens to mechanical energy in a real process.

What are the three forms of kinetic energy?

Kinetic energy is present whenever an object moves. The motion may involve a whole object changing position, an object turning, or a part moving repeatedly to and fro. These patterns distinguish translational, rotational and vibrational kinetic energy.

How can the motion be recognised?

The fan demonstrates why motion should not be identified solely by asking whether the entire appliance travels across the room. Its blades move around the axis even though the fan is fixed in place. Those moving blades possess rotational kinetic energy.

A ringing bell demonstrates a different pattern. The vibrating material moves to and fro. Sound can transfer energy away from the bell, so the vibration and the sound are connected parts of the explanation. Vibrational motion is not a requirement for the whole bell to travel elsewhere.

Translation and rotation can occur together. A rolling ball changes position and turns as it rolls. Recognising both motions avoids assuming that every moving object has only one kind of kinetic energy.

Note: The expression developed below calculates translational kinetic energy. Do not use it as a complete formula for the energy of rotation or vibration. Describe those forms through their motion and simple examples.

For numerical work, identify the motion being calculated before choosing the expression. A problem about the translational motion of a ball requires its mass and speed. A qualitative question about fan blades instead requires the correct name and explanation of rotational motion.

How is the expression for translational kinetic energy derived?

Let m denote the object's mass and v its final speed. Speed is the magnitude of velocity, which specifies how fast and in what direction position changes. Let u denote its initial velocity along a straight line.

Let a denote acceleration, the rate of change of velocity. Its SI unit is metre per second squared, written m/s². Velocity and speed are measured in metres per second, written m/s. Let K denote translational kinetic energy.

Derivation: Kinetic energy from work

Consider a body of constant mass moving along a straight line under a constant resultant force F. A resultant force is the combined effect of all forces on the body. Its acceleration remains constant, and its displacement in the force's direction is s.

1. Newton's second law relates resultant force to acceleration: F = ma.
2. For constant acceleration, the equation of motion is v² = u² + 2as, giving s = (v² − u²)/(2a) for non-zero acceleration.
3. The work done by the resultant force is W = mas. Substitution gives W = ½m(v² − u²).
4. The work-energy relation identifies this work with the change in kinetic energy. If the object starts from rest, u = 0 and its initial kinetic energy is zero.

K = ½mv². The kinetic energy acquired equals the work done to bring the body from rest to speed v under these conditions.

What does the expression tell us?

For the same speed, kinetic energy is directly proportional to mass. For the same mass, it is directly proportional to the square of speed. Therefore, doubling the speed of a vehicle makes its translational kinetic energy four times its original value.

Kinetic energy has no direction. The speed is squared, so changing the direction of motion without changing the speed does not change the kinetic energy. Energy must not be assigned a negative sign merely because motion is in a chosen negative direction.

When resultant work increases an object's speed, its kinetic energy increases. Braking can decrease its kinetic energy. In calculations involving a change, use the final kinetic energy minus the initial kinetic energy; distinguish this change from either energy value considered separately.

How are translational kinetic-energy problems solved?

Begin by identifying the mass and speed, and put them into compatible SI units. Square the speed, multiply by the mass, and then multiply by one half. The final energy unit is joule; a numerical answer without its unit is incomplete.

What if a speed conversion is needed?

Worked example 1. A cricket ball has an approximate mass of 0.2 kg and a delivery speed of about 154.8 km/h (kilometres per hour), equivalent to 43 m/s. Calculate its translational kinetic energy.

Given: m = 0.2 kg; the supplied speed conversion gives v = 43 m/s. Formula: v = 154.8 × 5/18 m/s; K = ½mv². Here km/h means kilometres per hour; the factor 5/18 converts its numerical value to metres per second.

Substitute: K = ½ × 0.2 × 43². Answer: K = 184.9 J, approximately, because the original mass and speed are approximate. Squaring the speed before multiplication preserves the intended order of calculation.

How can stopping work reveal the initial speed?

Worked example 2. A jet aircraft of mass 15000 kg is stopped over 100 m by an approximately constant backward force of 367500 N. Treat this as the resultant horizontal stopping force. Find its speed just before stopping.

Given: m = 15000 kg; F = 367500 N; s = 100 m; final speed = 0 m/s. Formula: K = Fs for the magnitude of kinetic energy removed; v = √(2K/m). The symbol √ means the positive square root.

Substitute: K = 367500 × 100 = 36750000 J; v = √(2 × 36750000/15000) = √4900. Answer: v = 70 m/s. The work done by the backward force is negative, while the initial kinetic energy is positive.

How is speed found directly from kinetic energy?

Worked example 3. A block of mass 10.0 kg has a translational kinetic energy of 180 J. Find its speed.

Given: m = 10.0 kg; K = 180 J. Formula: v = √(2K/m). Substitute: v = √(2 × 180/10.0) = √36. Answer: v = 6 m/s. This gives the speed magnitude; the energy value alone does not identify the direction of motion.

These examples use the same energy relation in different ways. The first calculates energy from a speed, the second finds energy from stopping work before calculating speed, and the third begins with an energy value. Identify what is given before rearranging the expression.

How can an object store potential energy?

An object can store energy even while it is stationary. Potential energy can arise from deformation or from the relative positions of interacting objects. Deformation means a change of shape, such as stretching or compressing a spring.

A stretched rubber band or bent bow can do work when released. The energy used to deform it is stored while it remains deformed. On release, it can transfer energy to an object in contact and set that object in motion.

What does the spring example show?

Three drawings show a spring fixed at its left end beside a red ball: the spring in its original shape, the spring compressed, and the spring returning towards its original shape while the ball moves right, indicated by an arrow.

See Fig. 7.14 in your NCERT textbook

The spring example distinguishes the source of the stored energy from its later effect. An applied force first does work to deform the spring. The spring then has the capacity to do work as it returns towards its original shape.

What is gravitational potential energy?

Gravitational potential energy is stored energy associated with the relative positions of objects that attract each other gravitationally. Gravity is their mutual attractive interaction. Raising a ball changes the positions of the ball and Earth relative to each other.

The Earth and ball together form the relevant system. Because Earth is much more massive, it hardly moves towards the ball. The system's stored energy is often simply referred to as the gravitational potential energy of the ball.

A stationary raised ball has no translational kinetic energy at that instant, but it can still have gravitational potential energy. Conversely, a moving object at the chosen zero-height level can have kinetic energy even though its gravitational potential energy is assigned zero.

Gravitational and elastic examples should not be confused. A spring's stored energy arises from its deformation. A raised ball's gravitational potential energy arises from its position relative to Earth. The height formula for the ball is not a formula for a compressed spring.

How is gravitational potential energy derived?

Let U denote gravitational potential energy, h the vertical height above a chosen reference level, and g the acceleration due to gravity. A reference level is the position at which gravitational potential energy is chosen to be zero.

The body's mass is m. Its weight, the gravitational force acting on it, has magnitude mg. Mass and weight are different: mass is measured in kilograms, while weight is a force measured in newtons.

Derivation: Energy gained on raising an object

Consider raising an object gradually through a vertical height h near Earth's surface, where g can be taken as constant. Choose the starting level as zero potential energy. The lifting process does not produce a change in kinetic energy.

1. Balance the weight with an upward lifting force: F = mg. The balanced forces allow the object to be raised without acceleration during the lift.
2. The displacement along the upward force is h. Work done by this force is W = Fh.
3. Substitute F = mg to obtain W = mgh.
4. The work supplied in raising the object is stored as gravitational potential energy relative to the starting level.

U = mgh. Use m in kilograms, g in metres per second squared and h in metres to obtain U in joules.

A block is shown at ground level and again above it. An upward force arrow is labelled F = mg, and the vertical separation between the two positions is labelled h.

See Fig. 7.18 in your NCERT textbook

Which conditions matter?

The expression applies near Earth's surface, where changes in g can be neglected. Further away, gravitational acceleration decreases, so the same constant-g expression cannot be extended without qualification.

The height is vertical height above the reference level, not the length of a sloping path. At fixed mass and g, a greater height means greater gravitational potential energy. At fixed height and g, a greater mass means greater gravitational potential energy.

For the same starting and finishing heights near Earth's surface, the gain in gravitational potential energy is independent of the path. The energy change is determined by the vertical separation, whether the object is lifted directly upwards or reaches the same level by another route.

How are gravitational potential-energy problems solved?

First state the reference level. Then identify the body's mass, the vertical height above that level, and the supplied value of acceleration due to gravity. Do not substitute the length of a staircase or sloping path for the vertical height.

How should a small mass be handled?

Worked example 4. A cricket ball of mass 200 g (grams), equivalent to 0.2 kg, is thrown to about 10 m above the ground. Calculate its gravitational potential energy there, taking the ground as zero and g = 10 m/s².

Given: m = 0.2 kg; h = 10 m; g = 10 m/s². Here g following a number in 200 g means grams, whereas g in the formula denotes gravitational acceleration. Formula: U = mgh.

Substitute: U = 0.2 × 10 × 10. Answer: U = 20 J, approximately because the height is about 10 m. Converting the mass to kilograms prevents a mismatch between the mass unit and the other SI units.

Does the route change the potential-energy gain?

Worked example 5. A student of mass 50 kg is slowly lifted in an elevator through a vertical height of 72.5 m. Taking g = 10 m/s², calculate the gain in gravitational potential energy relative to the starting level.

Given: m = 50 kg; h = 72.5 m; g = 10 m/s². Formula: U = mgh. Substitute: U = 50 × 10 × 72.5. Answer: the gain is 36250 J. The relevant height is the vertical separation of the starting and finishing levels.

Worked example 6. A student of mass 50 kg climbs stairs to a floor 72.5 m vertically above the starting level. Taking g = 10 m/s², calculate the gain in gravitational potential energy.

Given: m = 50 kg; h = 72.5 m; g = 10 m/s². Formula: U = mgh. Substitute: U = 50 × 10 × 72.5. Answer: the gain is 36250 J. It equals the elevator result because the mass and vertical height are unchanged.

The last two calculations compare the same gravitational energy change along different routes. They do not claim that every energy transfer inside the student's body is identical. The calculation concerns the gain in gravitational potential energy alone.

A statement about a raised object should therefore include the level from which height is measured. Choosing that level makes the calculation unambiguous and allows energy values at different positions to be compared consistently.

How do energy conversions connect the different forms?

An energy conversion chain records the starting form, any intermediate forms being considered, and the final form. An arrow means “is converted into”. Follow what physically happens to the object or device instead of merely listing unrelated energy names.

What happens during a fall?

For an object falling under gravity, gravitational potential energy decreases as height decreases. Its kinetic energy increases as it speeds up. When other energy transfers can be neglected, the gain in kinetic energy equals the loss of gravitational potential energy.

If E denotes total mechanical energy, E = U + K. Provided no energy is transferred into or out of these mechanical forms, this sum remains constant. The separate values of U and K can change while the sum stays the same.

This connects with the principle of conservation of energy: energy is neither created nor destroyed, but can change form. Mechanical energy is only part of the full energy account. Friction can reduce mechanical energy by transferring energy into thermal forms.

How should a conversion be explained?

1. Identify the object, device or system being considered.
2. Name the form in which energy is initially supplied or stored.
3. Describe the physical change, such as lifting, moving, heating or vibrating.
4. Name the resulting energy form and include other relevant transfers when needed.

A concise explanation of a ringing bell therefore connects the mechanical action, the bell's vibrations and sound energy. A concise explanation of falling water connects its initial height, the decrease in gravitational potential energy and the increase in kinetic energy.

Glossary

• Energy — The capacity to do work, possessed in several interconvertible forms.
• Work — Energy transferred when a force produces displacement in its direction.
• Joule — The SI unit used to measure both work and energy.
• Mechanical energy — Energy associated with motion and position, combining kinetic and potential energy.
• Kinetic energy — Energy an object possesses because it is in motion.
• Potential energy — Stored energy associated with deformation or relative positions of interacting objects.
• Gravitational potential energy — Energy associated with the relative positions of gravitationally attracting objects.
• Reference level — The chosen level at which gravitational potential energy is assigned zero.
• Translational motion — Motion in which an object changes position from one place to another.
• Rotational motion — Motion in which an object turns about an axis.
• Vibrational motion — Repeated to-and-fro motion of an object or part about its mean position.
• Chemical energy — Energy stored through chemical bonds in substances such as food and fuels.
• Electrical energy — Energy associated with the positions or motion of electric charges.
• Nuclear energy — Energy associated with atomic nuclei and released in nuclear changes.
• Energy conversion — A change from one form of energy into another form.

Common errors and misconceptions

• Misconception: A stationary object cannot possess energy. Correct: A raised ball or a compressed spring can possess potential energy while stationary.
• Misconception: Doubling speed doubles kinetic energy. Correct: For unchanged mass, doubling speed makes translational kinetic energy four times its original value.
• Misconception: Kinetic energy is negative when the object moves backwards. Correct: Kinetic energy has no direction and depends on the square of speed.
• Misconception: The distance travelled along stairs is h in U = mgh. Correct: Use the vertical height gained relative to the starting level.
• Misconception: Mass and weight can both be inserted as m. Correct: The symbol m denotes mass in kilograms; mg is weight in newtons.
• Misconception: Every form of potential energy is calculated using U = mgh. Correct: This expression describes gravitational potential energy near Earth's surface, not a spring's elastic energy.
• Misconception: A reduction in mechanical energy means energy is destroyed. Correct: Transfers into thermal and other forms can reduce mechanical energy without destroying total energy.

Exam-style questions with model answers

Q1. Define kinetic energy and potential energy. [2 marks]

1. Kinetic energy is the energy an object possesses because of its motion.
2. Potential energy is energy stored through deformation or the relative positions of interacting objects.

Q2. A cricket ball moves towards a wicket, fan blades rotate about their axis, and a bell vibrates to and fro. Name and explain the form of kinetic energy illustrated by each motion. [3 marks]

1. The ball illustrates translational kinetic energy because the described motion carries the ball from one position towards another. Its energy is associated with that motion.
2. The fan blades illustrate rotational kinetic energy because they turn about an axis even when the fan stays in place.
3. The bell illustrates vibrational kinetic energy because its material moves repeatedly to and fro. This vibration is associated with the production of sound.

Q3. A cricket ball has an approximate mass of 0.2 kg and a speed of about 154.8 km/h, equivalent to 43 m/s. Calculate its translational kinetic energy. [3 marks]

1. The mass is m = 0.2 kg and the supplied equivalent speed is v = 43 m/s. These values are in compatible SI units.
2. Using K = ½mv², substitute the data: K = ½ × 0.2 × 43². Square the speed before completing the multiplication.
3. The kinetic energy is approximately 184.9 J. The result is approximate because the given mass and speed are approximate.

Q4. A 50 kg student reaches a floor 72.5 m vertically above the starting level, first by elevator and later by stairs. Take g = 10 m/s². Find the gain in gravitational potential energy for each route and explain the comparison. [4 marks]

1. Choose the starting level as zero gravitational potential energy. The required height is the vertical rise, h = 72.5 m.
2. For the elevator, U = mgh = 50 × 10 × 72.5 = 36250 J.
3. For the stairs, the same mass reaches the same height, so U = 50 × 10 × 72.5 = 36250 J.
4. The gains are equal because gravitational potential-energy change depends on vertical height, not the path followed between these levels.

Q5. Derive K = ½mv² for a body of constant mass m accelerated from rest to speed v along a straight line by a constant resultant force F. Define any additional symbols and state the relevant condition. [5 marks]

1. Let a denote acceleration, s displacement along the force and W the work done by the resultant force. The body's initial speed is zero.
2. Newton's second law gives F = ma. Because the resultant force and mass are constant, the acceleration remains constant throughout the motion considered.
3. The constant-acceleration equation becomes v² = 2as, since the body starts from rest. Therefore s = v²/(2a), for non-zero acceleration.
4. Work done is W = Fs = ma × v²/(2a) = ½mv². The force and displacement are in the same direction.
5. This work equals the gain in kinetic energy. Initial kinetic energy is zero, so the final translational kinetic energy is K = ½mv².

Q6. Derive U = mgh for an object of mass m raised gradually through vertical height h near Earth's surface. Take g as constant, choose the starting level as zero potential energy and assume no change in kinetic energy. [5 marks]

1. The object's weight is the downward gravitational force mg, where g is gravitational acceleration. The mass m is measured in kilograms.
2. Raise the object with an upward force F equal in magnitude to its weight, so F = mg. There is no acceleration during the steady lift.
3. The displacement along this upward force is h. Therefore the lifting force does work W = Fh, where W denotes work.
4. Substitution gives W = mg × h = mgh. This work increases stored gravitational energy because the kinetic energy does not change.
5. With the starting level assigned zero potential energy, the final gravitational potential energy is U = mgh. The expression assumes effectively constant g near Earth's surface.

Q7. State the energy conversion in an electric water heater, an electric bulb and a ringing bell, explaining the output in each case. [3 marks]

1. An electric water heater converts electrical energy into thermal energy of the water. The conversion explains why the water becomes warmer as energy is supplied.
2. An electric bulb converts electrical energy into light energy. This identifies the light output without claiming it is the device's only energy transfer.
3. A ringing bell converts mechanical energy into sound energy. Mechanical action produces vibrations, and sound carries energy away through the surrounding medium.

Key takeaways

• Energy is the capacity to do work; work and energy are both measured in joules.
• Chemical, mechanical, thermal, electrical, nuclear, light and sound energy describe different forms that can be converted into one another.
• Translational, rotational and vibrational kinetic energy correspond to different patterns of motion; identify the motion before choosing an expression.
• Translational kinetic energy is K = ½mv², so doubling speed at fixed mass makes kinetic energy four times larger.
• Potential energy can be stored through deformation or the relative positions of interacting objects, even when they are stationary.
• Near Earth's surface, U = mgh measures gravitational potential energy relative to a chosen zero-height level.
• Gravitational potential-energy gain depends on vertical height and is independent of the route taken between the same levels.
• Energy conversions account for changes in energy form; a decrease in mechanical energy does not imply destruction of energy.

Test yourself

What does one joule of work mean?

A force of one newton displaces an object by one metre in the force's direction.

Why can a raised flowerpot possess energy while stationary?

Its position above the reference level gives it gravitational potential energy and the capacity to do work if it falls.

What happens to a vehicle's kinetic energy if its speed doubles while mass remains unchanged?

It becomes four times its original value because kinetic energy depends on the square of speed.

What distinguishes chemical energy from nuclear energy?

Chemical energy is associated with bonds between atoms; nuclear energy is associated with their nuclei.

What does h represent in the gravitational potential-energy expression?

It represents vertical height above the chosen zero-potential-energy level, not distance along a sloping route.

Why do rotating fan blades have kinetic energy even when the fan stays fixed?

The blades are moving about an axis, so they possess rotational kinetic energy.

What conversion occurs when a compressed spring pushes a ball?

The spring's stored potential energy is transferred into kinetic energy as it sets the ball in motion.

Under what condition can falling motion be described as an equal exchange between gravitational potential energy and kinetic energy?

The description applies when other energy transfers can be neglected, so the total mechanical energy remains constant.