SUMMARY: The chapter "Work and Energy" in Class 9 Science explores the concepts of work, energy, and their interrelation, including the laws governing them. KEY TOPICS: Work done by a force, energy and its forms, kinetic energy, potential energy, law of conservation of energy, power, commercial unit of energy, work-energy theorem, transformation of energy.
If a force of 10 N moves an object 5 m in the direction of the force, how much work is done?
A50 J
B5 J
C10 J
D15 J
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Correct answer: Option 1 — 50 J
Q31 Mark
Which of the following is a form of potential energy?
AA moving car
BA stretched spring
CA flowing river
DA spinning top
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Correct answer: Option 2 — A stretched spring
Q41 Mark
The work-energy theorem states that the work done on an object is equal to the change in its:
Amass
Bvelocity
Cenergy
Dmomentum
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Correct answer: Option 3 — energy
Q51 Mark
What is the formula for kinetic energy?
AKE = 1/2 mv^2
BKE = mv
CKE = mgh
DKE = 1/2 mgh
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Correct answer: Option 1 — KE = 1/2 mv^2
Q61 Mark
Which of the following statements is true regarding the law of conservation of energy?
AEnergy can be created and destroyed.
BEnergy can be transformed from one form to another but cannot be created or destroyed.
CEnergy is always lost in transformations.
DEnergy can only be stored, not transformed.
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Correct answer: Option 2 — Energy can be transformed from one form to another but cannot be created or destroyed.
Q71 Mark
What is the commercial unit of energy?
AJoule
BKilowatt-hour
CCalorie
DNewton-meter
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Correct answer: Option 2 — Kilowatt-hour
Q81 Mark
If an object is lifted to a height, what type of energy does it gain?
AKinetic energy
BThermal energy
CPotential energy
DMechanical energy
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Correct answer: Option 3 — Potential energy
Q91 Mark
Which of the following is an example of power?
AWork done in 10 seconds
BEnergy consumed in a day
CForce applied on an object
DWork done per unit time
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Correct answer: Option 4 — Work done per unit time
Q101 Mark
A car moving at a speed of 20 m/s has a kinetic energy of 400 J. What will be its kinetic energy if its speed is doubled?
A1600 J
B800 J
C400 J
D200 J
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Correct answer: Option 1 — 1600 J
Q111 Mark
What is the work done when a force of 15 N moves an object 3 m in the opposite direction?
A-45 J
B45 J
C0 J
D15 J
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Correct answer: Option 1 — -45 J
Q121 Mark
When a ball is thrown upwards, what happens to its kinetic energy as it rises?
AIncreases
BDecreases
CRemains constant
DBecomes potential energy
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Correct answer: Option 2 — Decreases
Q131 Mark
Which of the following is NOT a form of energy?
AMechanical energy
BChemical energy
CGravitational energy
DTime energy
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Correct answer: Option 4 — Time energy
Q141 Mark
If a machine does 500 J of work in 10 seconds, what is its power output?
A50 W
B500 W
C5 W
D100 W
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Correct answer: Option 1 — 50 W
Q151 Mark
What happens to the total mechanical energy of a system when it is subject to no external forces?
AIt increases
BIt decreases
CIt remains constant
DIt becomes zero
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Correct answer: Option 3 — It remains constant
Short Answer Questions10 questions
Q163 Marks
Define work done by a force. How is it calculated?
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Work done by a force is defined as the product of the force and the displacement in the direction of the force. It is calculated using the formula: Work = Force × Displacement × cos(θ), where θ is the angle between the force and the direction of displacement.
Q173 Marks
What is kinetic energy? Provide its formula.
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Kinetic energy is the energy possessed by an object due to its motion. The formula for kinetic energy is KE = 1/2 mv², where m is the mass of the object and v is its velocity.
Q183 Marks
Explain potential energy with an example.
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Potential energy is the energy stored in an object due to its position or configuration. For example, a book placed on a shelf has gravitational potential energy due to its height above the ground.
Q193 Marks
State the law of conservation of energy.
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The law of conservation of energy states that energy cannot be created or destroyed; it can only be transformed from one form to another. The total energy in a closed system remains constant.
Q203 Marks
What is the work-energy theorem?
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The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. Mathematically, it can be expressed as W = ΔKE = KE_final - KE_initial.
Q213 Marks
How is power defined in terms of work and time?
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Power is defined as the rate at which work is done. It can be calculated using the formula: Power = Work done / Time taken. The SI unit of power is the watt (W).
Q223 Marks
What are the commercial units of energy?
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The commercial units of energy include kilowatt-hour (kWh) and joules (J). Kilowatt-hour is commonly used for measuring electrical energy consumption.
Q233 Marks
Differentiate between kinetic energy and potential energy.
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Kinetic energy is the energy of an object in motion, while potential energy is the energy stored in an object due to its position or state. Kinetic energy depends on the velocity, whereas potential energy depends on the height or position of the object.
Q243 Marks
Describe an example of energy transformation.
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An example of energy transformation is a pendulum. At its highest point, it has maximum potential energy, which transforms into kinetic energy as it swings downwards. At the lowest point, kinetic energy is maximum and potential energy is minimum.
Q253 Marks
How does the angle between the force and displacement affect work done?
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The angle between the force and displacement affects the work done because it determines the component of the force that acts in the direction of displacement. If the angle is 0 degrees, work is maximized; if it is 90 degrees, no work is done.
Long Answer Questions6 questions
Q266 Marks
Define work done by a force. How is it calculated? Provide an example to illustrate your explanation.
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Work done by a force is defined as the product of the force applied and the displacement in the direction of the force. It is mathematically expressed as W = F × d × cos(θ), where W is work, F is the force applied, d is the displacement, and θ is the angle between the force and displacement vectors. For example, if a force of 10 N is applied to move an object 5 m in the direction of the force, the work done is W = 10 N × 5 m × cos(0°) = 50 J.
Q276 Marks
Explain the different forms of energy, providing examples for each form.
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Energy exists in various forms, including kinetic energy, potential energy, thermal energy, chemical energy, and electrical energy. Kinetic energy is the energy of an object in motion, calculated as KE = 1/2 mv², where m is mass and v is velocity. Potential energy is stored energy based on an object's position or state, such as gravitational potential energy, calculated as PE = mgh, where h is height. Thermal energy is related to the temperature of an object, while chemical energy is stored in chemical bonds, and electrical energy is associated with electric charges.
Q286 Marks
What is kinetic energy? Derive the formula for kinetic energy and explain its significance in real-life scenarios.
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Kinetic energy is the energy possessed by an object due to its motion. The formula for kinetic energy (KE) is derived from the work-energy theorem, which states that the work done on an object is equal to the change in its kinetic energy. The formula is KE = 1/2 mv², where m is the mass of the object and v is its velocity. This concept is significant in real life, as it helps us understand the energy involved in moving vehicles, sports, and various mechanical systems, emphasizing the importance of speed and mass in determining energy levels.
Q296 Marks
Define potential energy and explain how it differs from kinetic energy. Provide an example of potential energy in a gravitational field.
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Potential energy is the stored energy of an object based on its position or configuration. It differs from kinetic energy, which is the energy of motion. For example, in a gravitational field, an object raised to a height h has gravitational potential energy given by the formula PE = mgh, where m is mass, g is the acceleration due to gravity, and h is height. A classic example is a book placed on a shelf; it has potential energy due to its height above the ground, which can be converted to kinetic energy if it falls.
Q306 Marks
Discuss the law of conservation of energy and its implications in physical systems. Provide a practical example to support your explanation.
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The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. This principle implies that the total energy in a closed system remains constant. For instance, in a pendulum, the energy continuously transforms between potential energy at the highest point and kinetic energy at the lowest point, but the total mechanical energy remains constant, assuming no air resistance or friction. This law is fundamental in understanding energy transfer in various physical systems, including mechanical, thermal, and electrical systems.
Q316 Marks
What is the work-energy theorem? Explain how it relates work done on an object to its kinetic energy.
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The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. Mathematically, it can be expressed as W = ΔKE = KE_final - KE_initial. This theorem illustrates that when a net force acts on an object, causing it to accelerate, the work done by that force results in a change in the object's kinetic energy. For example, if a car accelerates from rest to a certain speed, the work done by the engine translates into an increase in the car's kinetic energy, demonstrating the direct relationship between work and energy.
Assertion–Reason Questions8 questions
Q321 Mark
Assertion (A): Work is done when a force acts on an object and causes displacement.
Reason (R): Work is defined as the product of the force and the displacement in the direction of the force.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q331 Mark
Assertion (A): Kinetic energy is the energy possessed by an object due to its motion.
Reason (R): Kinetic energy is given by the formula KE = 1/2 mv², where m is mass and v is velocity.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q341 Mark
Assertion (A): Potential energy is always greater than kinetic energy in a closed system.
Reason (R): Potential energy can be converted into kinetic energy, but they can exist independently in a system.
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Correct answer: Option 3 —
A is true, but R is false.
Q351 Mark
Assertion (A): The law of conservation of energy states that energy can neither be created nor destroyed.
Reason (R): Energy can change forms, but the total energy in a closed system remains constant.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q361 Mark
Assertion (A): Power is defined as the rate at which work is done.
Reason (R): Power is calculated as work done divided by time taken, hence it is a measure of how quickly work is done.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q371 Mark
Assertion (A): The commercial unit of energy is measured in joules.
Reason (R): The commercial unit of energy is actually measured in kilowatt-hours (kWh).
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Correct answer: Option 4 —
A is false, but R is true.
Q381 Mark
Assertion (A): The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy.
Reason (R): This theorem is a direct consequence of Newton's second law of motion.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Q391 Mark
Assertion (A): When an object is lifted to a height, its potential energy increases.
Reason (R): Potential energy is directly proportional to the height of the object above the ground.
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Correct answer: Option 1 —
Both A and R are true, and R is the correct explanation of A.
Statement-Based Questions8 questions
Q401 Mark
Statement 1: Work is done when a force acts on an object and causes displacement.
Statement 2: Work is independent of the direction of the force applied.
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Correct answer: Option 2 —
Only Statement 1 is true.
Q411 Mark
Statement 1: Kinetic energy is the energy possessed by a body due to its motion.
Statement 2: Potential energy is the energy possessed by a body due to its position.
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Correct answer: Option 1 —
Both statements are true.
Q421 Mark
Statement 1: The unit of work is Joule.
Statement 2: Power is defined as the rate of doing work.
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Correct answer: Option 1 —
Both statements are true.
Q431 Mark
Statement 1: The law of conservation of energy states that energy can be created and destroyed.
Statement 2: The total energy in an isolated system remains constant.
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Correct answer: Option 3 —
Only Statement 2 is true.
Q441 Mark
Statement 1: If an object is lifted to a height, its potential energy increases.
Statement 2: Potential energy decreases when the object is lifted to a height.
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Correct answer: Option 4 —
Both statements are false.
Q451 Mark
Statement 1: The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy.
Statement 2: The work-energy theorem applies only to objects at rest.
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Correct answer: Option 2 —
Only Statement 1 is true.
Q461 Mark
Statement 1: Energy can be transformed from one form to another without loss.
Statement 2: Mechanical energy is the sum of kinetic and potential energy.
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Correct answer: Option 1 —
Both statements are true.
Q471 Mark
Statement 1: Power is measured in Watts.
Statement 2: One Watt is equal to one Joule per second.
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Correct answer: Option 1 —
Both statements are true.
