work and energy
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Questions and Answers

What is the power output of a machine that raises a load of 750 N through a height of 16 m in 5 seconds?

  • 1200 watt
  • 2400 watt (correct)
  • 3000 watt
  • 1600 watt

If a motor has a power of 40 kW, what speed can it raise a load of 20,000 N?

  • 3 m/s
  • 2 m/s (correct)
  • 1 m/s
  • 4 m/s

How much work is done by a boy moving against a friction force of 5 N after travelling a distance of 1.5 km and additional distances?

  • 22000 J
  • 15000 J
  • 20000 J
  • 18500 J (correct)

What is the electric energy consumed by a 1200 W iron used for 30 minutes daily in April?

<p>18 kWh (B)</p> Signup and view all the answers

Calculate the equivalent power of an engine that pumps 2000 tonnes of water from a height of 20 m in a minute.

<p>6.6 × 10^6 W (D)</p> Signup and view all the answers

What is the speed of car B after car A collides with it and comes to rest?

<p>10 m/s (B)</p> Signup and view all the answers

How much work is done on a trolley of mass 5 kg given an initial velocity of 4 m/s over a distance of 16 m?

<p>320 J (C)</p> Signup and view all the answers

What force in newtons does a 150 kg car engine exert when moving at a speed of 20 m/s with a power output of 500 W/kg?

<p>3750 N (C)</p> Signup and view all the answers

If a person can jump 0.4 m high on Earth, how high can he jump on a planet where his weight is about half?

<p>0.8 m (D)</p> Signup and view all the answers

A ball dropped from a height of 10 m reduces its energy by 40% after striking the ground. How high can it bounce back?

<p>6 m (B)</p> Signup and view all the answers

What is the power required for an electric iron used for 30 minutes at 1200 W?

<p>216000 J (A)</p> Signup and view all the answers

What is the efficiency of a machine that does work of 1.323 × 10^7 J in 600 seconds?

<p>100% efficiency (D)</p> Signup and view all the answers

A girl having a mass of 35 kg sits on a trolley of 5 kg. How much work does she do?

<p>0 J (D)</p> Signup and view all the answers

What is the total mechanical energy of a freely falling body at point C just before it strikes the ground?

<p>mgH (A)</p> Signup and view all the answers

In the scenario where a light object and a heavy object have the same momentum, which object has the greater kinetic energy?

<p>The heavy object (D)</p> Signup and view all the answers

How much work do the men do while holding a 250 kg box without moving it?

<p>0 J (B)</p> Signup and view all the answers

If a car is traveling at a steady speed of 30 m/s against a resistive force of 2000 N, how much energy is used in 1 second?

<p>60000 J (A)</p> Signup and view all the answers

What is the gravitational potential energy increase when a 36 kg package is raised by 2.4 m?

<p>864 J (B)</p> Signup and view all the answers

What happens to the operation of a conveyor belt if a package with a mass greater than 36 kg is raised through a height of 2.4 m?

<p>The time taken increases and speed decreases (A)</p> Signup and view all the answers

What type of energy is primarily utilized by the car engine to drive the vehicle?

<p>Chemical energy (D)</p> Signup and view all the answers

How much is the minimum power that the car engine has to deliver to maintain speed while overcoming a resistive force of 2000 N?

<p>60 kW (C)</p> Signup and view all the answers

What happens to the kinetic energy when the velocity of a vehicle of mass 500 kg increases from 36 km/h to 72 km/h?

<p>It increases (A)</p> Signup and view all the answers

What is the work done in lifting a mass of 200 kg through a vertical distance of 6 m against gravity?

<p>12000 J (C)</p> Signup and view all the answers

Which formula correctly expresses kinetic energy for a moving body with mass m and velocity v?

<p> rac{1}{2}mv^2 (C)</p> Signup and view all the answers

What is required for a person to feel tired while holding a 250 kg box, even when doing no mechanical work?

<p>Muscle exertion against gravity (D)</p> Signup and view all the answers

If a man of mass 60 kg runs up a flight of 30 steps each 20 cm high in 15 seconds, what is the power developed?

<p>240 W (B)</p> Signup and view all the answers

What is the ratio of the initial kinetic energy to the final kinetic energy when the velocity of the rocket is suddenly tripled?

<p>1 : 9 (A)</p> Signup and view all the answers

Which of the following describes the power produced by Avinash and Kapil correctly?

<p>Avinash produces more power than Kapil. (C)</p> Signup and view all the answers

When a constant force is applied to increase the velocity of a body, what does this force accomplish?

<p>Increases the kinetic energy equal to the work done. (B)</p> Signup and view all the answers

During the pole-vault performance, which energy type is primarily transformed to potential energy as the athlete rises?

<p>Elastic energy from the pole. (D)</p> Signup and view all the answers

What is the gravitational potential energy change for a hammer of mass 2.0 kg dropped from a height of 4.8 m?

<p>96 J (B)</p> Signup and view all the answers

What is the work done by the force of gravity on an object that returns to the same height?

<p>Zero work since initial and final heights are equal. (A)</p> Signup and view all the answers

How is the total mechanical energy of a freely falling body calculated?

<p>It remains constant throughout the fall. (A)</p> Signup and view all the answers

What energy transformation takes place when a hammer drops and hits the ground?

<p>Potential to heat and sound. (D)</p> Signup and view all the answers

Which of the following correctly defines gravitational potential energy?

<p>Energy due to an object's position relative to the ground. (D)</p> Signup and view all the answers

When comparing power and energy between Anil and Ashok, what can be concluded?

<p>Anil has greater power; Ashok has more energy. (B)</p> Signup and view all the answers

What is the work done on a mass 'm' raised to a height 'h' against gravity?

<p>$mgh$ (B)</p> Signup and view all the answers

What happens to the kinetic energy of an object as it falls freely under gravity?

<p>Increasing linearly over time. (B)</p> Signup and view all the answers

What kind of energy is primarily involved when the pole is bent during the vault?

<p>Elastic potential energy. (D)</p> Signup and view all the answers

If a workman lifts a hammer weighing 520 N up 3.0 m in 5.0 s, what is the useful power exerted by his legs?

<p>104 W (A)</p> Signup and view all the answers

Flashcards

Work is done

When a force causes an object to move, work is being done. To have work done there must be a force acting and the object must be displaced.

Positive Work

When a force acts in the same direction as the movement of an object, work is positive. For example, when you pull a wagon, the force of the pull is in the same direction as the wagon's motion.

Zero Work

When a force is perpendicular to the direction of movement, no work is done. A common example is a person carrying a load. The person applies an upward force, but the load is moving horizontally.

Kinetic Energy

The energy possessed by a body because of its motion. The faster the object is moving, the more kinetic energy it has.

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Potential Energy

The energy stored in a body because of its position or configuration. A stretched rubber band or a book held above the ground both have potential energy.

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Law of Conservation of Energy

Energy cannot be created or destroyed, it can only be transformed from one form to another.

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Negative Work

Work done against a force, such as friction. For example, when you push a heavy box across a rough floor, you are doing negative work against friction.

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What is the difference between moment of force and work done by a force?

Moment of force causes rotation, while work done by a force causes linear motion.

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Elastic Potential Energy

A type of potential energy stored in an object that has been deformed, like a stretched spring or a compressed rubber band.

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Energy Transformation

The process of changing one form of energy into another, like converting chemical energy into electrical energy.

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What is work?

Work is done when a force causes a displacement of an object. It is a measure of energy transfer.

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Factors affecting work done

Work done on a body depends on two factors: the force applied and the displacement caused by the force.

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Power

The rate at which work is done, or the amount of energy transferred per unit of time.

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Kilowatt (kW)

The unit of power, equal to 1000 watts.

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Kilowatt-Hour (kWh)

The commercial unit of energy, representing the energy used in one hour at the rate of 1000 watts.

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Energy transformation in electric heater

Electrical energy is transformed into heat energy.

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Energy transformation in solar battery

Solar energy (light) is transformed into electrical energy.

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Energy transformation in a Dynamo

Mechanical energy (like rotation) is transformed into electrical energy.

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Tripling Velocity on Kinetic Energy

If an object's velocity triples, its kinetic energy becomes nine times greater.

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Work Done by a Force

The energy transferred when a force moves an object over a distance. It's the product of force and displacement.

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Conservation of Mechanical Energy

In a system without external forces, the total mechanical energy (potential + kinetic) remains constant.

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Gravitational Potential Energy

Energy stored in an object due to its position relative to a gravitational field.

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Energy Changes in a Pole Vault

Chemical energy is converted to kinetic energy, then elastic energy, then potential energy, then kinetic energy again, finally becoming heat and sound.

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Calculating Potential Energy Change

To calculate a change in potential energy, use the formula: ΔPE = mgh, where m is mass, g is gravitational acceleration, and h is the height change.

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Work Done by Gravity

When a body moves vertically, the work done by gravity depends on the change in its vertical position.

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Power vs. Energy

Power is the rate at which work is done (how quickly). Energy is the total work done.

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Work Done in Lifting an Object

The work done in lifting an object vertically is equal to the increase in its gravitational potential energy.

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Freely Falling Body

A body that falls under the influence of gravity only, with no air resistance.

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What is the relationship between force, work, and energy?

Force causes displacement, which results in work being done. This work done is equal to the energy transferred to the object.

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Explain how a pole vaulter converts kinetic energy into potential energy.

As the pole vaulter runs, they gain kinetic energy. This energy is then transferred to the pole, storing it as elastic potential energy. The pole then releases this energy, launching the vaulter upward, converting it into gravitational potential energy as they reach their highest point.

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Velocity at point B

The velocity of a freely falling body at point B, calculated using the equation v² = u² + 2as, where u = 0, a = g, and s = x (the distance fallen from the initial point).

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Kinetic Energy at point B

The energy possessed by the body at point B due to its motion, calculated as (1/2)mv² where v is the velocity at point B.

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Potential Energy at point B

The stored energy of the body at point B due to its position relative to the ground, calculated as mg(H - x) where H is the initial height and x is the distance fallen.

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Total Energy at point B

The sum of kinetic and potential energy of the body at point B, which remains constant throughout the fall.

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Velocity at point C

The velocity of the body just before it hits the ground, calculated using v² = u² + 2as, where u = 0, a = g, and s = H (the total height).

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Kinetic Energy at point C

The energy of the body at point C due to its maximum velocity, calculated as (1/2)mv².

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Total Energy at point C

The total energy of the body at point C, which is equal to the kinetic energy since potential energy is zero.

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Momentum and Kinetic Energy Relationship

Objects with the same momentum can have different kinetic energies if their masses are different. The lighter object with the higher velocity will have a larger kinetic energy.

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Work Done in Lifting

The work done in lifting an object is equal to the force required to lift it (its weight) multiplied by the distance it is lifted.

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Work Done in Holding

No work is done when an object is held stationary, as there is no displacement.

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Why Muscles Get Tired Holding

Muscles get tired while holding an object because they are constantly working against the force of gravity, even though the object is not moving.

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Kinetic Energy of a Moving Car

The energy possessed by a car due to its motion, calculated as (1/2)mv² where m is the car's mass and v is its speed.

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Energy Used against Resistive Force

The energy lost by a moving car due to forces opposing its motion (like air resistance), calculated as force × distance.

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Minimum Power Output of an Engine

The minimum power needed to maintain a car's speed is calculated as energy used divided by the time taken.

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Fuel's Energy Source

The energy stored in fuel is chemical energy, which is released during combustion to provide the power to move the car.

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What is power?

Power is the rate at which work is done, or the amount of energy transferred per unit of time.

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What is the unit of Power?

The unit of power is the watt (W).

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How to calculate power?

Power can be calculated by dividing the work done by the time taken (Power = Work / Time).

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What is work done?

Work done is the product of the force applied and the displacement caused by the force in the direction of the force.

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How to calculate work?

Work can be calculated by multiplying the force and the displacement (Work = Force × Displacement).

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What is potential energy?

Potential energy is the energy stored in a body because of its position or configuration.

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What is kinetic energy?

Kinetic energy is the energy possessed by a body because of its motion.

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What's the relationship between power, work, and time?

Power is the rate at which work is done, meaning the amount of work done in a given time.

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What is the difference between work and power?

Work is the energy transferred by a force causing displacement, while power is the rate at which that work is done.

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How is power related to speed?

Power is directly proportional to speed when the force applied is constant.

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How can we measure power in a real-world scenario?

Power can be measured in units like watts (W) or kilowatts (kW).

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What is the formula for power in terms of force and velocity?

Power is equal to the product of force and velocity (Power = Force × Velocity).

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What is the relationship between potential energy and kinetic energy?

When an object falls, its potential energy is converted into kinetic energy.

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Energy transformation in a hydroelectric dam.

Potential energy of stored water is transformed into kinetic energy as it falls and then into electrical energy by a turbine.

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How does an engine work?

An engine uses the energy from fuel to create motion, transforming chemical energy into mechanical energy.

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Mechanical Energy and Momentum

An object can have mechanical energy even with zero momentum if it has potential energy. However, zero mechanical energy means zero potential and kinetic energy, leading to zero momentum.

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Work and Circular Motion

Work done by a force on an object moving in a circle is zero. This occurs because the force is always perpendicular to the displacement, meaning no energy is transferred in the direction of motion.

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Pendulum Energy Transformation

A pendulum released from a high point loses potential energy and gains kinetic energy as it swings down. The energy is eventually transformed into heat due to air resistance, causing the pendulum to stop.

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Factors Affecting Work

Work done on an object depends on the applied force and the displacement caused by that force.

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Study Notes

Work, Power, and Energy - Class 9 Extra Questions

  • Work: Work is done when a force acts on an object and the object is displaced.

    • Conditions for Work: A force must act, and the object must move.
    • Positive Work: Force acts in the direction of motion (e.g., pulling a lawn roller forward).
    • Zero Work: Force is perpendicular to the direction of motion (e.g., a porter carrying a load).
    • Negative Work: Force acts opposite to the direction of motion (e.g., work done against friction).
    • Calculating Work: Work (W) = Force (F) × Displacement (s) × cos(θ), where θ is the angle between force and displacement.

  • Energy: The capacity of a body to do work. Units are Joules (J) and Kilojoules (kJ).

    • Source of Energy for Life Processes: Food.

  • Kinetic Energy (KE): Energy of motion.

    • Units: Joules (J)
    • Relationship with Speed: KE ∝ v² (Kinetic energy is proportional to the square of velocity). Doubling the speed quadruples the kinetic energy.

  • Potential Energy (PE): Stored energy due to position or shape.

    • Gravitational Potential Energy: Energy due to position (PE = mgh, where m=mass, g=gravity on Earth, h=height).
    • Elastic Potential Energy: Energy stored in a stretched or compressed object (e.g., a spring).

  • Law of Conservation of Energy: Energy can neither be created nor destroyed, only transformed from one form to another (verified experimentally by James Prescott Joule).

  • Power: Rate of doing work. Unit is Watts (W) or Kilowatts (kW). Power = (Work / Time).

  • Commercial unit of Energy: Kilowatt-hour (kWh) ; 1 kWh = 3.6 × 106 J.

  • Converting Forms of Energy:

    • Electricity to light/heat (e.g., electric heater)
    • Mechanical to electrical (e.g., dynamo)
    • Heat to mechanical (e.g., steam engine)
    • Mechanical to electrical (e.g., hydroelectric power plant)
    • Chemical to electrical (e.g., battery)

Additional Notes

  • Work done by a constant force is zero when the object does not move (displacement = 0).
  • Constant velocity means net force (and hence work done) is zero.
  • Potential energy doesn't change if there's no change in height on a level surface.
  • Kinetic Energy depends on mass and square of velocity: KE =1/2 * m*v^2.

Differences

  • Moment of a force causes rotational motion. Work done causes translational motion.
  • Power measures the rate of doing work.
  • Energy measures the capacity to do work.

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