9th Class Physics Chapter 5 Work, Energy and Power Long Questions

Q: What is power? Write its units and show that power = force × velocity.

Definition Power: The rate of doing work: P = W ÷ t. Units watt: 1 W = 1 J s⁻¹; kilowatt: 1 kW = 10³ W; megawatt: 1 MW = 10⁶ W Power in terms of force and velocity P = W ÷ t, and W = F × S.; So P = F × S ÷ t.; S ÷ t = v, the speed.; Therefore P = Fv. P = Fv Example Work = mgh = 40 × 10 × 10 = 4000 J. P = 4000 ÷ 8. 500 W The same work done in less time means more power.

Q: What is potential energy? Derive Ep = mgh and describe the types of potential energy.

Definition Potential energy: The energy a body has because of its position or deformation. Derivation of gravitational potential energy Lift a body of mass m at constant speed from the ground to a height h.; The force needed equals its weight: F = mg.; Work done = F × h = mgh.; This work is stored in the body as gravitational potential energy. Ep = mgh Types Gravitational, Height above the reference level, Water in a dam; Elastic, Stretching or compressing, A wound clock spring, a bow; Chemical, Arrangement of atoms in molecules, Food, petrol, batteries Reference level Height is measured from a chosen level (usually the ground), so potential energy is always relative to that level. Ep = 0.18 × 10 × 12. 21.6 J

Q: Explain energy transformations with examples from daily life.

Meaning Energy is useful because it can change from one form to another. In every change the total energy stays the same, though some becomes heat that cannot be used again. Examples Photosynthesis, Light → chemical (food); Our body, Chemical (food) → kinetic + heat; Electric bulb, Electrical → light + heat; Electric motor, fan, Electrical → kinetic; Generator, dynamo, Kinetic → electrical; Loudspeaker, Electrical → sound; Solar cell, Light → electrical; Car engine, Chemical → heat → kinetic; Bow and arrow, Elastic potential → kinetic Energy is never lost, but some is always wasted as heat.

Q: Describe solar energy. How are solar cells and solar panels used? Give the advantages and disadvantages of solar energy.

Solar energy Definition: The energy we get from sunlight. The Sun is the biggest source of energy for the Earth. Uses of solar energy Solar heaters and cookers: black surfaces absorb sunlight to heat water or cook food.; Solar cells (photovoltaic cells): change light directly into electricity.; Solar panels: one cell gives a very small voltage, so many cells are joined in series to give a useful voltage, for homes, street lights, tube wells and calculators. Advantages Free and renewable.; No pollution or noise.; Useful in remote villages far from power lines. Disadvantages No output at night and little on cloudy days.; Panels and batteries are costly.; Large areas are needed for large amounts of power. Pakistan gets strong sunshine most of the year, so solar energy is a good choice here.

Question 1

What is meant by kinetic energy? State its unit. Describe how it is determined.

Accepted Answer

Definition Kinetic energy: The energy a body has because of its motion. SI unit: joule (J). Principle A moving body can do work as it is brought to rest. So its kinetic energy equals the work done by the force that stops it. Derivation A body of mass m moves with speed v. A steady opposing force F stops it over a distance S, so Ek = F × S ...(i); By Newton's second law, F = ma, where a is the deceleration.; The speed falls uniformly from v to 0, so the average speed is v/2 and S = (v/2) × t.; Putting these in (i): Ek = ma × (v/2) × t ...(ii); From the velocity-time graph, a = v/t. Putting this in (ii): Ek = m × (v/t) × (v/2) × t = ½mv². Ek = ½mv² Example Ek = ½ × 0.05 × 300². 2250 J Kinetic energy depends on mass and on the square of the speed.

Question 2

State the law of conservation of energy. Explain it with the help of a body falling from a certain height, in terms of its potential and kinetic energy.

Accepted Answer

Statement Law of conservation of energy: Energy can neither be created nor destroyed; it can only change from one form to another, and the total amount stays constant. Diagram At point A (top) The body is at rest, so Ek = 0 and Ep = mgh. Total = mgh. At point B (height x) Ep = mgx. It has fallen h − x, so v² = 2g(h − x) and Ek = ½mv² = mg(h − x). Total = mgx + mg(h − x) = mgh. At point C (ground) Ep = 0. It has fallen h, so v² = 2gh and Ek = ½m(2gh) = mgh. Total = mgh. A, mgh, 0, mgh; B, mgx, mg(h − x), mgh; C, 0, mgh, mgh Potential energy changes into kinetic energy, but the total stays mgh. On hitting the ground the kinetic energy becomes heat and sound; in real falls some energy also goes into air resistance.

Question 3

Differentiate between renewable and non-renewable sources of energy. Give three examples of each.

Accepted Answer

Renewable sources Definition: Sources that are replaced naturally after use and will not run out. Rain and snow keep refilling the reservoirs of dams, the Sun keeps shining and the wind keeps blowing, so these sources last. Non-renewable sources Definition: Sources that are used up and cannot be replaced in a human lifetime. Coal, oil and gas formed from plants and animals buried for millions of years. The stock is limited; once used, it takes millions of years to form again. Differences Replaced naturally, Not replaced in our lifetime; Will not run out, Will run out; Usually clean, Fossil fuels pollute and add CO₂; Solar, wind, hydroelectric (also tidal, geothermal, biomass), Coal, oil, natural gas (also nuclear fuel) Pakistan has strong sunshine and wind along the coast, so solar and wind power can reduce our dependence on imported oil.

Question 4

Explain what is meant by the efficiency of a machine. How is it calculated? Why is there a limit to the efficiency of a machine?

Accepted Answer

Definition Efficiency: The ratio of the useful output energy of a machine to the total energy put into it. Formula Efficiency = (useful output energy ÷ total input energy) × 100% Efficiency = (useful power output ÷ total power input) × 100% Diagram Example Efficiency = 700 ÷ 1000 × 100. 70%; 300 J is wasted Why there is a limit In every machine, moving parts rub, and friction turns some energy into heat.; Some energy is lost as sound and vibration.; So the useful output is always less than the input, and efficiency is always below 100%.; For the same reason a perfect (perpetual-motion) machine that runs for ever without input cannot be built. Efficiency can be raised (by lubrication, for example) but never to 100%.

Question 5

Describe the process of electricity generation by drawing a block diagram of the process in the following cases: (i) hydroelectric power generation (ii) fossil fuels.

Accepted Answer

(i) Hydroelectric power Water stored behind a dam at a height has gravitational potential energy. It flows down through tunnels (penstocks), gaining kinetic energy, and turns the blades of a turbine. The turbine turns a generator, which produces electrical energy. (ii) Fossil fuels Coal, oil or gas is burnt in a furnace. The heat boils water in a boiler to make high-pressure steam. The steam turns a turbine, which turns a generator to produce electricity. The steam is cooled, condensed back to water and used again. Block diagrams Starting energy, Gravitational potential energy of water, Chemical energy of fuel; What turns the turbine, Falling water, Steam; Renewable?, Yes, No; Pollution, None in use, Smoke and CO₂ In both, a turbine turns a generator; only the way the turbine is driven differs.

Question 6

Define work. Explain W = FS cos θ, and the cases in which work is positive, zero and negative.

Accepted Answer

Definition Work: Work is done when a force moves a body through a distance in the direction of the force. SI unit: joule. Force at an angle If the force F acts at an angle θ to the displacement S, resolve F into F cos θ along the motion and F sin θ at right angles to it. Only F cos θ moves the body, so: W = (F cos θ) × S = FS cos θ Positive, zero and negative work Positive work, 0° ≤ θ < 90°, Pulling a trolley forward; Zero work, θ = 90°, Carrying a bag on a level road; gravity on a satellite; Negative work, 90° < θ ≤ 180°, Friction on a sliding box; brakes on a car W = 20 × 3 × cos 60° = 20 × 3 × 0.5. 30 J

Question 7

What is power? Write its units and show that power = force × velocity.

Accepted Answer

Definition Power: The rate of doing work: P = W ÷ t. Units watt: 1 W = 1 J s⁻¹; kilowatt: 1 kW = 10³ W; megawatt: 1 MW = 10⁶ W Power in terms of force and velocity P = W ÷ t, and W = F × S.; So P = F × S ÷ t.; S ÷ t = v, the speed.; Therefore P = Fv. P = Fv Example Work = mgh = 40 × 10 × 10 = 4000 J. P = 4000 ÷ 8. 500 W The same work done in less time means more power.

Question 8

What is potential energy? Derive Ep = mgh and describe the types of potential energy.

Accepted Answer

Definition Potential energy: The energy a body has because of its position or deformation. Derivation of gravitational potential energy Lift a body of mass m at constant speed from the ground to a height h.; The force needed equals its weight: F = mg.; Work done = F × h = mgh.; This work is stored in the body as gravitational potential energy. Ep = mgh Types Gravitational, Height above the reference level, Water in a dam; Elastic, Stretching or compressing, A wound clock spring, a bow; Chemical, Arrangement of atoms in molecules, Food, petrol, batteries Reference level Height is measured from a chosen level (usually the ground), so potential energy is always relative to that level. Ep = 0.18 × 10 × 12. 21.6 J

Question 9

Explain energy transformations with examples from daily life.

Accepted Answer

Meaning Energy is useful because it can change from one form to another. In every change the total energy stays the same, though some becomes heat that cannot be used again. Examples Photosynthesis, Light → chemical (food); Our body, Chemical (food) → kinetic + heat; Electric bulb, Electrical → light + heat; Electric motor, fan, Electrical → kinetic; Generator, dynamo, Kinetic → electrical; Loudspeaker, Electrical → sound; Solar cell, Light → electrical; Car engine, Chemical → heat → kinetic; Bow and arrow, Elastic potential → kinetic Energy is never lost, but some is always wasted as heat.

Question 10

Describe solar energy. How are solar cells and solar panels used? Give the advantages and disadvantages of solar energy.

Accepted Answer

Solar energy Definition: The energy we get from sunlight. The Sun is the biggest source of energy for the Earth. Uses of solar energy Solar heaters and cookers: black surfaces absorb sunlight to heat water or cook food.; Solar cells (photovoltaic cells): change light directly into electricity.; Solar panels: one cell gives a very small voltage, so many cells are joined in series to give a useful voltage, for homes, street lights, tube wells and calculators. Advantages Free and renewable.; No pollution or noise.; Useful in remote villages far from power lines. Disadvantages No output at night and little on cloudy days.; Panels and batteries are costly.; Large areas are needed for large amounts of power. Pakistan gets strong sunshine most of the year, so solar energy is a good choice here.

Question 11

Explain nuclear, geothermal, wind, tidal and biomass energy, with one advantage and one disadvantage of each.

Accepted Answer

Nuclear energy When the nucleus of a heavy atom such as uranium splits (fission), a large amount of heat is released. It boils water into steam to drive turbines. Geothermal energy Deep underground the rocks are very hot. Water pumped down turns into steam, which comes up to drive turbines. Wind energy Moving air turns the large blades of a wind turbine, which drives a generator. Tidal and wave energy The Moon's pull makes the sea rise and fall twice a day. Water flowing in and out through a barrage turns turbines; waves can also drive generators. Biomass energy Wood, crop waste and animal dung are burnt for heat, or allowed to rot in a digester to make biogas. Comparison Nuclear, Huge energy from little fuel, Dangerous radioactive waste; Geothermal, Works day and night, Found only in a few places; Wind, No fuel, no pollution, Depends on the wind; noisy; Tidal, Predictable, Only on coasts; costly; Biomass, Uses waste, Smoke when burnt All of these except nuclear are renewable.

Renewable	Non-renewable
Replaced naturally	Not replaced in our lifetime
Will not run out	Will run out
Usually clean	Fossil fuels pollute and add CO₂
Solar, wind, hydroelectric (also tidal, geothermal, biomass)	Coal, oil, natural gas (also nuclear fuel)

	Hydroelectric	Fossil fuel
Starting energy	Gravitational potential energy of water	Chemical energy of fuel
What turns the turbine	Falling water	Steam
Renewable?	Yes	No
Pollution	None in use	Smoke and CO₂

Case	Angle	Example
Positive work	0° ≤ θ < 90°	Pulling a trolley forward
Zero work	θ = 90°	Carrying a bag on a level road; gravity on a satellite
Negative work	90° < θ ≤ 180°	Friction on a sliding box; brakes on a car

Type	Stored by	Example
Gravitational	Height above the reference level	Water in a dam
Elastic	Stretching or compressing	A wound clock spring, a bow
Chemical	Arrangement of atoms in molecules	Food, petrol, batteries

Device or process	Energy change
Photosynthesis	Light → chemical (food)
Our body	Chemical (food) → kinetic + heat
Electric bulb	Electrical → light + heat
Electric motor, fan	Electrical → kinetic
Generator, dynamo	Kinetic → electrical
Loudspeaker	Electrical → sound
Solar cell	Light → electrical
Car engine	Chemical → heat → kinetic
Bow and arrow	Elastic potential → kinetic

Source	Advantage	Disadvantage
Nuclear	Huge energy from little fuel	Dangerous radioactive waste
Geothermal	Works day and night	Found only in a few places
Wind	No fuel, no pollution	Depends on the wind; noisy
Tidal	Predictable	Only on coasts; costly
Biomass	Uses waste	Smoke when burnt

Long Questions with Answers

Set A: Textbook comprehensive questions 5 questions

What is meant by kinetic energy? State its unit. Describe how it is determined.

Definition

Principle

Derivation

Example

State the law of conservation of energy. Explain it with the help of a body falling from a certain height, in terms of its potential and kinetic energy.

Statement

Diagram

At point A (top)

At point B (height x)

At point C (ground)

Differentiate between renewable and non-renewable sources of energy. Give three examples of each.

Renewable sources

Non-renewable sources

Differences

Explain what is meant by the efficiency of a machine. How is it calculated? Why is there a limit to the efficiency of a machine?

Definition

Formula

Diagram

Example

Why there is a limit

Describe the process of electricity generation by drawing a block diagram of the process in the following cases: (i) hydroelectric power generation (ii) fossil fuels.

(i) Hydroelectric power

(ii) Fossil fuels

Block diagrams

Set B: More important long questions 6 questions

Define work. Explain W = FS cos θ, and the cases in which work is positive, zero and negative.

Definition

Force at an angle

Positive, zero and negative work

What is power? Write its units and show that power = force × velocity.

Definition

Units

Power in terms of force and velocity

Example

What is potential energy? Derive Ep = mgh and describe the types of potential energy.

Definition

Derivation of gravitational potential energy

Types

Reference level

Explain energy transformations with examples from daily life.

Meaning

Examples

Describe solar energy. How are solar cells and solar panels used? Give the advantages and disadvantages of solar energy.

Solar energy

Uses of solar energy

Advantages

Disadvantages

Explain nuclear, geothermal, wind, tidal and biomass energy, with one advantage and one disadvantage of each.

Nuclear energy

Geothermal energy

Wind energy

Tidal and wave energy

Biomass energy

Comparison

The rest of Chapter 5

Long Questions from other Physics chapters