Chapter 4 Part 2: WORK AND ENERGY S. Chand Class 9 Solution.

Very Short Answer Type Questions

1. Name the commercial unit of measurement of energy.

The commercial unit of measurement of energy is the kilowatt-hour (kWh).

2. Define kilowatt-hour.

Kilowatt-hour is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h).

3. Name two units of power bigger than watt.

Two units of power bigger than watt are kilowatt (kW) and megawatt (MW).

4. Define the term ‘watt’.

A watt is the SI unit of power, defined as one joule per second (1 W = 1 J/s). It measures the rate of energy conversion or transfer.

5. How many watts equal one horse power ?

One horsepower is equivalent to approximately 746 watts.

6. Name the physical quantity whose unit is watt.

The physical quantity whose unit is watt is power.

7. What is the power of a body which is doing work at the rate of one joule per second ?

The power of a body which is doing work at the rate of one joule per second is one wat

8. A body does 1200 joules of work in 2 minutes. Calculate its power.

To calculate the power, we use the formula: $\text{Power}=\frac{\text{Work}}{\text{Time}}$ Given that the work is 1200 joules and the time is 2 minutes (which is $2\times 60=120$ seconds): $\text{Power}=\frac{1200\text{ J}}{120\text{ s}}=10\text{ watts}$

9. How many joules are there in one kilowatt-hour ?

One kilowatt-hour (kWh) is equal to 3.6 million joules (3.6 × 10^6 J).

10. Name the quantity whose unit is :

(a) kilowatt

(b) kilowatt-hour

(a) The quantity whose unit is kilowatt is power. (b) The quantity whose unit is kilowatt-hour is energy.

11. What is the common name of ‘1 kWh’ of electrical energy ?

The common name of '1 kWh' of electrical energy is "unit" or "electricity unit."

12. A cell converts one form of energy into another form. Name the two forms.

A cell converts one form of energy into another form. The two forms are chemical energy (stored in the cell's chemicals) and electrical energy (produced when the cell operates).

13. Name the device which converts electrical energy into mechanical energy.

The device which converts electrical energy into mechanical energy is an electric motor.

14. Name the devices or machines which convert :

(a) Mechanical energy into electrical energy.

(b) Chemical energy into electrical energy.

(c) Electrical energy into heat energy.

(d) Light energy into electrical energy.

(e) Electrical energy into light energy.

(a) Mechanical energy into electrical energy: Generator or dynamo. (b) Chemical energy into electrical energy: Battery or fuel cell. (c) Electrical energy into heat energy: Electric heater or resistor. (d) Light energy into electrical energy: Photovoltaic cell or solar panel. (e) Electrical energy into light energy: Light bulb or LED

15. Name the devices or machines which convert :

(i) Electrical energy into sound energy.

(ii) Heat energy into kinetic energy (or mechanical energy).

(iii) Chemical energy into kinetic energy (or mechanical energy).

(iv) Chemical energy into heat energy.

(v) Light energy into heat energy.

(i) Electrical energy into sound energy: Loudspeaker or buzzer. (ii) Heat energy into kinetic energy (or mechanical energy): Steam engine or turbine. (iii) Chemical energy into kinetic energy (or mechanical energy): Engine (like internal combustion engine). (iv) Chemical energy into heat energy: Combustion (burning fuel). (v) Light energy into heat energy: Absorbing surfaces or objects.

16. Fill in the following blanks with suitable words :

(a) Power is the rate of doing work. (b) 1 watt is a rate of working of one joule per second.

(c) The electricity meter installed in our home measures electric energy in the units of kilowatt-hour (kWh).

(d) The principle of conservation of energy says that energy can be transformed from one form to another, but it cannot be created or destroyed.

(e) When a ball is thrown upwards, potential energy is transformed into kinetic energy.

Short Answer Type Questions

17. A trolley is pushed along a road with a force of 400 N through a distance of 60 m in 1 minute. Calculate the

power developed.

17. To calculate power, we use the formula: $\text{Power}=\frac{\text{Work}}{\text{Time}}$

Given that force = 400 N, distance = 60 m, and time = 1 minute = 60 seconds:

$\text{Work}=\text{Force}\times \text{Distance}=400\text{N}\times 60\text{m}=24000\text{J}$

$\text{Power}=\frac{24000\text{J}}{60\text{s}}=400\text{W}$

So, the power developed is 400 watts.

18. What kind of energy transformations take place at a hydroelectric power station ?

At a hydroelectric power station, the energy transformation involves converting the potential energy of water stored in reservoirs to kinetic energy by allowing it to flow through turbines. The turbines then convert this kinetic energy into mechanical energy, which drives generators to produce electrical energy.

19. What kind of energy transformations take place at a coal-based thermal power station ?

At a coal-based thermal power station, the energy transformation involves burning coal to produce heat energy. This heat energy is then used to boil water and produce steam, which drives turbines. The turbines, in turn, convert the kinetic energy of the steam into mechanical energy, which drives generators to produce electrical energy.

20. A man weighing 500 N carried a load of 100 N up a flight of stairs 4 m high in 5 seconds. What is the

power ?

18. To calculate power, we use the formula: $\text{Power}=\frac{\text{Work}}{\text{Time}}$

Given that weight = 500 N, load = 100 N, height = 4 m, and time = 5 seconds:

$\text{Work}=(\text{Weight}+\text{Load})\times \text{Height}=(500\text{N}+100\text{N})\times 4\text{m}=2400\text{J}$

$\text{Power}=\frac{2400\text{J}}{5\text{s}}=480\text{W}$

So, the power is 480 watts.

21. The power output of an engine is 3 kW. How much work does the engine do in 20 s ?

21. Given that power output = 3 kW and time = 20 s:

$\text{Power}=\frac{\text{Work}}{\text{Time}}$

$\text{Work}=\text{Power}\times \text{Time}=3\text{kW}\times 20\text{s}=60\text{kJ}$

So, the engine does 60 kJ of work in 20 s.

22. An electric heater uses 600 kJ of electrical energy in 5 minutes. Calculate its power rating.

22. To calculate power rating, we use the formula: $\text{Power}=\frac{\text{Energy}}{\text{Time}}$

Given that energy = 600 kJ and time = 5 minutes = 300 seconds:

$\text{Power}=\frac{600\text{kJ}}{300\text{s}}=2000\text{W}$

So, the power rating is 2000 watts.

23. How much electrical energy in joules does a 100 watt lamp consume :

(a) in 1 second ?

(b) in 1 minute ?

(a) For a 100-watt lamp in 1 second: $\text{Energy}=\text{Power}\times \text{Time}=100\text{W}\times 1\text{s}=100\text{J}$

(b) For a 100-watt lamp in 1 minute = 60 seconds: $\text{Energy}=\text{Power}\times \text{Time}=100\text{W}\times 60\text{s}=6000\text{J}$

24. Five electric fans of 120 watts each are used for 4 hours. Calculate the electrical energy consumed in kilowatt-

hours.

Total power of 5 fans=5×120W=600W $\text{Total time}=4\text{hours}=4\times 60\text{minutes}=240\text{minutes}$

$\text{Total energy consumed}=\text{Power}\times \text{Time}=600\text{W}\times 240\text{minutes}=144000\text{J}$

Converting joules to kilowatt-hours: $\text{Total energy consumed}=\frac{144000\text{J}}{3600000}=0.04\text{kWh}$

So, the electrical energy consumed by the five electric fans is 0.04 kWh.

25. Describe the energy changes which take place in a radio.

In a radio, electrical energy from a power source (such as a battery or mains electricity) is converted into various forms:

• Electrical energy is transformed into sound energy in the speakers.
• Electrical energy is transformed into electromagnetic waves (radio waves) in the antenna.
• Electrical energy is transformed into light and heat energy in the electronic components due to resistive losses.

26. Write the energy transformations which take place in an electric bulb (or electric lamp).

In an electric bulb (or electric lamp), energy transformations occur as follows:

• Electrical energy from the power source is converted into light energy and heat energy.
• The filament of the bulb, heated due to the flow of electricity, emits light through incandescence.
• A portion of electrical energy is also transformed into heat energy due to the resistance of the filament.

27. Name five appliances or machines which use an electric motor.

Five appliances or machines which use an electric motor are:

• Washing machines
• Refrigerators
• Electric fans
• Blenders
• Vacuum cleaners

28. A bulb lights up when connected to a battery. State the energy change which takes place : (i) in the battery. (ii) in the bulb.

When a bulb lights up when connected to a battery:

(i) In the battery: Chemical energy stored in the battery is converted into electrical energy.

(ii) In the bulb: Electrical energy is converted into light energy and heat energy in the filament of the bulb.

29. The hanging bob of a simple pendulum is displaced to one extreme position B and then released. It swings towards centre position A and then to the other extreme position C. In which position does the bob have : (i) maximum potential energy ? (ii) maximum kinetic energy ? Give reasons for your answer.

For a simple pendulum swinging from position B to A to C:

(i) Maximum potential energy is at position B, the highest point of the swing, as potential energy is directly proportional to height. At this point, the bob is farthest from the ground.

(ii) Maximum kinetic energy is at position A, the lowest point of the swing, as kinetic energy is directly proportional to velocity. At this point, the bob has the highest velocity and thus maximum kinetic energy.

30. A car of weight 20000 N climbs up a hill at a steady speed of 8 m/s, gaining a height of 120 m in 100 s. Calculate : (a) work done by the car. (b) power of engine of car.

For the car climbing a hill:

(a) Work done by the car is equal to the change in its potential energy. It is given by the formula: $\text{Work}=\text{Force}\times \text{Distance}=\text{Weight}\times \text{Height}$ $\text{Work}=20000\text{N}\times 120\text{m}=2,400,000\text{J}$

(b) Power of the engine of the car can be calculated using the formula: $\text{Power}=\frac{\text{Work}}{\text{Time}}$ $\text{Power}=\frac{2400000\text{J}}{100\text{s}}=24000\text{W}$ $\text{Power}=24\text{kW}$

Long Answer Type Questions

31. (a) What do you understand by the term “transformation of energy” ? Explain with an example.

(b) Explain the transformation of energy in the following cases :

(i) A ball thrown upwards.

(ii) A stone dropped from the roof of a building.

(a) The term "transformation of energy" refers to when energy changes from one form to another. Energy can be in different forms like movement, stored energy, heat, electricity, and more. When energy changes from one form to another, it's called an energy transformation. For example, when you turn on a light bulb, electrical energy is transformed into light and heat energy. (b) (i) When you throw a ball upwards: At first, the ball has energy because it's moving. This energy is called kinetic energy. As the ball goes up, it slows down because gravity pulls it back down. So, its kinetic energy decreases, but its potential energy increases because it's higher up. At the top, the ball stops moving for a moment. This is when it has the most potential energy and the least kinetic energy. As the ball falls back down, its potential energy decreases, but its kinetic energy increases until it reaches the ground again. So, energy transforms between kinetic and potential energy. (ii) When you drop a stone from a building: The stone starts with potential energy because it's high up. As it falls, gravity pulls it down, and its potential energy changes into kinetic energy because it's moving faster. Right before it hits the ground, all of its potential energy has turned into kinetic energy. When it hits the ground, some of its kinetic energy changes into sound and heat energy. So, here, energy transforms from potential to kinetic energy, and some of it becomes sound and heat energy upon impact.

32. (a) State and explain the law of conservation of energy with an example.

(b) Explain how, the total energy a swinging pendulum at any instant of time remains conserved. Illustrate

your answer with the help of a labelled diagram.

(a) The law of conservation of energy states that the total energy of an isolated system remains constant over time. Energy can neither be created nor destroyed; it can only change from one form to another. This means that the total energy before a process is equal to the total energy after the process, even though the energy may change forms during the process.

For example, consider a roller coaster at the top of a hill. At this point, it has a high potential energy due to its height above the ground and zero kinetic energy because it's not moving. As the roller coaster descends the hill, its potential energy decreases while its kinetic energy increases because it gains speed. However, according to the law of conservation of energy, the total mechanical energy (potential energy + kinetic energy) remains constant throughout the ride. Some of the potential energy is converted into kinetic energy as the roller coaster moves, but the total energy remains the same.

(b) The total energy of a swinging pendulum remains conserved because the pendulum's motion involves the interconversion of potential energy (due to its height) and kinetic energy (due to its motion). At any instant during its swing, the sum of its potential and kinetic energy remains constant, assuming no energy losses due to friction or air resistance.

Let's illustrate this with a labelled diagram of a swinging pendulum:

At the highest point (position A) of its swing, the pendulum has maximum potential energy due to its height above the lowest point (position B). However, it has minimum kinetic energy because it momentarily stops moving at this point.

At the lowest point (position B) of its swing, the pendulum has maximum kinetic energy because it's moving at its fastest. However, it has minimum potential energy because it's at its lowest height above the ground.

As the pendulum swings back and forth, the potential energy is converted into kinetic energy and vice versa. However, the sum of the pendulum's potential and kinetic energy remains constant, demonstrating the conservation of energy.

33. (a) What is the meaning of the symbol kWh ? What quantity does it represent ? (b) How much electric energy in kWh is consumed by an electrical appliance of 1000 watts when it is switched on for 60 minutes ?

(a) The symbol kWh stands for kilowatt-hour. It represents a unit of electrical energy. One kilowatt-hour is the amount of energy consumed by a device with a power of one kilowatt (1000 watts) operating for one hour.

(b) To calculate the electric energy consumed by the appliance in kWh, we first need to convert the time from minutes to hours, as one hour is the standard unit for measuring energy in kilowatt-hours.

Given: Power of the electrical appliance = 1000 watts Time it is switched on = 60 minutes

Converting time from minutes to hours: $\text{Time in hours}=\frac{\text{Time in minutes}}{60}=\frac{60}{60}=1\text{ hour}$

Now, to find the electric energy consumed: $\text{Energy}=\text{Power}\times \text{Time}$

$\text{Energy}=1000\text{W}\times 1\text{h}$

$\text{Energy}=1000\text{Wh}$

Since 1 kilowatt-hour (kWh) is equal to 1000 watt-hours (Wh): $\text{Energy in kWh}=\frac{1000\text{Wh}}{1000}=1\text{kWh}$

So, the electrical appliance consumes 1 kWh of electric energy when it is switched on for 60 minutes.

34. (a) Derive the relation between commercial unit of energy (kWh) and SI unit of energy (joule). (b) A certain household consumes 650 units of electricity in a month. How much is this electricity in joules ?

(a) To derive the relation between the commercial unit of energy (kWh) and the SI unit of energy (joule), we need to understand that 1 kilowatt-hour (kWh) is equivalent to the energy consumed by a device with a power of 1 kilowatt (1000 watts) operating for one hour.

First, let's find the energy in joules represented by 1 kWh: $1\text{kWh}=1\text{kilowatt}\times 1\text{hour}$

Using the definition of power: $\text{Power}=\frac{\text{Energy}}{\text{Time}}$

We know that 1 kilowatt (kW) = 1000 watts (W), so 1 kWh = 1000 W for 1 hour.

$\text{Energy}=\text{Power}\times \text{Time}$

$\text{Energy}=1000\text{W}\times 3600\text{s}$

$\text{Energy}=3,600,000\text{J}$

So, 1 kWh is equal to 3,600,000 joules (J).

(b) To find out how much electricity in joules is consumed in a month, we need to multiply the number of units consumed by the conversion factor we derived above.

Given: Electricity consumption = 650 units (kWh)

$\text{Electricity consumption in joules}=650\text{units}\times 3,600,000\text{J/unit}$

$\text{Electricity consumption in joules}=650\times 3,600,000\text{J}$

$\text{Electricity consumption in joules}=2,340,000,000\text{J}$

So, the electricity consumption for the month is 2,340,000,000 joules.

35. (a) Define power. Give the SI unit of power. (b) A boy weighing 40 kg carries a box weighing 20 kg to the top of a building 15 m high in 25 seconds. Calculate the power. (g = 10 m/s2)

(a) Power is defined as the rate at which work is done or the rate at which energy is transferred or converted. Mathematically, power (P) is calculated as the amount of work done (W) or energy transferred (E) divided by the time (t) taken to do the work or transfer the energy:

$�=\frac{�}{�}$

or

$�=\frac{�}{�}$

The SI unit of power is the watt (W), named after the Scottish engineer James Watt. One watt is defined as one joule per second (1 W = 1 J/s).

(b) To calculate the power, we first need to determine the work done by the boy in carrying the box to the top of the building. Work is calculated as the force exerted (weight) multiplied by the distance traveled vertically (height):

$\text{Work}=\text{Force}\times \text{Distance}$

Given: Boy's weight (force exerted) = 40 kg Box's weight = 20 kg Height of the building = 15 m Time taken = 25 seconds Acceleration due to gravity (g) = 10 m/s²

Total weight lifted = Boy's weight + Box's weight = 40 kg + 20 kg = 60 kg

$\text{Work}=\text{Force}\times \text{Distance}=(60\text{kg}\times 10{\text{m/s}}^2)\times 15\text{m}$

$\text{Work}=60\times 10\times 15\text{kg}{\text{m}}^2\mathrm{/}{\text{s}}^2=9000\text{J}$

Now, to calculate power: $�=\frac{�}{�}$

$�=\frac{9000\text{J}}{25\text{s}}$

$�=360\text{W}$

So, the power exerted by the boy in carrying the box to the top of the building is 360 watts.

Multiple Choice Questions (MCQs)

36. When an object falls freely towards the earth, then its total energy :

(a) increases (b) decreases

(c) remains constant (d) first increases and then decreases

37. Which one of the following is not the unit of energy ?

(a) joule (b) newton-metre (c) kilowatt (d) kilowatt-hour

38. Which of the following energy change involves frictional force ?

(a) chemical energy to heat energy (b) kinetic energy to heat energy

(c) potential energy to sound energy (d) chemical energy to kinetic energy

39. Which one of the following statements about power stations is not true ?

(a) hydroelectric power stations use water to drive turbines

(b) in a power station, turbines drive generators

(c) electricity from thermal power stations differs from that produced in hydroelectric power stations

(d) in hydroelectric power stations and thermal power stations, alternators produce electricity

40. An electric motor raises a load of 0.2 kg, at a constant speed, through a vertical distance of 3.0 m in 2 s. If

the acceleration of free fall is 10 m/s2, the power in W developed by the motor in raising the load is :

(a) 0.3 (b) 1.2 (c) 3.0 (d) 6.0

To find the power developed by the motor, we can use the formula:

$\text{Power}=\text{Force}\times \text{Velocity}$

Given that the load is raised at a constant speed, we know that the net force exerted by the motor is equal to the weight of the load. The weight of the load is given by:

$\text{Force}=\text{mass}\times \text{acceleration due to gravity}$

$\text{Force}=0.2\text{kg}\times 10{\text{m/s}}^2$

$\text{Force}=2\text{N}$

The velocity is given by the formula:

$\text{Velocity}=\frac{\text{Distance}}{\text{Time}}$

$\text{Velocity}=\frac{3.0\text{m}}{2\text{s}}$

$\text{Velocity}=1.5\text{m/s}$

Now, we can find the power:

$\text{Power}=2\text{N}\times 1.5\text{m/s}$

$\text{Power}=3\text{W}$

Therefore, the power developed by the motor in raising the load is 3 watts.

41. An object is falling freely from a height x. After it has fallen a height 2

x , it will possess :

(a) only potential energy (b) only kinetic energy

(c) half potential and half kinetic energy (d) less potential and more kinetic energy

42. The commercial unit of energy is :

(a) watt (b) watt-hour (c) kilowatt-hour (d) kilowatt

43. How much energy does a 100 W electric bulb transfer in 1 minute ?

(a) 100 J (b) 600 J (c) 3600 J (d) 6000 J

Given: Power of the electric bulb = 100 W Time = 1 minute = 60 seconds

First, let's convert the power from watts to joules per second (W to J/s): $1\text{ watt}=1\text{ joule/second}$

So, the power of the electric bulb in joules per second (or watts) is 100 J/s.

Now, to find the energy transferred: $\text{Energy}=\text{Power}\times \text{Time}$ $\text{Energy}=100\text{J/s}\times 60\text{s}$ $\text{Energy}=6000\text{J}$

Therefore, a 100 W electric bulb transfers 6000 joules of energy in 1 minute.

44. The device which converts mechanical energy into energy which runs our microwave oven is :

(a) electric motor (b) alternator (c) turbine (d) electric heater

45. A microphone converts :

(a) electrical energy into sound energy in ordinary telephone

(b) microwave energy into sound energy in a mobile phone

(c) sound energy into mechanical energy in a stereo system

(d) sound energy into electrical energy in public address system

Questions Based on High Order Thinking Skills (HOTS)

46. The following data was obtained for a body of mass 1 kg dropped from a height of 5 metres :

Distance above ground Velocity

5 m 0 m/s

3.2 m 6 m/s

0 m 10 m/s

Show by calculations that the above data verifies the law of conservation of energy (Neglect air resistance).

(g = 10 m/s2).

To verify the law of conservation of energy for the given data, we'll calculate the potential energy and kinetic energy at each point and ensure that the total mechanical energy (sum of potential and kinetic energies) remains constant.

The potential energy (PE) of the body at a height h above the ground is given by: $��=��ℎ$ where:

• m is the mass of the body (1 kg),
• g is the acceleration due to gravity (10 m/s²), and
• h is the height above the ground.

The kinetic energy (KE) of the body with a velocity v is given by: $��=\frac{1}{2}�{�}^2$

Let's calculate the potential energy (PE) and kinetic energy (KE) at each point:

1. At a height of 5 m above the ground:

   • PE = mgh = (1 kg)(10 m/s²)(5 m) = 50 J
   • KE = 0.5mv^2 = 0.5(1 kg)(0 m/s)² = 0 J
   • Total mechanical energy (PE + KE) = 50 J + 0 J = 50 J

2. At a height of 3.2 m above the ground:

   • PE = mgh = (1 kg)(10 m/s²)(3.2 m) = 32 J
   • KE = 0.5mv^2 = 0.5(1 kg)(6 m/s)² = 18 J
   • Total mechanical energy (PE + KE) = 32 J + 18 J = 50 J

3. At the ground level (0 m above the ground):

   • PE = mgh = (1 kg)(10 m/s²)(0 m) = 0 J
   • KE = 0.5mv^2 = 0.5(1 kg)(10 m/s)² = 50 J
   • Total mechanical energy (PE + KE) = 0 J + 50 J = 50 J

As we can see, the total mechanical energy remains constant at 50 J at each point. This verifies the law of conservation of energy for the given data.

47. A ball falls to the ground as shown below : A potential energy = 80 J kinetic energy = 0 B kinetic energy = 48 J C potential energy = 0 (a) What is the kinetic energy of ball when it hits the ground ? (b) What is the potential energy of ball at B ? (c) Which law you have made use of in answering this question ?

(a) When the ball hits the ground, all its potential energy is converted into kinetic energy. Given that the potential energy at point C is 0 J, the kinetic energy at point C (when it hits the ground) will be equal to the initial potential energy at point A.

So, the kinetic energy of the ball when it hits the ground is 80 J.

(b) At point B, the kinetic energy is given as 48 J. Since the total mechanical energy (sum of potential energy and kinetic energy) remains constant, we can calculate the potential energy at point B by subtracting the kinetic energy at point B from the total mechanical energy at point A.

Total mechanical energy at point A (A potential energy) = 80 J Kinetic energy at point B = 48 J

Potential energy at B = Total mechanical energy at A - Kinetic energy at B Potential energy at B = 80 J - 48 J = 32 J

So, the potential energy of the ball at point B is 32 J.

(c) The law used in answering this question is the law of conservation of mechanical energy. According to this law, the total mechanical energy (sum of kinetic energy and potential energy) of a system remains constant if only conservative forces are acting on it. In this case, as the ball falls, its potential energy decreases while its kinetic energy increases, but the total mechanical energy remains constant. This allows us to calculate the unknown energies at different points using the given data.

48. In an experiment to measure his power, a student records the time taken by him in running up a flight of steps on a staircase. Use the following data to calculate the power of the student : Number of steps = 28 Height of each step = 20 cm Time taken = 5.4 s Mass of student = 55 kg Acceleration = 9.8 m s–2 due to gravity

To calculate the power of the student, we need to determine the work done by the student in climbing the stairs and then divide it by the time taken.

First, let's calculate the total vertical distance climbed by the student: $\text{Total vertical distance}=\text{Number of steps}\times \text{Height of each step}$ $\text{Total vertical distance}=28\text{steps}\times 0.20\text{m/step}$ $\text{Total vertical distance}=5.6\text{m}$

Now, let's calculate the work done by the student against gravity: $\text{Work}=\text{Force}\times \text{Distance}$ $\text{Force}=\text{Mass}\times \text{Acceleration}$ $\text{Force}=55\text{kg}\times 9.8{\text{m/s}}^2$ $\text{Force}=539\text{N}$

$\text{Work}=\text{Force}\times \text{Distance}$ $\text{Work}=539\text{N}\times 5.6\text{m}$ $\text{Work}=3018.4\text{J}$

Now, we can calculate the power: $\text{Power}=\frac{\text{Work}}{\text{Time}}$ $\text{Power}=\frac{3018.4\text{J}}{5.4\text{s}}$ $\text{Power}\approx 558.67\text{W}$

Therefore, the power of the student is approximately 558.67 watts.

49. In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m. (a) How much work does he do in lifting one box ? (b) How much energy is transferred when one box is lifted ? (c) If the man lifts 4 boxes per minute, at what power is he working ? (g = 10 m s–2)

(a) To calculate the work done in lifting one box, we use the formula for work: $\text{Work}=\text{Force}\times \text{Distance}$ Given that the force exerted (the weight of one box) is 100 N and the distance lifted is 1.5 m, we have: $\text{Work}=100\text{N}\times 1.5\text{m}$ $\text{Work}=150\text{J}$

So, the man does 150 joules of work in lifting one box.

(b) The energy transferred when one box is lifted is the same as the work done, which is 150 joules.

(c) To find the power at which the man is working, we use the formula for power: $\text{Power}=\frac{\text{Work}}{\text{Time}}$ Given that the man lifts 4 boxes per minute, we need to convert this to seconds: $\text{Time}=\frac{4\text{boxes}}{1\text{minute}}\times \frac{1\text{minute}}{60\text{s}}$ $\text{Time}=\frac{4}{60}\text{s/box}$ $\text{Time}=\frac{1}{15}\text{s/box}$

Now, let's calculate the power: $\text{Power}=\frac{150\text{J}}{\frac{1}{15}\text{s/box}}$ $\text{Power}=150\times 15\text{W}$ $\text{Power}=2250\text{W}$

So, the man is working at a power of 2250 watts.

50. Name the energy transfers which occur when : (a) an electric bell rings (b) someone speaks into a microphone (c) there is a picture on a television screen (d) a torch is on

(a) When an electric bell rings, the energy transfers involved include electrical energy being converted into sound energy and mechanical energy. The electrical energy powers the electromagnet, causing it to attract the metal striker, which produces the sound.

(b) When someone speaks into a microphone, the energy transfer involves sound energy being converted into electrical energy. The sound waves from the person's voice cause the microphone diaphragm to vibrate, which generates electrical signals.

(c) When there is a picture on a television screen, the energy transfer involves electrical energy being converted into light energy and electromagnetic radiation. The electrical signals from the television signal are translated into varying intensities of light by the phosphors on the screen, which emit light to create the visual image.

(d) When a torch is on, the energy transfer involves chemical energy being converted into light energy and heat energy. The chemical energy stored in the battery is converted into electrical energy, which powers the light bulb (converting electrical energy into light energy). Some of the energy is also dissipated as heat due to the resistance in the circuit and the heating of the light bulb filament.

The provided answers are accurate based on my understanding. However, there is always a possibility of errors, so if you notice any inaccuracies, please feel free to let me know by commenting in the comment box below. We can work together to resolve any issues.