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# Selina solutions Selina ICSE Concise Physics for Class 10 chapter 2 Work, Energy and Power

## Chapter 2 - Work, Energy and Power

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Define work. Is work a scalar or a vector?

How is the work done by a force measured when (i) force is in direction of displacement, (ii) force is at an angle to the direction of displacement?

A force F acts on a body and displaces it by a distance S in a direction at an angle θ with the direction of force. (a) Write the expression for the work done by the force. (b) what should be the angle between the force and displacement to get the (i) zero work (ii) maximum work?

A body is acted upon by a force. State two condition when the work done is zero.

State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the help of examples.

A body is moved in a direction opposite to the direction of force acting on it. State whether the work is done by the force or work is done against the force

When a body moves in a circular path, how much work is done by the body? Give reason.

A satellite revolves around the earth in a circular orbit. What is the work done by the force of gravity? Give reason.

In which of the following cases, is work being done?
(i) A man pushing a wall.
(ii) a coolie standing with a load of 12 kgf on his head.
(iii) A boy climbing up a staircase.

A coolie carrying a load on his head and moving on a frictionless horizontal platform does no work. Explain the reason

The work done by a fielder when he takes a catch in a cricket match, is negative Explain.

Give an example when work done by the force of gravity acting on a body is zero even though the body gets displaces from its initial position.

What are the S.I. and C.G.S units of work? How are they related? Establish the relationship.

State and define the S.I. unit of work.

Express joule in terms of erg.

A body of mass m falls down through a height h. Obtain an expression for the work done by the force of gravity.

A boy of mass m climbs up a staircase of vertical height h.
(a) What is the work done by the boy against the force of gravity?
(b) What would have been the work done if he uses a lift in climbing the same vertical height?

Define the term energy and state its S.I. unit.

What physical quantity does the electron volt (eV) measure? How is it related to the S.I. unit of that quality?

Complete the following sentence:
1 J = Calorie

Name the physical quantity which is measured in calorie. How is it related to the S.I. unit of the quality?

Define a kilowatt hour. How is it related to joule?

Define the term power. State its S.I. unit.

State two factors on which power spent by a source depends. Explain your answer with examples.

Differentiate between work and power.

Differentiate between energy and power.

State and define the S.I. unit of power.

What is horse power (H.P)? How is it related to the S.I. unit of power

Differentiate between watt and watt hour.

Name the quality which is measured in kWh.

Name the quality which is measured in kW.

Name the quality which is measured in Wh.

Name the quality which is measured in eV

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MULTIPLE CHOICE TYPE:

One horse power is equal to:
(a) 1000 W
(b) 500 W
(c) 764 W
(d) 746 W

MULTIPLE CHOICE TYPE:

kWh is the unit of:
(a) power
(b) force
(c) energy
(d) none of these

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A body, when acted upon by a force of 10 kgf, gets displaced by 0.5 m. Calculate the work done by the force, when the displacement is (i) in the direction of force, (ii) at an angle of 60° with the force, and (iii) normal to the force. (g = 10 N kg-1)

A boy of mass kg runs upstairs and reaches the 8 m high floor in 5 s Calculate:
the force of gravity acting on the boy.
(i) the work done by him against gravity.
(ii) the power spent by boy.
(Take g = 10 m s-2)

It takes 20 s for a person A to climb up the stairs, while another person B does the same in 15 s. Compare the (i) Work done and (ii) power developed by the persons A and B.

A boy weighing 350 N runs up a flight of 30 steps, each 20 cm high in 1 minute, Calculate:
(i) the work done and
(ii) power spent.

A man spends 6.4 KJ energy in displacing a body by 64 m in the direction in which he applies force, in 2.5 s Calculate:
(i) the force applied and
(ii) the power Spent (in H.P) by the man.

A weight lifter a load of 200 kgf to a height of 2.5 m in 5 s. Calculate: (i) the work done, and (ii) the power developed by him. Take g = 10 N kg-1

A machine raises a load of 750 N through a height of 16 m in 5 s. calculate:
(i) energy spent by machine,
(ii) power at which the machine works.

An electric heater of power 3 KW is used for 10 h. How much energy does it consume? Express your answer in (i) kWh, (ii) joule.

A boy of mass 40 kg runs up a flight of 15 steps each 15 cm high in 10 s. Find:
(i) the work done and
(ii) the power developed by him

Take g =10 N   kg^-1

A water pump raises 50 litres of water through a height of 25 m in 5 s. Calculate the power which the pump supplies.

(Take g = 10 N kg^-1 and density of water = 1000 kg m^-3)

A man raises a box of mass 50 kg to a height of 2 m in 2 minutes, while another man raises the
same box to the same height in 5 minutes. Compare:
(i) the work done and
(ii) the power developed by them.

A pump is used to lift 500 kg of water from a depth of 80 m in 10 s. calculate:
(a) the work done by the pump
(b) the power a which the pump works,
(c) the power rating of the pump if its efficiency is 40% (Take g = 10 m s^-2)

An ox can apply a maximum force of 1000 N. It is taking part in a cart race and is able to pull the cart at a constant speed of 30   M   S^-1 while making its best effort. Calculate the power developed by the ox.

If the power of a motor is 40 kw, at what speed can it raise a load of 20,000 N?

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What are the two forms of mechanical energy?

Name the forms of energy which a wound-up watch spring possesses.

Name the type of energy (kinetic energy K or potential energy U) possessed in the given cases:

A moving cricket ball

Name the type of energy (kinetic energy K or potential energy U) possessed in the given cases:

A compressed spring

Name the type of energy (kinetic energy K or potential energy U) possessed in the given cases:

A moving bus

Name the type of energy (kinetic energy K or potential energy U) possessed in the given cases:

The bob of a simple pendulum at its extreme position.

Name the type of energy (kinetic energy K or potential energy U) possessed in the given cases:

The bob of a simple pendulum at its mean position.

Name the type of energy (kinetic energy K or potential energy U) possessed in the following case:
A piece of stone places on the roof.

When an arrow is shot from a bow, it has kinetic energy in it. Explain briefly from where does it get its kinetic energy?

Define the term potential energy of a body.

State different forms of potential energy and give one example of each.

A ball is placed on a compressed spring. What form of energy does the spring possess? On releasing the spring, the ball flies away. Give a reason.

What is meant by the gravitational potential energy? Derive expression for it.

Write an expression for the potential energy of a body of mass m places at a height h above the earth’s surface.

Name the form of energy which a body may possess even when it is not in motion. Give an example to support your answer.

What do you understand by the kinetic energy of a body?

A body of mass m is moving with a velocity v. Write the expression for its kinetic energy.

State the work energy theorem.

A body of mass m is moving with a uniform velocity u. A force is applied on the body due to
which its velocity changes from u to v. How much work is being done by the force.

A light mass and a heavy mass have equal momentum. Which will have more kinetic energy?
(Hint : Kinetic energy K = P2/2m where P is the momentum)

Name the three forms of kinetic energy and give on example of each.

Differentiate between the potential energy (U) and the kinetic energy (K)

Complete the following sentence:
The kinetic energy of a body is the energy by virtue of its………….

Complete the following sentence:
The potential energy of a body is the energy by virtue of its ……………….

Is it possible that no transfer of energy may take place even when a force is applied to a body?

Name the form of mechanical energy, which is put to use.

In what way does the temperature of water at the bottom of a waterfall differ from the temperature at the top? Explain the reason.

Name six different forms of energy?

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:
the unwinding of a watch spring

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

a loaded truck when started and set in motion.

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

a car going uphill

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

photosynthesis in green leaves.

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

Charging of a battery.

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

respiration

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

burning of a match stick

Energy can exist in several forms and may change from one form to another. For the following, state the energy changes that occur in:

explosion of crackers.

State the energy changes in the following case while in use:
loudspeaker

State the energy changes in the following case while in use:
a steam engine

State the energy changes in the following case while in use:
microphone

State the energy changes in the following case while in use:
washing machine

State the energy changes in the following case while in use:
an electric bulb

State the energy changes in the following case while in use:
burning coal

State the energy changes in the following case while in use:
a solar cell

State the energy changes in the following case while in use:
bio-gas burner

State the energy changes in the following case while in use:
an electric cell in a circuit

State the energy changes in the following case while in use:
a petrol engine of a running car

State the energy changes in the following case while in use:
an electric toaster.

State the energy changes in the following case while in use:
a photovoltaic cell.

State the energy changes in the following case while in use:
an electromagnet.

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MULTIPLE CHOICE TYPE
A body at a height possesses:
(a) kinetic energy
(b) potential energy
(c) solar energy
(d) heat energy

MULTIPLE CHOICE TYPE

In an electric cell which in use, the change in energy is from:
(a) electrical to mechanical
(b) electrical to chemical
(c) chemical to mechanical
(d) chemical to electrical

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Two bodies of equal masses are placed at heights h and 2h. Find the ration of their gravitational potential energies.

Find the gravitational potential energy of 1 kg mass kept at a height of 5 m above the ground if g = 10 m s-2.

A box of weight 150 kgf has gravitational potential energy stored in it equal to 14700 J. Find the height of the box above the ground. (Take g = 9.8 N kg-1)

A body of mass 5 kg falls from a height of 10 m to 4 m. Calculate: (i) the loss in potential energy of the body, (ii) the total energy possessed by the body at any instant? (Take g = 10 m s-2)

Calculate the height through which a body of mass 0.5 kg is lifted if the energy spent in doing so is 1.0 J. Take g = 10 m s-2.

A boy weighing 25 kgf climbs up from the first floor at height 3 m above the ground to the third floor at height 9m above the ground. What will be the increase in his gravitational potential energy? (Take g = 10 N kg -1)

A vessel containing 50 kg of water is placed at a height 15 m above the ground. Assuming the gravitational potential energy at ground to be zero, what will be the gravitational potential energy of water in the vessel? (g = 10 m s-2)

A man of mass 50 kg climbs up a ladder of height 10 m. Calculate: (i) the work done by the man, (ii) the increase in his potential energy. (g = 9.8 m s-2)

A block A, whose weight is 200 N, is pulled up a slope of length 5 m by means of a constant force F (= 150 N) as illustrated in Fig 2.13

(a) what is the work done by the force F in moving the block A, 5 m along the slope?
(b) By how much has the potential energy of the block A increased?
(c) Account for the difference in work done by the force and the increase in potential energy of the block.

Find the kinetic energy of a body of mass 1 kg moving with a uniform velocity of 10 m s-1.

If the speed of a car is halved, how does its kinetic energy change?

Two bodies of equal masses are moving with uniform velocities v and 2v. Find the ratio of their kinetic energies.

A car is running at a speed of 15 km h-1 while another similar car is moving at a speed of 30 km h-1. Find the ration of their kinetic energies.

A bullet of mass 0.5 kg slows down from a speed of 5 m s-1 to that of 3 m s-1. Calculate the change in kinetic energy of the ball.

A cannon ball of mass 500 g is fired with a speed of 15 m s-1. Find: (i) its kinetic energy and (ii) its momentum.

A bullet of mass 50 g is moving with a velocity of 500 m s-1. It penetrated 10 cm into a still target and comes to rest. Calculate: (a) the kinetic energy possessed by the bullet, (b) the average retarding force offered by the target.

A body of mass 10 kg is moving with a velocity 20 m s-1. If the mass of the body is doubled and its velocity is halved, find the ratio of the initial kinetic energy to the final kinetic energy.

A truck weighing 1000 kgf changes its speed from 36 km h-1 to 72 km h-1 in 2minutes. Calculate: (i) the work done by the engine and (ii) its power/ (g = 10 m s-2)

A body of mass 60 kg has the momentum 3000 kg m s-1. Calculate: (i) the kinetic energy and (ii) the speed of the body.

How much work is needed to be done on a ball of mass 50 g to give it s momentum of 500 g cm s-1?

How much energy is gained by a box of mass 20 kg when a man
(a) carrying the box waits for 5 minutes for a bus?
(b) runs carrying the box with a speed of 3 m s-1 to catch the bus?
(c) Raises the box by 0.5 m in order to place it inside the bus? (g = 10 m s-2)

A spring is kept compressed by a small trolley of mass 0.5 kg lying on a smooth horizontal surface as shown in the adjacent fig. when the trolley is released, it is found to move at a speed v = 2 m s-1. What potential energy did the spring possess when compressed?

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State two characteristic which a source of energy must have.

Name the two groups in which various sources of energy are classified. State on what basis are they classified.

What is meant by the renewable and non-renewable sources of energy? Distinguish between them giving two examples of weach.

Select the renewable and non-renewable sources of energy from the following:
Coal

Select the renewable and non-renewable sources of energy from the following:
Wood

Select the renewable and non-renewable sources of energy from the following:
Water

Select the renewable and non-renewable sources of energy from the following:
Diesel

Select the renewable and non-renewable sources of energy from the following:
Wind

Select the renewable and non-renewable sources of energy from the following:
Oil

Why is the use of wood as a fuel not advisable although wood is a renewable source of energy?

Name five renewable sources of energy.

Name three non-renewable sources of energy.

What is tidal? Explain in brief.

What is ocean? Explain in brief.

What is geo thermal energy? Explain in brief.

What is the main source of energy for earth?

What is solar energy?

How is the solar energy used to generate electricity in a solar power plant?

What is a solar cell?

State whether a solar cell produces a.c. or d.c.

Give one disadvantage of using a solar cell.

State two advantages of producing electricity from solar energy.

State two disadvantages of producing electricity from solar energy.

What is wind energy?

How is wind energy used to produce electricity?

How much electric power is generated in India using the wing energy?

State two advantages of using wind energy for generating electricity.

State two disadvantages of using wind energy for generating electricity.

What is hydro energy?

Explain the principle of generating electricity from hydro energy.

How much hydroelectric power is generated in India?

State two advantage of producing hydroelectricity.

State two disadvantages of producing hydroelectricity.

What is nuclear energy?

Explain the principle of producing electricity using the nuclear energy.

State the energy transformation on the following:

Electricity is obtained from solar energy.

Name two places in india where electricity is generated from nuclear power plants.

State two advantages of using nuclear energy for producing electricity.

State two disadvantages of using nuclear energy for producing electricity.

State the energy transformation on the following:

Electricity is obtained from solar energy.

State the energy transformation on the following:

Electricity is obtained from wind energy.

State the energy transformation on the following:

Electricity is obtained from hydro energy.

State the energy transformation on the following:

Electricity is obtained from nuclear energy.

State four ways for the judicious use of energy.

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MULTIPLE CHOICE TYPE:

The ultimate source of energy is:
(a) wood
(b) wind
(c) water
(d) sun

MULTIPLE CHOICE TYPE:

Renewable source of energy is:
(a) Coal
(b) fossil fuels
(c) natural gas
(d) sun

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State the law of conservation of energy.

What do you understand by the conservation of mechanical energy?

State the condition under which the mechanical energy is conserved.

Name two examples in which the mechanical energy of a system remains constant.

A body is thrown vertically upwards. Its velocity keeps on decreasing. What happens to its kinetic energy as its velocity becomes zero?

A body falls freely under gravity from rest. Name the kind of energy it will possess at the point from where it falls.

A body falls freely under gravity from rest. Name the kind of energy it will possess while falling.

A body falls freely under gravity from rest. Name the kind of energy it will possess on reaching the ground.

Show that the sum of kinetic energy and potential energy (i.e., total mechanical energy) is always conserves in the case of a freely falling body under gravity (with air resistance neglected) from a height h by finding it when (i) the body is at the top, (ii) the body has fallen a distance x, (iii) the body has reached the ground.

A pendulum is oscillating on either side of its rest position. Explain the energy changes that takes place in the oscillating pendulum. How does the mechanical energy remains constant in it? Draw the necessary diagram.

A pendulum with bob of mass m is oscillating on either side from its resting position A between the extremes B and C at a vertical height h and A. what is the kinetic energy K and potential energy U when the pendulum is at position (i) A, (ii) B and (iii) C?

What do you mean by degradation of energy?

Explain degradation of energy by taking two examples of your daily life.

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MULTIPLE CHIOCE TYPE:

A ball of mass m is thrown vertically up with an initial velocity so as to reach a height h. The correct statement is:
(a) Potential energy of the ball at the ground in mgh.
(b) Kinetic energy imparted to the ball at the ground is zero.
(c) Kinetic energy of the ball at the highest point is mgh.
(d) potential energy of the ball at the highest point is mgh.

MULTIPLE CHIOCE TYPE:

A pendulum is oscillating on either side of its rest position. The correct statement is:
(a) It has only the kinetic energy.
(b) it has the maximum kinetic energy at its extreme position.
(c) it has the maximum potential energy at its rest position.
(d) The sum of its kinetic and potential energies remains constant throughout the motion.

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A ball of mass 0.20 kg is thrown vertically upwards with an initial velocity of 20 m s-1. Calculate the maximum potential energy it gains as it goes up.

A stone of mass 500g is thrown vertically upwards with a velocity of 15 m s-1. Calculate: (a) the potential energy at the greatest height, (b) the kinetic energy on reaching the ground, (c) the total energy at its half-way point.

A metal ball of mass 2 kg is allowed to fall freely from rest from a height of 5m above the ground.
(Take g = 10 m s-2)

(a) Calculate the potential energy possessed by the ball when initially at rest.

(b) what is the kinetic energy of the ball just before it hits the ground?

(c) what happens to the mechanical energy after the ball hits the ground and comes to rest?

The diagram given below shows a ski jump. A skier weighing 60 kgf stands at A at the top of ski jump. He moves from A to B and takes off for his jump at B.

(a) Calculate the change in the gravitational potential energy of the skier between A and B.

(b) If 75% of the energy in part (a) becomes kinetic energy at B. Calculate the speed at which the skier arrives at B.
(Take g=10 m s-2)

A hydroelectric power station takes its water from a lake whose water level if 50 m above the turbine. Assuming an overall efficiency of 40%, calculate the mass of water which must flow through the turbine each second to produce power output of 1 MV.

The bob of a simple pendulum is imparted a velocity 5 m s-1 when it is at its mean position. To what maximum vertical height will it rise on reaching to its extreme position if 60% of its energy is lost in overcome friction of air?

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