Selina solutions for Physics [English] Class 10 ICSE chapter 2 - Work, Energy and Power [Latest edition]

Define work.

When is work said to be done by a force?

How is the work done by a force measured when the force is in the direction of displacement? 

How is work done by a force measured when the force is in 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.

Write the expression for the work done by the force. 

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.

What should be the angle between force and displacement so that the work done is

1. Zero
2. Maximum

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

State the condition when the work done by a force is positive.  Explain with the help of examples.

State the condition when the work done by a force is 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.

(Hint: The body is acted upon by the centripetal force.)

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

State whether work is done or not, by writing yes or no, in the following case:

A man pushes a wall.

In the following case write yes, if the work is being done and no if no work is being done.

A coolie standing with a heavy load on his head

State whether work is done or not, by writing yes or no, in the following case:

A boy climbs up 20 stairs.

A coolie X Carrying a load on his head climbs up a slope and another coolie Y carrying the identical load on his head move the same distance on a frictionless horizontal platform. Who does more 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 displaced 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.

1. What is the work done by the boy against the force of gravity?
2. What would have been the work done if he uses a lift in climbing the same vertical height?

Define the term energy.

State the S.I. unit of energy.

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

1 J = ______ calorie.

1 kWh = ______ J.

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

Define kilowatt hour.

How is kilowatt hour related to joule?

Define power.

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.

Name the physical quantity measured in terms of horse power.

How is horse power related to the S. I. unit of power?

Differentiate between watt and watt hour.

Name the quantity which is measured in kWh.

Name the quantity which is measured in kW.

Name the quantity which is measured in Wh.

Name the quantity which is measured in eV.

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

One horse power is equal to ______.

kWh is the unit of ______.

A body, when acted upon by a force of 20 kgf, gets displaced by 1 m. Calculate the work done by the force, when the displacement is

1. in the direction of force,
2. at an angle of 60° with the force, and
3. normal to the force. (g = 10 N kg^-1)

A boy of mass 40 kg climbs up the stairs and reaches the roof at a height 8 m in 5 s. Calculate:

1. the force of gravity acting on the boy,
2. the work done by him against the force of gravity,
3. the power spent by boy. (Take g = 10 m s^-2)

A man spends 7.4 KJ energy in displacing a body by 74 m in the direction in which he applies force in 2.5 s. Calculate the force applied.

A man spends 7.4 kJ energy in displacing a body by 74 m in the direction in which he applies force, in 2.5 s. Calculate the power spent (in H.P) by the man.

A weight lifter lifted a load of 200 kgf to a height of 2.5 m in 5 s. Calculate:

1. the work done, and
2. 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:

1. energy spent by machine,
2. 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

1. kWh,
2. joule.

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

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

A pump is used to lift 600 kg of water from a depth of 75 m in 10s. Calculate:

1. The work done by the pump,
2. the power at which the pump works, and
3. The power rating of the pump if its efficiency is 40%. (Take g = 10m s^-2).

`["Hint" : "Efficiency" = "useful power"/"power input"]`

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 ms^-1 while making its best effort. Calculate the power developed by the ox.

The power of a motor is 40 kW. At what speed can the motor raise a load of 20,000 N?

Rajan exerts a force of 150 N in pulling a cart at a constant speed of 10 m/s^-1. Calculate the power exerted.

A boy weighing 350 N climbs up 30 steps, each 20 cm high in 1 minute, Calculate:

1. the work done and
2. power spent.

It takes 20 s for a person A of mass 50 kg to climb up the stairs, while another person B does the same in 15 s. Compare the Work done.

It takes 20 s for a person A of mass 50 kg to climb up the stairs, while another person B of same mass does the same in 15 s. Compare the power developed by the persons A and B.

A boy weighing 40 kgf climbs up a stair of 30 steps each 20 cm high in 4 minutes and a girl weighing 30 kgf does the same in 3 minutes. Compare:

1. The work done by them.
2. The power developed by them.

A man raises a box of mass 50 kg to a height of 2 m in 20 s, while another man raises the same box to the same height in 50 s. Compare:

1. the work done, and
2. the power developed by them. Take g = 10 N kg^-1

A man raises a box of mass 50 kg to a height of 2 m in 20s, while another man raises the same box to the same height in 50s. Calculated:

1. the work done, and
2. the power developed by each man. Take g = 10 N kg^-1.

A boy takes 3 minutes to lift a 20 litre water bucket from a 20 m deep well, while his father does it in 2 minutes. Compare:

1. the work, and
2. power developed by them.

A boy takes 3 minutes to lift a 20 litre water bucket from a 20 m deep well, while his father does it in 2 minutes.

How much work each does? Take density of water = 10³ kg m^-3 and g = 9.8 N kg^-1.

What are the two kinds of mechanical energy?

Name the form of energy stored in a wound up spring of a watch.

Name the type of energy (kinetic energy K or potential energy U) possessed in the following case:

A moving cricket ball

Name the type of energy (kinetic energy K or potential energy U) possessed in the following case:

A compressed spring

Name the type of energy (kinetic energy K or potential energy U) possessed in the following case:

A moving bus

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

A stretched wire

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

An arrow shot out of a bow.

Name the type of energy (kinetic energy K or potential energy U) possessed in the following case:

A piece of stone placed on the roof.

Define the term potential energy of a body.

Name two forms of potential energy and give one example of each.

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 is meant by the gravitational potential energy?

Derive expression for gravitational potential energy for a body placed at a height above the ground.

Write an expression for the potential energy of a body of mass m placed at a height h above the earth's surface. State the assumptions made, if any.

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.

Show that the quantity 2K/v² has the unit of mass, where K is the kinetic energy of the body.

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.)

Two bodies A and B of masses m and M (M ≫ m) have same kinetic energy. Which body will have more momentum?

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

What is the difference between potential energy and kinetic energy?

The kinetic energy of a body is the energy by virtue of its ______.

The potential energy of a body is the energy by virtue of its ______.

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

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.

A pebble is thrown up. It goes to a height and then comes back on the ground. State the different changes in form of energy during its motion.

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 the form of energy in which potential energy can change.

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

Name six different forms of energy?

Energy can exist in several forms and may change from one form to another. 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. 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. 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. 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. State the energy changes that occur in the Charging of a battery.

Energy can exist in several forms and may change from one form to another. State the energy changes that occur in respiration.

Energy can exist in several forms and may change from one form to another. State the energy changes that occur in the burning of a match stick.

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

A glowing 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 iron

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

A ceiling fan

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

An electromagnet.

Name the process used for producing electricity using nuclear energy.

Is it practically possible to convert a form of energy completely into the other useful form? Explain your answer.

What is degraded energy?

What do you mean by degradation of energy?

Explain the degradation of energy by taking one example of your daily life.

The conversion of part of energy into an undesirable form is called ______.

A body at a height possesses ______.

In an electric cell while in use, the change in energy is from ______.

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

Find the gravitational potential energy of 1 kg mass kept at a height of 5 m above the ground. Calculate its kinetic energy when it falls and hits the ground. Take g = 10 m s^-2.

A box of mass 300 kg has gravitational potential energy stored in it equal to 29400 J. Find the height of the box above the ground. (Take g = 9·8 N kg^-1).

A body of mass 10 kg falls from a height of 20 m to 8 m. Calculate:

1. the loss in potential energy of the body, and
2. 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 50 kgf climbs up from the first floor at a height of 3 m above the ground to the third floor at a height of 9 m 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 15 m. Calculate:

1. the work done by the man,
2. the increase in his potential energy. (g = 9.8 m s^-2)

A block A, whose weight is 100 N, is pulled up a slope of length 5 m by means of a constant force F (= 100 N) as illustrated in the following figure.

1. What is the work done by force F in moving block A, 5 m along the slope?
2. What is the increase in potential energy of the block A?
3. 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 5 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.

Two bodies have masses in the ratio 5 : 1 and their kinetic energies are in the ratio 125 : 9. Calculate the ratio of their velocities.

A car is running at a speed of 15 km h^-1 while another similar car is moving at a speed of 45 km h^-1. Find the ratio of their kinetic energies.

A ball of mass 0.5 kg slows down from a speed of 10 m s^-1 to that of 6 m s^-1. Calculate the change in the kinetic energy of the ball.

A cannon ball of mass 500 g is fired with a speed of 15 m s^-1. Find:

1. its kinetic energy.
2. its momentum.

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:

1. the initial kinetic energy, and
2. the final kinetic energy.

A truck weighing 1000 kgf changes its speed from 36 km h^-1 to 72 km h^-1 in 2 minutes. Calculate:

1. the work done by the engine and
2. its power (g = 10 m s^-2)

A body of mass 60 kg has the momentum 3000 kg m s^-1. Calculate:

1. the kinetic energy.
2. the speed of the body.

How much work is needed to be done on a ball of mass 50 g to give it a momentum of 5 kg cm s^-1?

How much energy is gained by a box of mass 20 kg when a man, carrying the box waits for 5 minutes for a bus?

How much energy is gained by a box of mass 20 kg when a man, runs carrying the box with a speed of 3 m/s^-1 to catch the bus?

How much energy is gained by a box of mass 20 kg when a man, raises the box by 0.5 m in order to place it inside the bus? (g=10 m/s^-2)

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:

1. the kinetic energy possessed by the bullet, and
2. the average retarding force offered by the target.

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?

State the principle 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 conserved in the case of a freely falling body under gravity (with air resistance neglected) from a height h by finding it when

1. The body is at the top.
2. the body has fallen a distance x.
3. 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 above A. What is the kinetic energy K and potential energy U when the pendulum is at position?

1. A,
2. B and
3. C

Name the type of energy possessed by the bob of a simple pendulum when it is at

1. the extreme position,
2. the mean position, and
3. between the mean and extreme positions.

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 pendulum is oscillating on either side of its rest position. The correct statement is: 

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 500 g is thrown vertically upwards with a velocity of 15 m/s. Calculate:

1. the potential energy at the greatest height,
2. the kinetic energy on reaching the ground, and
3. 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 5 m above the ground. Taking g = 10 m s^-2, calculate:

1. the potential energy possessed by the ball when it is initially at rest.
2. the kinetic energy of the ball just before it hits the ground?

A metal ball of mass 2 kg is allowed to fall freely from rest from a height of 5 m above the ground. 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 and takes off for his jump at B.

1. Calculate the change in the gravitational potential energy of the skier between A and B.
2. If 75% of the energy in part
   1. becomes kinetic energy at B.
   2. 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 is 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 MW. (Take g = 10 m s^-2)

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? (Take g = 10 m s^-2)

The figure below shows a simple pendulum of mass 200 g. It is displaced from the mean position A to the extreme position B. The potential energy at the position A is zero. At the position B the pendulum bob is raised by 5 m.

1. What is the potential energy of the pendulum at the position B?
2. What is the total mechanical energy at point C?
3. What is the speed of the bob at the position A when released from position B?
   (Take g = 10 ms^-2, given that there is no loss of energy.)