Question:
whether or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in a pond.
(b) A donkey is carrying a load on its back.
(c) A wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the Sun.
(g) A sailboat is moving due to wind energy.
Answer:
(a)Suma is doing work as she moves (displaces) herself by applying force with the movement of her arms and legs in the water.
(b) A donkey is not doing any work (in the technical sense) as the weight he is carrying (Le.t force) and the displacement are perpendicular to each other.However, he does work against friction between his hoofs and the ground. Further, work is also done by the donkey if he climbs up a mountain as in that case force and displacement are not perpendicular to each other.
(c) The kinetic energy of the wind is imparted to the wind-mill which lifts water from the well.Thus,a wind-mill works against the force of gravity to lift water vertically upward.
(d) Photosynthesis is the process by which green plants use Sun’s energy to build up carbohydrate reserves.In this case,both force (F) and displacement (s) are zero and as such work done is zero.
(e) When the engine pulls train,it does work (i)in pulling the train and (ii) against the force of friction between the wheels of the train and the railway track.
(f) When the food grains are dried in the Sun,solar radiation takes moisture out of the food grains.Since there is no force (F) and no displacement (s) is involved in the process,work done is zero.
(g) The force,F exerted by the wind on the sailboat moves it through a distance (s) in its direction.Hence,work is done by the wind energy in moving the sailboat.
Work and Energy
Q 1.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 2.
Name a machine that transforms muscular energy into useful mechanical work.
Q 3.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 4.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 5.
Two girls,each of weight 400 N,climb up a rope through a height of 8 m.We name one of the girls A and the other B.Girl A takes 20 s while B takes 50 s to accomplish this task.What is the power expended by each girl ?
Q 6.
Define potential energy.Derive an expression for the potential energy of a body of mass m,at a height h above the surface of the Earth.
Q 7.
How does the wind move the blades of a wind-mill ?
Q 8.
An object of mass 15 kg is moving with a uniform velocity of 4 m/s.What is the kinetic energy possessed by the object ?
Q 9.
What is the form of energy possessed by a running car?
Q 10.
Name two forms of mechanical energy.
Q 11.
What do the machines need for their working ?
Q 12.
When is work done by a force zero ?
Q 13.
Why do some engines require fuels like petrol and diesel ?
Q 14.
Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Q 15.
Name the device which converts electrical energy into mechanical energy.
Q 16.
A boy of mass 55 kg runs up a flight of 40 stairs,each measuring 0.15 m.Calculate the work done by the boy
Q 17.
Fill in the blanks :
Ability to do work is called _______.
Q 18.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 19.
Name the type of energy possessed by a raised hammer?
Q 20.
A force of 7 N acts on an object.The displacement is,say 8 m,in the direction of the force,[in Fig.].Let us take it that the force acts on the object throughout the displacement What is the work done in this case ?
Q 21.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 22.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 23.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 24.
A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground.Calculate the work done by him on the luggage.
Q 25.
Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Q 26.
The mass of a ball A is double the mass of another ball B.The ball A moves at half the speed of the ball B.Calculate the ratio of the kinetic energy of A to the kinetic energy of B.
Q 27.
Can there be displacement of an object in the absence of any force acting on it ? Think.Discuss this question with your friends and teacher.
Q 28.
The velocity of a body moving in a straight line is increased by applying a constant force F,for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
Q 29.
Fill in the blanks :
A force acting on a body shows a displacement in the direction of the force. It is said _______ is done on the body.
Q 30.
A boy of mass 50 kg runs up a staircase of 45 steps in 9 s.If the height of each step is 15 cm,find his power.Take g =
.
Q 31.
Is a person doing any work by holding a suitcase ?
Q 32.
Define average power.
Q 33.
A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Q 34.
Give examples for the following scenarios of energy transformations. You may choose the answer(s) from the following list of words
solar cell, turbine, mixer, fan, steam engine, automobiles, green plants, nuclear reactor, electric bulb, torch, electric bulb, microphone, loudspeaker, geyser, heater, car battery, inverter, LPG
| SNo. |
Energy Transformation
From ⟶ To |
Give Example |
| 1. |
Mechanical ⟶ Electrical |
_________ |
| 2. |
Electrical ⟶ Mechanical |
_________ |
| 3. |
Electrical ⟶ Heat |
_________ |
| 4. |
Heat ⟶ Mechanical |
_________ |
| 5. |
Electrical ⟶ Light |
_________ |
| 6. |
Electrical ⟶ Sound |
_________
|
| 7. |
Chemical ⟶ Heat |
_________ |
| 8. |
Electrical ⟶ Chemical |
_________ |
| 9. |
Sound ⟶ Electrical |
_________ |
| 10. |
Chemical ⟶ Electrical |
_________ |
| 11. |
Light ⟶ Electric |
_________ |
| 12. |
Chemical ⟶ Mechanical |
_________ |
| 13. |
Nuclear ⟶ Electrical |
_________ |
| 14. |
Light ⟶ Chemical |
_________ |
Q 35.
How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?
Q 36.
A coolie carries a load of 50 kg on his head and walks on a level road upto 100 m.What is the work done by him ?
Q 37.
Is work a scalar quantity or a vector quantity ?
Q 38.
Can a body have energy without having momentum ?
Q 39.
Fill in the blanks :
An object moving in a circular orbit. The work done by the centrepetal force acting on the object is ______ (zero/non-zero).
Q 40.
What are other forms of energies apart from mechanical energy? Does Law of Conservation of Energy apply to all of them?
Q 41.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 42.
How is energy stored in a clock ?
Q 43.
Name a few forms of energy and their transformation into each other.
Q 44.
Two friends Ram and Shyam, each having weight of 40 kg, go for rock climbing. Ram climbs to a height of 3 m in 10 s. and Shyam covers the same height in 12 s. Is the work done by Ram and Shyam equal ? or not ? Compare the power of Ram and Shyam.
Q 45.
A heavy stone is lowered to the ground with the help of rope. Is the work done by the applied force positive or negative?
Q 46.
A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Q 47.
Can kinetic energy of an object be negative?
Q 48.
A body of mass 120 g is taken vertically upwards to reach the height of 5m. Calculate work done. (Take g = 10 m/s
2)
Q 49.
A battery lights a bulb.Describe the energy changes involved in the process
Q 50.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
