Question:
A box is pulled across a floor by applying a force of 100N at an angle of 60 degrees above the horizontal. How much work is done by the applying force in pulling the object to a distance of 8m?
Answer:
Consider the following figure,
A force of 100 N is applied to a box of weight W. Force is a vector quantity applied at angle 60. It has two components, horizontal component (100 cos 60) and vertical component (100 sin 60).
The horizontal component of the force should be considered here, because it causes the displacement in horizontal direction.
∴ W = 100 cos 60 × 8 = 100 × 0.5 × 8 = 400 J
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.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 3.
Name a machine that transforms muscular energy into useful mechanical work.
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.
How does the wind move the blades of a wind-mill ?
Q 7.
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 8.
What is the form of energy possessed by a running car?
Q 9.
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 10.
When is work done by a force zero ?
Q 11.
Name two forms of mechanical energy.
Q 12.
What do the machines need for their working ?
Q 13.
Name the device which converts electrical energy into mechanical energy.
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.
Why do some engines require fuels like petrol and diesel ?
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.
Name the type of energy possessed by a raised hammer?
Q 19.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 20.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 21.
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 22.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 23.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
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.
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 26.
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 27.
Write an expression for the work done when a force is acting on an object in the direction of its displacement.
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.
Is a person doing any work by holding a suitcase ?
Q 30.
Define average power.
Q 31.
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 32.
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 33.
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 34.
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 35.
Can a body have energy without having momentum ?
Q 36.
How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?
Q 37.
How is energy stored in a clock ?
Q 38.
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 39.
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 40.
What is Kilowatt hour?
Q 41.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Q 42.
What do you mean by the term potential energy ? Give some examples of bodies possessing potential energy ?
Q 44.
A boy of mass 45 kg climbs up 20 steps in 20 second. If each step is 25 cm high, calculate the power used in climbing
Q 45.
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 46.
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 47.
A person throws a stone of mass 3kg. It crosses the top of a wall of 2m high at a speed of 4m/s. What is the total mechanical energy possessed by the stone when it crosses the wall?(Take g = 10 m/s)
Q 48.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q 49.
A motor pump does same amount of water by lifting underground water in shorter time than a hand pump. What do you infer from this statement?
Q 50.
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)
