# 11th Class Physics Chapter 4 Work and Energy Short Question Answers ## 11th Class Physics Chapter 4 Work and Energy Short Question Answers Below

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1.A person holds a bag of groceries while standing still talking to a friend.A car is stationary with its engine running.From the standpoint of work how are these two situations similar?
In both of these two cases,the value of the displacements is zero.According to definition of work,
w= Fd
Here    d=0,therefore
W=F×0=0
Thus in both the situations the work done  is zero.Therefore from the standpoint of work these two situations are similar.
2. Calculate the work done in kilo joules in lifting a mass of 10kg through a vertical height of 10m? 3.In which case is more work done?When 50kg bag of book is lifted through 50 cm,or when 50kg crate is pushed through 2m across the floor with a force of 50N? 4.An object has IJ of potential energy.Explain what does it mean?
An object having IJ of potential energy means that the work done stored in the object in the form of potential energy has the capacity to do work of I.J.
5.A ball of mass m is held at a height h1 above a table.The table top is at a height h2 above the floor.One student says that the ball has potential energy mgh1 but another says that it is mg.Who is correct?
If the table top is considered as reference point the potential energy of a ball of mass ‘m’at a  height ‘h’ from above the table top is magh1.If the floor is taken as reference point,the potential energy of the same ball with respect to the floor is mg because now the ball is at a height of from the floor.Thus both the students are correct because one student has chosen the table top as a point of reference and the other is telling with respect to the floor as reference point.
6.When a rocket re-enters the atmosphere the nose cone becomes very hot,where does this energy come from?
There is a large number of dust particles and water vapours present in the air.When a rocket re-enters the atmosphere it has to face the resistance due to particles. Some K,E of the rocket is converted into heat energy.Therefore the cone nose of the rocket becomes very hot due to the heat energy produced by the fluid frictions of atmosphere.
7.What sort of energy is in the following? (a)Compressed spring (b) Water in a high dam (c) A moving car
(a) Compressed spring: It has elastic potential energy. (b) Water in a high dam:Water stored in a high dam possesses the gravitational potential energy. (c) A moving car:It has kinetic energy due to its motion.
8.A girl drops a cup from a certain height which breaks into pieces.What energy changes are involved?
When a cup is dropped from a certain height,its potential energy is converted into kinetic energy.When the cup collides with the floor it breaks into pieces and makes a noise.The kinetic energy of cup becomes zero on the floor.Some of K.E. is used to break the cup into pieces and transferred as K,E. Of pieces the remaining K.E. is converted into sound energy which produces the noise and heat energy.
9.A boy uses a catapult to throw a stone which accidentally smashes a green house window.List the possible energy changes?
When a boy throws a stone by a catapult the stone possesses kinetic energy.A part of this energy is used in breaking a greenhouse window and also transferred as K,E. To the broken pieces of window.The reaming part of the K.E. is converted into sound energy which causes the noise and also heat energy.
10.Write the expression of work done by a force that acts at an angle 60ᵒ to a moving body?
The work done by a force  is expressed as
W=F.d= Fd cos0
Where 0 is the angle between force F and displacement d.
Hence,0=60ᵒ so the expression becomes
W=Fd  cos60ᵒ
11.A body of 1 kg is lifted to a height of 10m above the ground whan is the work done on the body?
Solution:-
Mass  of body =m= 1kg
Height =h=10m
Acceleration due to gravity =g=-9.8ms-2
Work done = P.E =mgh
Putting the values we get
w=1×9.8×10=98j
Hence W=98J Ans
12.What are the essential conditions for conservative field?
Gravitational field is essential for conservative field. There must be a closed path in a gravitational field. Work done along a closed path in the gravitational field must be zero. Work done by a body in the field should be independent of the path followed.
13.What is the dimension of power?
As Power =F.V
Power= (mass×acceleration)(velocity)
[M×L/T2][L/T]
[ML2/T3]=[ML2T-3]
14.A masons of 100N weight is climbing on a 10 m high ladder.Find his P.E.at the middle of ladder?
Weight of the mason= W=100N
Height up to middle of ladder=h= h//2=5m
Using the formula of P.E.
P.E.=W×h, putting the values,we get
P.E= 100×5=500J
P.E=500j
15.What is meant by Dissipation of energy?
When the body comes to rest after hitting the ground the K.E.possessed by the body is coveted into sound and heat energy after striking against the ground.This is called Dissipation of energy.
16.Differentiate between renewable and non-renewable resources of energy?
Renewable resources:-The sources of energy which can be renewed are called renewable resources.For example hydroelectric wind tides geothermal biomass sunlight and ethanol. Nonrenewable resources:The sources of energy which can not be renewed are called nonrenewable resources of energy.For example coal natural gas oil uranium oil shale and tar sands.
17.A ball of weight 98N is moving on a friction-less surface with velocity of 1 ms-1 .Compute its K.E. 18.A diver of 50g mass dives from a 10m high diving board in a swimming pool.Calculate its P.E before jump? 19.Find the average power in lifting 100kg of mass in 1 s up to height of 10 m? 20.What type of energy is stored in water dam?
Water stored in a high dam possesses the gravitational potential energy.
21.When an arrow is shot from its bow it has kinetic energy.From where does it get the K.E.?
Some work is done in pulling the string back through a certain distance.This work is stored as the elastic potential energy in the string.When the arrow is shot from its bow this P.E is converted into K,E. Of the arrow.
22.What type of energy is stored in the spring of watch?
The work done in winding the watch is used up in compressing the spring and is stored in the form of elastic potential energy in the spring of watch.
23.A bucket is taken to the bottom of a well does the bucket possess any potential energy?
Yes,when the bucket is taken to the bottom of the well some work is to be done against the upward thrust of water.This work done is stored as P.E. in the bucket.As soon as the applied force is removed the bucket comes up to the surface of water since the P.E is converted into K.E.
24.The force between a pair of protons is that of repulsion.Does the P.E increase as the protons are brought closer?
Yes the potential energy increases when the two protons are brought closer to each other because the work has to be done against the force of repulsion.This work will appear as the increase in their potential energy.
25.A meteor when enters into Earth’s Atmosphere burns.What happens to its energy and momentum?
When a meteor enters Earth’s atmosphere it will burn due to the friction between the particles of the atmosphere and meteorite.Hence the energy possessed by a meteor is converted into heat energy and its momentum is transferred to the particles of atmosphere.
26.A man rowing boat upstream is at rest with respect to shore is he doing work?
When the man rowing the boat up stream is at rest with respect to shore he is not doing any work because the displacement of boat is zero and the work is the product of force and displacement(W=Fd=F×0=0),Here the work done by the man on the boat upstream is counter balanced by work done on the boat by current of water downstream.So net work done by the man is zero.
27.Due to smaller value of ‘g’a man can jump higher on the surface of the moon.Can he run faster on the account?
No,he cannot run faster in moon than on earth due to smaller value of ‘g’ in running man pushes the earth backward with his roof and the reaction of the earth pushes the man in the forward direction.As the value of ‘g’ is less on the surface of the moon as compared to that on the earth so the reaction force of the moon will push the man forward with smaller force.Hence a man cannot run faster in moon on this account.
28.Why does the bob of a swinging pendulum come to rest after some time?
Ans:The to and fro motion of the bob is opposed by two forces,air resistance and friction in the string at the point of suspension.Due to these opposing forces the motion of the bob is retarded and after some time it comes to rest.The energy of the bob is used against the opposing force and it is converted into heat.
29.What happens to the kinetic energy of a bullet when it penetrates into a target
When a bullet strikes a target it penetrates into it and comes to rest after covering a small distance.A major part of its kinetic energy is used in doing work against the resistance of the target and rest of energy is converted into heat. Loss of the bullet = work done against the resistance of target + heat produced due to friction.
30.Is kinetic energy a vector quantity?
Kinetic energy is simply the capacity of a body to do work by virtue of its motion.Since work is scalar quantity therefore kinetic energy is also a scalar quantity and not actor quantity.
31.A car moving with constant speed along a level road with no net force acting on it.Is any work being done on the car?
When a car is moving with constant speed along a level road with no net force acting on it the work done on it is zero.But the engine of the cat exerts a force and does work continuously against friction to keep the motion steady.
32.At what point the energy of a swinging pendulum is wholly potential.Where is its K.E maximum?
P,E of swinging Pendulum:At the extreme positions the whole energy of a simple pendulum lies in from of potential energy. K,E. Of swinging pendulum: At the mean point the kinetic energy of the swinging pendulum is maximum.
33.Name two conservative forces?
Conservative forces: There are three conservative forces. (i) Gravitational force (ii) Electric force (iii) Elastic spring force,
34.What is meant by kwh i.e.Show that kwh=3.6 MJ
Def:- It is the amout of work done when a power of one kilowatt is maintained for one hour.
It is commercial unit of electric energy.
To show that 1 KWH=3.6 MJ
1KWH =1000 watt×3600 s
36×105 watt –s
=3.6×106j
Hence 1KWH =3.6Mj
35.Distinguish between average power and instantaneous power? 36.Show that work done against frictional force is negative?
The work done is expressed as
W=F.d.=Fd cos0……………… (i)
Since frictional force is always apposite to the direction of motion of a body,Therefore the angle between force and displacement will be 180ᵒ.
Now equation (1) can be written
W=Fd cos 180ᵒ
=Fd (-1)
or W=-Fd
Hence,the work done against frictional force is negative.
37.What is absolute potential energy? 38.What is meant by conservative field?
Def: A field in which work done along a closed path is zero is called conservative field.
39.State law of conservation of energy?
It states that energy can neither be created nor destroyed but it can change from one form to another but the total amount of energy remains constant. Mathematically it can be written as Total Energy=P.E + K.E= constant.
40.What are conservative and non-conservative forces?Give the examples of non-conservative force. Define non-conservative field?
Conservative Force: The force due to conservative field is called conservative force. Non conservative force:The frictional force is an example of non Conservative force because if a body is moved iver a rough surface between two points along different paths the work done against the frictional force certainly depends on the path followed. Non conservative field:The field in which the work done in moving a body between two points depends upon the path followed is called non conventional field.
41.What do you mean by non conventional energy sources?
The energy sources which are not very common these days and may be available to the world energy demand of future are called non conventional sources.Some of non conventional energy sources are. (i)Geo-thermal energy (ii) Solar energy (iii) Energy from waves (iv) Energy from tides (v) Energy from biomass.
42.How can we gain the energy from waves?
The tides and winds produce strong water waves.The waves energy makes duck float move relative to balance float and electrical energy is generated.The energy of these waves can be used to generate electricity.One such device was invented by prof.Salter known as Salter ’s duck.
43.How can we gain energy from tides?
Gravitational interaction between the earth and the moon serves as the source of tide energy.Due to gravitational pull of moon the water rises twice a day along the coasts.The dam is filled at high tides and water is released in a controlled way at allow tide to drive the turbines and produces electricity.
44.Describe some uses of solar cells?
(i) They have low running cost (ii) Solar calculators are being used now-a days.(iii) Although the solar cells are costly but remain in use for a long time. (iv) They are used in remote ground based weather stations and rain for cat communication system.(v) solar cells are used to power satellites having a large solar panels which are kept facing the sun.
45.What is solar energy?What is the function of solar cells?
The sun is a great source of energy.Its energy is called solar energy.The earth receives hugs amount of energy directly from the sun the sun supplies 1.4 kwm-2 energy at outer atmosphere.It is called solar constant.There is a direct method in which sum light is directly converted into electricity by using semi- conductor devices called solar cells.Solar cells are thin  plates made from silicon. A large number of solar cells are connected in series forming a solar cell panel so that high voltage may be obtained for practical use.
46.Describe the methods to obtain solar energy?
Following are the methods for obeying the solar energy. (i) By solar cells (ii) By heating the water directly (iii) By collecting sun rays with help of mirrors or lenses.
47.How can the waste products be converted into useful energy?
Waste product like wood waste crop residue and solid waste are used to get energy by direct conversion.They are burnt in a confined container.In this way heat produced is used in boiler to produce steam that can run turbine generator.
48.What are common methods to convent biomass into energy?
There are two common methods to convert biomass into energy. (i)Direct combustion (ii) Fermentation

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