Question:
In physics,energy is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems.However,this must be understood as an overly simplified definition as we shall come to know in higher classes that not all energy can perform work.
(a) How did the concept of energy emerge ?
(b) How much electric energy is generated in a typical lightning strike ? What are the forms to which this energy gets converted ?
(c) What is the kinetic energy of a passenger seated in a moving plane :(i) with respect to the plane (ii) with respect to the Earth.
Answer:
(a) The concept of energy emerged out of the idea of Vis Viva (living force) and it was believed that total Vis Viva was conserved.
(b)500 mega joules.Light energy,sound energy,thermal energy,etc.
(c) (i)Zero (ii)Non-zero kinetic energy (and higher total energy).
Work and Energy
Q 1.
Name a machine that transforms muscular energy into useful mechanical work.
Q 2.
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 3.
What do the machines need for their working ?
Q 4.
How does the wind move the blades of a wind-mill ?
Q 5.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 6.
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 7.
When is work done by a force zero ?
Q 8.
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 9.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 10.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 11.
What is the form of energy possessed by a running car?
Q 12.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 13.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 14.
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 15.
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 16.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 17.
A force of 5 N is acting on an object.The object is displaced through 2 m in the direction of the force.[in Fig.].If the force acts all through the displacement,find the work done by the force.
Q 18.
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 19.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 20.
A man does 200 J of work in 10 s and a boy does 100 J of work in 4 s.(a) Who is delivering more power ?
(b) Find the ratio of the power delivered by the man to that delivered by the boy.
Q 21.
Define average power.
Q 22.
Name two forms of mechanical energy.
Q 23.
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 24.
Name the type of energy possessed by a raised hammer?
Q 25.
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 26.
A battery lights a bulb.Describe the energy changes involved in the process
Q 27.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 28.
A certain household has consumed 250 units of energy during a month.How much energy is this in joules ?
Q 29.
When is work done by a force negative ?
Q 30.
What are the units of work and energy ?
Q 31.
Name the device which converts electrical energy into mechanical energy.
Q 32.
Why do some engines require fuels like petrol and diesel ?
Q 33.
What is the difference between potential energy and kinetic energy ?
Q 34.
What do you understand by the term energy ? Give its scientific conception.
Q 35.
Name a few forms of energy and their transformation into each other.
Q 36.
Can a body have energy without having momentum ?
Q 37.
Is it possible to havfc a situation where mechanical energy (E) - potential energy (
) Is negative,i.e.,(E-
)< 0.
Q 38.
The global climate has always fluctuated.Millions of years ago,some parts of the world that are now quite warm, were covered with ice,and over recent centuries,average temperatures have risen and fallen in cycles.What is new,however,is that current and future climate change will be caused not just by natural events but also by activities of human beings.Suggest three simple ways to help save the climate of our planet.
Q 39.
Fill in the blanks :
A form of energy an object has because of its motion is called ________.
Q 40.
A stone of mass 2Kg is tied with a string and is whirled with a centripetal force of 10N in a circular path. How much work is done for making one revolution?
Q 41.
Calculate the amount of work done in drawing a bucket of water weighing 10kg from a well of depth 20m.
Q 42.
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?
Q 43.
Fill in the blanks :
Work done against a force is stored as _____.
Q 44.
Fill in the blanks :
Ability to do work is called _______.
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.
Can a body have energy without possessing momentum?
Q 47.
Fill in the blanks :
Work done on an object is _____ if displacement is zero.
Q 48.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 49.
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 50.
An object of mass m is raised from the the earth to an height of equal to the radius of the earth (R). Calculate the gain in potential energy by the object. Assume gravity of the earth is g.
