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.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 2.
How do you differentiate between energy and power ?
Q 3.
Define kinetic energy.Give some examples of bodies possessing kinetic energy.
Q 4.
An electric bulb of 60 W is used for 6 h per day.Calculate the ‘units’ of energy consumed in one day by the bulb
Q 6.
Name the type of energy possessed by a raised hammer.
Q 7.
Name a machine that transforms muscular energy into useful mechanical work.
Q 8.
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 9.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 10.
What do you mean by the term potential energy ? Give some examples of bodies possessing potential energy ?
Q 11.
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 12.
What do you mean by the law of conservation of energy ?
Q 13.
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 14.
A momentum of the body is increased by 20%. What is the percentage increase in its kinetic energy?
Q 15.
What do you mean by work ?Give its scientific conception.
Q 16.
A bullet of mass 20 g is found to pass two points 30 m apart in 4 s ? Assuming the -speed to be constant,find its kinetic energy.
Q 17.
Define average power.
Q 18.
Calculate the power of an engine required to lift 105 kg of coal per hour from a mine 360 m deep.
Q 19.
A certain household has consumed 250 units of energy during a month.How much energy is this in joules ?
Q 20.
Can a body have energy without having momentum ?
Q 21.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 22.
What do the machines need for their working ?
Q 23.
Is Work a scalar or vector quantity?
Q 24.
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 25.
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 26.
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 27.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
Q 28.
When is work done by a force zero ?
Q 29.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 30.
How does the wind move the blades of a wind-mill ?
Q 31.
What is the difference between potential energy and kinetic energy ?
Q 32.
What do you mean by kinetic energy ? Derive an expression for the kinetic energy of an object of mass m moving with velocity,v.
Q 33.
What is the form of energy possessed by a running car?
Q 34.
A bullet of mass 0.03 kg moving with a speed of 400 m/s penetrates 12 cm into fixed block of wood.Calculate the average force exerted by the wood on the bullet.
Q 35.
A man exerts a force of 200 N in pulling a cart at a constant speed of 16 m/s.Calculate the power spent by the man.
Q 36.
What do you understand by the term energy ? Give its scientific conception.
Q 38.
Fill in the blanks :
Work done against a force is stored as _____.
Q 39.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 40.
Fill in the blanks :
Work done against gravity in lifting a mass is stored as ______. Numerically, it is equal to m ×_ ×__.
Q 41.
What is Kilowatt hour?
Q 42.
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 43.
Write an expression for kinetic energy of an object
Q 44.
Is work a scalar quantity or a vector quantity ?
Q 45.
Give an example where potential energy is acquired by a body due to change of its shape.
Q 46.
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 47.
Fill in the blanks :
Work is a ______ (scalar/vector) quantity.
Q 48.
Fill in the blanks :
Work done is positive when the angle between force and displacement is ____ (acute/obtuse)
Q 49.
Fill in the blanks :
If the mass is doubled while its speed remains the same, the kinetic energy will be _____ (be same/double/four times) of the initial kinetic energy.
Q 50.
A heavy stone is lowered to the ground with the help of rope. Is the work done by the applied force positive or negative?
