Question:
What is Kilowatt hour?
Answer:
Kilo watt hour is a unit of energy. It define the relation between work and power. 1 kW hour means one killowatt of power is supplied for an hour.
⇒ 1 kWh = 1000J/s × 3600 s = 3.6 × 106 J
Work and Energy
Q 1.
Name a machine that transforms muscular energy into useful mechanical work.
Q 2.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 3.
What do the machines need for their working ?
Q 4.
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 5.
How does the wind move the blades of a wind-mill ?
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.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 9.
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 10.
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 11.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 12.
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 13.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 14.
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 15.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 16.
What is the form of energy possessed by a running car?
Q 17.
Name two forms of mechanical energy.
Q 18.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 19.
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 20.
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 21.
Can a body have energy without having momentum ?
Q 22.
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 23.
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 24.
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 25.
Define average power.
Q 26.
Name the device which converts electrical energy into mechanical energy.
Q 27.
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 28.
Is a person doing any work by holding a suitcase ?
Q 29.
Why do some engines require fuels like petrol and diesel ?
Q 30.
What is a conservative forces? Give an example.
Q 31.
Name the type of energy possessed by a raised hammer?
Q 32.
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 33.
When an arrow is shot from a bow,it has kinetic energy.From where does it get the kinetic energy ?
Q 34.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 35.
Fill in the blanks :
A form of energy an object has because of its motion is called ________.
Q 36.
Fill in the blanks :
Work done is positive when the angle between force and displacement is ____ (acute/obtuse)
Q 37.
From where the falling raindrops gains kinetic energy?
Q 38.
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 39.
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 40.
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 41.
How is work done by a force measured ?
Q 42.
What do you mean by the law of conservation of energy ?
Q 43.
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.
Q 44.
Fill in the blanks :
Work done against a force is stored as _____.
Q 45.
A force of 10 N moves a body with a constant speed of 2 m/s. Calculate the power of the body.
Q 46.
When a player hits a football it moves along the curved path and then falls to the ground. Calculate the work done by the force of gravity on the football.
Q 47.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 48.
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 49.
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 50.
The work done by the heart is I J per beat.Calculate the power of the heart if it beats 72 times/min.
