Question:
Define average power.
Answer:
Average power=
The concept of average power is useful when the power of an agent varies with time and it does work at different rates during different intervals of time.
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.
What do the machines need for their working ?
Q 4.
How does the wind move the blades of a wind-mill ?
Q 5.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 6.
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 7.
Name a machine that transforms muscular energy into useful mechanical work.
Q 8.
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 9.
What is the form of energy possessed by a running car?
Q 10.
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 11.
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 12.
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 13.
Name two forms of mechanical energy.
Q 14.
Why do some engines require fuels like petrol and diesel ?
Q 15.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 16.
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 17.
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 18.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 19.
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 20.
Is a person doing any work by holding a suitcase ?
Q 21.
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 22.
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 23.
Fill in the blanks :
Ability to do work is called _______.
Q 24.
Name the device which converts electrical energy into mechanical energy.
Q 25.
Write an expression for kinetic energy of an object
Q 26.
When is work done by a force zero ?
Q 27.
Can a body have energy without having momentum ?
Q 28.
How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?
Q 29.
Name the type of energy possessed by a raised hammer?
Q 30.
A body of mass 15 kg undergoes downward displacement of 40m under the effect of gravitational force. How much work is done? (take g = 10 m/s
2)
Q 32.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q 33.
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 34.
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 35.
The work done by the heart is I J per beat.Calculate the power of the heart if it beats 72 times/min.
Q 36.
Define average power.
Q 37.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 38.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
Q 39.
What do you mean by power of an agent ? Give an expression for power.
Q 40.
Can a body have energy without possessing momentum?
Q 41.
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 42.
The kinetic energy of an object of mass, m moving with a velocity of 5 m/s is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
Q 43.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 44.
Assuming the earth revolves around the sun in a perfect circular orbit due to centripetal force exerted by the Sun. Is the work done by the Sun positive or negative.
Q 45.
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 46.
A boy of mass 20 kg climbs up a ladder in 20 seconds. If the height of a ladder is 10 metres, calculate amount of power used.
(Take g = 10 m/s
2)
Q 47.
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 48.
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 49.
Calculate the power of an engine required to lift 105 kg of coal per hour from a mine 360 m deep.
Q 50.
Write an expression for the work done when a force is acting on an object in the direction of its displacement.
