Question:
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Answer:
When a freely falling body eventually stops on reaching the ground,its kinetic energy appears in the form of:
(i) heat (the body and the ground become warmer due to collision),
(ii) sound (produced due to collision of the body with the ground),
(iii) potential energy of configuration of the body and the ground(the body may be deformed and the ground may be depressed at the place of collision).
This process in which the kinetic energy of a freely falling body is lost in an unproductive chain of energy changes is called dissipation of energy.
Work and Energy
Q 1.
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 2.
Name a machine that transforms muscular energy into useful mechanical work.
Q 3.
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 4.
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 5.
What do the machines need for their working ?
Q 6.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 8.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 9.
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 10.
When is work done by a force zero ?
Q 11.
Fill in the blanks :
1 Joule = ____ × ____.
Q 12.
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 13.
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 14.
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 15.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
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.
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 18.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 19.
How does the wind move the blades of a wind-mill ?
Q 20.
Is Work a scalar or vector quantity?
Q 21.
Fill in the blanks :
A force acting on a body shows a displacement in the direction normal to the direction of the force. It is said _______ done on the body.
Q 22.
A roller coaster is shown in figure, at points i.e. ➊ and ➋, comparatively at what point (i) potential energy is maximum (ii) kinetic energy is maximum
Q 23.
A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Q 24.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q 25.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 26.
What are the units of work and energy ?
Q 27.
What do you understand by the term energy ? Give its scientific conception.
Q 28.
What do you mean by power of an agent ? Give an expression for power.
Q 29.
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 30.
How do you differentiate between energy and power ?
Q 31.
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 32.
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 33.
A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Q 34.
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 35.
Fill in the blanks :
The sum of P.E. and K.E. is called __________.
Q 36.
What is the relationship between kinetic energy and work?
Q 37.
Certain force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. Calculate the work done by the force.
Q 38.
Can kinetic energy of an object be negative?
Q 39.
From where the falling raindrops gains kinetic energy?
Q 40.
Identify the kind of energy possessed by a running athlete.
Q 41.
What is the form of energy possessed by a running car?
Q 42.
Calculate potential energyof a person having 60 kg mass on the summit of Mt. Everest. Height of Mt. Everest is 8848 m from sea level. (g = 9.8 m/s
2)
Q 43.
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 44.
A momentum of the body is increased by 20%. What is the percentage increase in its kinetic energy?
Q 45.
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 46.
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 47.
Define average power.
Q 48.
A battery lights a bulb.Describe the energy changes involved in the process
Q 49.
A certain household has consumed 250 units of energy during a month.How much energy is this in joules ?
Q 50.
When is work done by a force negative ?
