Question:
whether or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in a pond.
(b) A donkey is carrying a load on its back.
(c) A wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the Sun.
(g) A sailboat is moving due to wind energy.
Answer:
(a)Suma is doing work as she moves (displaces) herself by applying force with the movement of her arms and legs in the water.
(b) A donkey is not doing any work (in the technical sense) as the weight he is carrying (Le.t force) and the displacement are perpendicular to each other.However, he does work against friction between his hoofs and the ground. Further, work is also done by the donkey if he climbs up a mountain as in that case force and displacement are not perpendicular to each other.
(c) The kinetic energy of the wind is imparted to the wind-mill which lifts water from the well.Thus,a wind-mill works against the force of gravity to lift water vertically upward.
(d) Photosynthesis is the process by which green plants use Sun’s energy to build up carbohydrate reserves.In this case,both force (F) and displacement (s) are zero and as such work done is zero.
(e) When the engine pulls train,it does work (i)in pulling the train and (ii) against the force of friction between the wheels of the train and the railway track.
(f) When the food grains are dried in the Sun,solar radiation takes moisture out of the food grains.Since there is no force (F) and no displacement (s) is involved in the process,work done is zero.
(g) The force,F exerted by the wind on the sailboat moves it through a distance (s) in its direction.Hence,work is done by the wind energy in moving the sailboat.
Work and Energy
Q 1.
Fill in the blanks :
Work is a ______ (scalar/vector) quantity.
Q 2.
Fill in the blanks :
Work done on an object is _____ if displacement is zero.
Q 3.
A force of 10 N moves a body with a constant speed of 2 m/s. Calculate the power of the body.
Q 4.
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 5.
Fill in the blanks :
Work done by gravity on a falling body is available as ______ (kinetic/potential) energy on reaching the ground.
Q 6.
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 7.
Fill in the blanks :
Work done is defined as the product of _____ and ______ in the direction of force.
Q 8.
Fill in the blanks :
Work done is negative when the angle between force and displacement is ____ (acute/obtuse)
Q 9.
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 10.
Write an expression for the kinetic energy of an object.
Q 11.
Write an expression for the kinetic energy of an object in terms (i) velocity (
v) of an object (ii) in terms of momentum (
p) of an object.
Q 12.
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 13.
A battery lights a bulb.Describe the energy changes involved in the process
Q 14.
What do you understand by the term energy ? Give its scientific conception.
Q 15.
What are other forms of energies apart from mechanical energy? Does Law of Conservation of Energy apply to all of them?
Q 16.
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 objjgft by the gravitational force ? Explain your answer.
Q 17.
Two bodies of masses m
1 and m
2 have equal kinetic energies. If p
1 and p
2 are their respective momentum, then ratio p
1 : p
2 is ___?
Q 18.
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Q 19.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Q 20.
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 21.
If the momentum of a body is increased
n times, its kinetic energy increases (a) n times (b) 2n times (c) √n times (d) n
2 times
Q 22.
How do you differentiate between energy and power ?
Q 23.
Is it possible to havfc a situation where mechanical energy (E) - potential energy (
) Is negative,i.e.,(E-
)< 0.
Q 24.
Fill in the blanks :
Work done against a force is stored as _____.
Q 25.
Does the work done on a load depend on the time taken to raise the load?
Q 26.
Can there be displacement of an object in the absence of any force acting on it?
Q 27.
Two bodies of equal masses move with uniform velocities v and 3 v respectively.Find the ratio of their kinetic energies.
Q 28.
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 29.
Define 1 J of work.
Q 30.
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 increased three times ?
Q 31.
What are the units of work and energy ?
Q 32.
Name the type of energy possessed by a raised hammer.
Q 33.
What is meant by the term potential energy ? Give its two examples.
Q 34.
Define kinetic energy.Give some examples of bodies possessing kinetic energy.
Q 35.
Fill in the blanks :
Ability of a body to do work due to its position or configuration is known as _________ of the body.
Q 36.
What is mechanical energy? What are its types?
Q 37.
What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
Q 38.
What is the kinetic energy of an object ?
Q 39.
What are the various energy transformations that occur when you are riding a bicycle ?
Q 40.
A certain household has consumed 250 units of energy during a month.How much energy is this in joules ?
Q 41.
What is the difference between potential energy and kinetic energy ?
Q 42.
What do you mean by the term potential energy ? Give some examples of bodies possessing potential energy ?
Q 43.
Name a few forms of energy and their transformation into each other.
Q 44.
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 45.
Fill in the blanks :
A form of energy an object has because of its motion is called ________.
Q 46.
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 47.
A boy of mass 45 kg climbs up 20 steps in 20 second. If each step is 25 cm high, calculate the power used in climbing
Q 48.
Is it possible that a force acts on a body still the work done is zero? Explain with an example.
Q 49.
Fill in the blanks :
SI Unit of work is _______.
Q 50.
Can kinetic energy of an object be negative?
