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A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force as shown in figure. Let us take it that the force acts on the object through the displacement.What is the work done in this case?
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When do we say that work is done?
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Write an expression for the work done when a force is acting on an object in the direction of its displacement.
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Define 1 J of work.
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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?
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What is the kinetic energy of an object?
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Write an expression for the kinetic energy of an object.
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The kinetic energy of an object of mass m moving with a velocity of 5 ms-1 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?
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What is power?
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Define 1 watt of power.
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A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
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Define average power
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Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.
•Suma is swimming in a pond.
•A donkey is carrying a load on its back.
•A wind-mill is lifting water from a well.
•A green plant is carrying out photosynthesis.
•An engine is pulling a train.
•Food grains are getting dried in the sun.
•A sailboat is moving due to wind energy.
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An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
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A battery lights a bulb. Describe the energy changes involved in the process.
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Certain force acting on a 20 kg mass changes its velocity from 5 ms-1 to 2 ms-1. Calculate the work done by the force.
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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.
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The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
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What are the various energy transformations that occur when you are riding a bicycle?
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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?
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A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
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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.
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What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.
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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.
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A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.
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An electric heater is rated 1500 W. How much energy does it use in 10 hours?
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Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
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An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?
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Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km h-1 ?
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The sign of minus indicates that the work is being done against the moving car. In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
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Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
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Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.
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A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
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question_answer34)
When a body falls freely towards the earth, then its total energy
(a) increases (b) decreases
(c) remains constant
(d) first increases and then decreases
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question_answer35)
A car is accelerated on a leveled road and attains a
velocity 4 times of its initial velocity. In this process, the potential energy
of the car
(a) does not change
(b) becomes twice to that of initial
(c) becomes 4 times that of initial d)
becomes 16 times that of initial
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question_answer36)
In case of negative work, + the angle between the force
and displacement is
(a) 0 (b) (c)
(d)
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question_answer37)
An iron sphere of mass 10 kg has the same diameter as an aluminium
sphere of mass 3.5 kg. Both spheres are dropped simultaneously from a tower.
When they are 10 m above the ground, they have the same
(a) acceleration (b) momenta (c)
potential energy (d) kinetic energy
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question_answer38)
A girl is carrying a school bag of 3 kg mass on her back
and moves 200 m on a leveled road. The work done against the gravitational
force will be
(a) (b) (c) (d)zero
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question_answer39)
Which one of the following is not the unit of energy?
(a) Joule (b) Newton metre (c)
Kilowatt (d) Kilowatt hour
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question_answer40)
The work done on an object does not depend upon the
(a) displacement (b) force applied
(c) angle between force and displacement
(d) initial velocity of the object
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question_answer41)
Water stored in a dam possesses
(a) no energy (b) electrical energy (c)
kinetic energy (d) potential energy
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question_answer42)
A body is falling from a height h, After it has fallen a
height , it will possess
(a) only potential energy
(b) only kinetic energy
(c) half potential and half kinetic energy
(d) more kinetic and less potential energy
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question_answer43)
A rocket is moving up with a velocity v. If the velocity
of this rocket is suddenly tripled, what will be the ratio of two kinetic
energies?
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question_answer44)
Avinash can run with a speed of against the
frictional force of 10 N and Kapil can move with a speed of against the
frictional force of 25 N. Who is more powerful and why?
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question_answer45)
A boy is moving on a straight road against a frictional force of 5 N.
After travelling a distance of 1.5 km, he forgot the correct path at a
roundabout of radius 100 m as shown in figure. However, he moves on the circular
path for one and half cycle and then he moves forward up to 2.0 km. calculate
the work done by him.
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question_answer46)
Can any object have mechanical energy even if its momentum is zero?
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question_answer47)
Can any object have momentum even if its mechanical energy
is zero? Explain.
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question_answer48)
The power of a motor pump is 2 kW. How much water per
minute, the pump can raise to a height of 10 m? [given, ]
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question_answer49)
The weight of a person on a planet A is about half that on
the earth. He can jump up to 0.4 m height on the surface of the earth. How high
he can jump on the planet A?
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question_answer50)
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.
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question_answer51)
Is it possible that an object is in the state of
accelerated motion due to external force acting on it, but no work is being
done by the force?
Explain it with an example.
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question_answer52)
A ball is dropped from a height of 10 m. If the energy of
the ball reduces by 40% after striking the ground, how much high can the ball
bounce back?
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question_answer53)
If an electric iron of 1200 W is used for 30 min everyday,
find electric energy consumed in the month of April.
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question_answer54)
A light and a heavy object have the same momentum. Find
out the ratio of their kinetic energies. Which one has a larger kinetic energy?
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question_answer55)
An automobile engine propels a 1000 kg car A along a leveled
road at a speed of 36 . Find
the power if the opposing frictional force is 100 N. Now, suppose after
travelling a distance of 200 m, this car collides with another stationary car B
of same mass and comes to rest.
Let its engine also stop at the same time. Now car B
starts moving on the same level road without getting its engine started. Find
the speed of the car B just after the collision.
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question_answer56)
A girl having mass of 35 kg sits on a trolley of mass 5
kg. The trolley is given an initial velocity of by appling a
force. The trolley comes to rest after traversing a distance of 16 m. (a) How
much work is done on the trolley? (b) How much work is done by the girl?
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question_answer57)
Four men lift a 250 kg box to a height of 1 m and hold it
without raising or lowering it.
(a) How much work is done by the men in lifting the box?
(b) How much work do they do in just holding it? (c) Why
do they get tired while holding it? [given = 10 ms-2].
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question_answer58)
What is power? How do you differentiate kilowatt from
kilowatt hour?
The Jog Falls in Karnataka state are nearly 20 m high.
2000 tonnes of water falls from it in a minute. Calculate the equivalent power
if all this energy can be utilized?
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question_answer59)
How is the power related to the speed at which a body can
be lifted?
How many kilograms will a man working at the power of 100
W, be able to lift at constant speed of 1 ms-1 vertically?
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question_answer60)
Define watt. Express kilowatt in terms of joule per
second. A 150 kg car engine develops 500 W for each kg. What force does it
exert in moving the car at speed of 20 ms-1?
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question_answer61)
Compare the power at which each of the following is moving
upwards against the force of gravity? [given, ]
(i) A butterfly of mass 1.0 g that flies upward at a rate
of 0.5 ms-1.
(ii) A 250 g squirrel climbing upon a tree at a rate of
0.5 ms-1 .
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