THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE PATTERN
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Conceptual Questions
Q. Define Work in Physics?
Or
When do we say that work is done?
Ans: Work is said to be done, when force produces motion in the body.
Work is equal to product of force exerted on the body and distance moved by it.
Work = Force × Distance
W = F × S
The S.I unit of work is Newton.meter (Nm) i.e Joule.
1 joule of work is done when 1 newton force displaces an object by 1 meter.
Ans: Work is said to be done, when force produces motion in the body.
Work is equal to product of force exerted on the body and distance moved by it.
Work = Force × Distance
W = F × S
The S.I unit of work is Newton.meter (Nm) i.e Joule.
1 joule of work is done when 1 newton force displaces an object by 1 meter.
Q. Write an expression for the work done when a force is acting on
an object in the direction of its displacement.
Ans: The expression for the work done when a force acts on an object in the direction of its displacement will be given as W = F.S
Where, W is the work done, F is the Force and S is the displacement of the object.
Ans: The expression for the work done when a force acts on an object in the direction of its displacement will be given as W = F.S
Where, W is the work done, F is the Force and S is the displacement of the object.
Q. Define 1 J of work.
Ans: One joule of work done is when the force
of one newton is applied on the body and it is displaced by one meter
in the direction of force.
Q. What is Energy?
Ans: Energy of the body is said to be its ability to do work.
It is a scaler quantity. Its SI unit is Joules.
The various forms of energy are Kinetic energy, Potential energy, Light energy, Heat energy etc.
Q. Define Kinetic energy? Derive the relation for kinetic energy?
Ans: The energy of a body due to its motion is called Kinetic energy.
All objects in motion possess energy e.g. falling cocunut, moving bus
Kinetic Energy,
2
K.E. = 1/2 m v
Derivation:
We know that,
Work done = Force × Distance
W = F × S ........... I
The third equation of motion says,
2 2
V - U = 2 A S
2 2
S = V - U .......... II
2 A
Applying equation II in equation I
2 2
W = F × V - U
2 A
We know that F = M × A ........III
Applying above equation, we get:
2 2
W = M × A × V - U
2 A
2 2
W = 1 M ( V - U )
2
If Object starts from the stationary position, then U = 0, so the formula is
2
W = 1/2 m v
2
K.E. = 1/2 m v
Q. Define potential energy? Derive the relation for potential energy.
Ans: The energy of the body due to its position is called potential energy.
Potential energy is the energy stored due to work done on the body.
Derivation:
Suppose a body of mass "m" is raised to height "h"
The force acting on the body is F = m × g
Where g= universal gravitational constant.
To lift the body up, we need to act against the gravity,
So W = F × h
W = m × g × h
Therefore, P.E = mgh.
Q. What is power? Write down the SI unit of power.
Ans: Power is defined as rate of doing work.
Power is work done in particular time.
Power = Work done
Time taken
In terms of energy, power is rate at which energy is consumed.
Power is scaler quantity. Its SI unit is Watt or Joules/second.
1 watt power is defined as when one joule of work is done in 1 second time.
Q. Define 1 watt of power.
Q. What is the commercial unit of energy?
Ans: The commercial unit of energy is Kilowatt.hour (Kwh).
One Kilowatthour is amount of electric energy consumed when an electric item of 1000 watts is used for 1 hour.
1 Kwh = 3,600,000 Joules
One kilowatthour is also called one unit.
Ans: 1 watt of power is defined as one joule of work done in 1
second.
1 Watt = 1 Joule/1 Sec
Q. What is the commercial unit of energy?
Ans: The commercial unit of energy is Kilowatt.hour (Kwh).
One Kilowatthour is amount of electric energy consumed when an electric item of 1000 watts is used for 1 hour.
1 Kwh = 3,600,000 Joules
One kilowatthour is also called one unit.
Q. Define average power?
Q. What do you mean by Law of conservation of energy?
Ans: Law of conservation of energy states that energy can neither be created nor destroyed but converted from one form to another.
In other words, sum of Kinetic energy and potential energy remains same.
Textual Question:
Ans: Average power is defined the ration of total work done to the
total time taken.
Q. What do you mean by Law of conservation of energy?
Ans: Law of conservation of energy states that energy can neither be created nor destroyed but converted from one form to another.
In other words, sum of Kinetic energy and potential energy remains same.
Textual Question:
Q. A force of 7 N acts on the object. The displacement is 8 m, in the direction of force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Ans:
Force acting on object, F = 7 N
Displacement, S = 8 m
Work done, W = F × S
W = 7 × 8
W = 56 Joules
Q. 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 is ploughing the length of field?
Ans:
Force applied by bullock, F = 140 N
Distance to plough, S = 15 m
Work done, W = F × S
W = 140 × 15
W = 2100 Joules.
Q. The kinetic energy of an object of mass, m moving with the 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?
Ans:
Kinetic energy of object = 25 J.
Velocity = 5 m/s.
2
K.E = 1/2 m v
2
25 = 1/2 m (5)
25 = 1/2 m × 25
m = 25 × 2
25
25
m = 2 K.g
Now, if velocity doubles, i.e v = 10
2
K.E = 1/2 m v
K.E = 1 × 2 × 100
2
K.E = 100 Joules
If velocity tripled; i.e V = 15
2
K.E = 1/2 m v
K.E = 1 × 2 × 225
2
K.E = 225 Joules
Q. A lamp consumes 1000 J of electric energy in 10 seconds. What is its power?
Ans:
Electric energy consumed by lamp,W= 1000 J
Time taken, t = 10 s
Therefore Power = Work done
Time taken
P = W
t
P= 1000
10
P = 100 Watt.
Q. The Potential energy of freely falling object decreases progressively. Does this violate law of conservation of energy?
Ans: As the potential energy of freely falling object decreases, its kinetic energy increases.
The total mechanical energy remains constant. Therefore it does not violate law of conservation of energy.
Q. A certain household has consumed 250 units of energy during a month. How much energy is this in Joules?
Ans:
Energy consumed by household= 250 units
250 units = 250 kwh
1 kilowatt hour = 1000 watt × 3600 s
Therefore, 250 kwh = 250 × 1000 × 3600 s
250 kwh = 900, 000, 000 Joules.
Now, if velocity doubles, i.e v = 10
2
K.E = 1/2 m v
K.E = 1 × 2 × 100
2
K.E = 100 Joules
If velocity tripled; i.e V = 15
2
K.E = 1/2 m v
K.E = 1 × 2 × 225
2
K.E = 225 Joules
Q. A lamp consumes 1000 J of electric energy in 10 seconds. What is its power?
Ans:
Electric energy consumed by lamp,W= 1000 J
Time taken, t = 10 s
Therefore Power = Work done
Time taken
P = W
t
P= 1000
10
P = 100 Watt.
Q. The Potential energy of freely falling object decreases progressively. Does this violate law of conservation of energy?
Ans: As the potential energy of freely falling object decreases, its kinetic energy increases.
The total mechanical energy remains constant. Therefore it does not violate law of conservation of energy.
Q. A certain household has consumed 250 units of energy during a month. How much energy is this in Joules?
Ans:
Energy consumed by household= 250 units
250 units = 250 kwh
1 kilowatt hour = 1000 watt × 3600 s
Therefore, 250 kwh = 250 × 1000 × 3600 s
250 kwh = 900, 000, 000 Joules.
Q. What is the work done by the force of gravity on satallite moving round the earth? Justify your answer.
Ans: The work done by force of gravity on satallite moving around the earth is zero.
This satellite moves in a circular orbit. The direction of displacement of the satellite is perpendicular to the force of gravity.
Hence work done is zero.
Q. Can there be displacement of an object in the absence of any force acting on it? Think
Ans: Yes there can the displacement of an object in absence of any force acting on it.
If the object is already moving with the uniform velocity.
Q. An electric heater is rated 1500 W. How much energy does it use in 10 hours.
Ans:
Ans: The work done by force of gravity on satallite moving around the earth is zero.
This satellite moves in a circular orbit. The direction of displacement of the satellite is perpendicular to the force of gravity.
Hence work done is zero.
Q. Can there be displacement of an object in the absence of any force acting on it? Think
Ans: Yes there can the displacement of an object in absence of any force acting on it.
If the object is already moving with the uniform velocity.
Q. An electric heater is rated 1500 W. How much energy does it use in 10 hours.
Ans:
Energy consumed by heater = Power × Time
Energy consumed = 1500 × 10
Energy consumed = 15000 W
Energy consumed = 15 KWh
Q. An object of mass, m is moving with constant velocity, v. How much work should be done on an object in order to bring the object to rest?
Ans:
In order to bring the object to rest, its kinetic energy has to be reduced to zero.
This can be done by doing negative work on the object which is equal to its kinetic energy i.e.
2
_ 1 m. V
2
Q. Find the energy in KWh consumed in 10 hours by four devices of power 500 w each.
Ans:
Power, P=500 W i.e. 0.5 kW
Time, t=10 hours
Energy consumed by 4 devices, E= 4 × P× t
E = 4 × 0.500 × 10
E = 20 kWh
Q. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Ans: When the free falling object is in air, ifs Potential energy converts into kinetic energy.
When that object hits the ground, its kinetic energy converts into sound energy, heat energy.
Energy consumed = 1500 × 10
Energy consumed = 15000 W
Energy consumed = 15 KWh
Q. An object of mass, m is moving with constant velocity, v. How much work should be done on an object in order to bring the object to rest?
Ans:
In order to bring the object to rest, its kinetic energy has to be reduced to zero.
This can be done by doing negative work on the object which is equal to its kinetic energy i.e.
2
_ 1 m. V
2
Q. Find the energy in KWh consumed in 10 hours by four devices of power 500 w each.
Ans:
Power, P=500 W i.e. 0.5 kW
Time, t=10 hours
Energy consumed by 4 devices, E= 4 × P× t
E = 4 × 0.500 × 10
E = 20 kWh
Q. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
Ans: When the free falling object is in air, ifs Potential energy converts into kinetic energy.
When that object hits the ground, its kinetic energy converts into sound energy, heat energy.
Ans: We know that,
Work done on an object by the force of gravity is given by Potential energy i.e. mgh where h is the vertical distance travelled by the object.
In the above case, the initial and the final points of an object lie on the same horizontal plane and the object has travelled 0 vertical distance.
Therefore, work done by the gravity = mgh
Work done by the gravity = mg x 0
Work done by the gravity = 0.
Q. A battery lights a bulb. Describe the energy changes involved in the process.
Ans: When a battery lights a bulb, the chemical energy is converted into the electrical energy.
Q. 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.
Ans: Given,
Initial Velocity (U) = 5 m/s
Final Velocity (V) = 2 m/s
Mass of object = 20 kg
We know,
Work done to change velocity is equal to change in the Kinetic energy of the object.
W = ½ MV2 – ½ MU2
W = ½ 20 . (2)2 – ½ 20 (5)2
W = 40 – 250
W = - 210 J
The Minus sign indicates that the work is done in the direction opposite to the direction of motion.
Q. 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.
Ans: Work done on an object by the force of gravity is given by Potential energy i.e. mgh where h is the vertical distance travelled by the object.
Since, the object moves in the horizontal direction, therefore Zero/No work is done by the gravity.
Q. What are the various energy transformations that occur when you are riding a bicycle?
Ans: When we ride the bicycle, our muscular energy is converted into the kinetic energy which propels the bicycle.
Q. 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?
Ans: When we push a huge rock and fail to move it, no transfer of energy takes place.
When we push the rock, our muscular energy gets converted to heat energy in our body.
Q. 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.
Ans: Given,
Mass = 40 kg
Height = 5 m
Acceleration due to gravity = 9.8 m/s2
We Know,
Potential Energy = Mgh
P.E = 40 x 9.8 x 5
P.E = 1960 Joules
Kinetic energy of an object when it is half way down:
According to the law of conservation of energy, the energy can neither be created nor destroyed but converted from one form to another. In other words, the total energy of the system remains constant.
Hence when the object falls, its Potential energy is converted into Kinetic energy.
Therefore, Kinetic energy is an object when it is half way down will be the equal to the Potential energy at that point.
But, Potential energy half way down = 1960/2 = 980 J
Thus, Kinetic energy half way down = 980 Joules.
Q. 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.
Ans: When a person holds a bundle of hay over his head for 30 minutes, then there is no displacement. Hence,, the work done by the person on the bundle is F x 0 i.e. Zero.
Q.
a. 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.
Ans: When a pendulum bob is allowed to oscillate, its kinetic energy is converted to potential energy and vice versa.
b. Why does the bob eventually come to rest?
b. Why does the bob eventually come to rest?
c. What happens to its energy eventually?
d. Is it a violation of the law of conservation of energy?
Ans: No, it is not the violation of law of conservation of energy since the energy is transferred to the surrounding air.
Q. Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
Ans: Given,
Mass of the Car = 1500 kg
Velocity of the car = 60 km/h i.e. 16.66 m/s
We know, work done to stop an object is equal and opposite to the kinetic energy of the object.
W = - ½ MV^2 (The minus sign indicates that the work is done in the opposite direction of the motion)
W = - ½ (1500)(16.66)^2
W = - 750 x 277.55
W = - 208166.7 Joules.
Q. 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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Ans:
Case 1: The Force acts Perpendicular to the direction of motion of the body. Thus, work done is Zero.
Case 2: The Force acts in the same direction as the motion of the body. Thus, work done is Positive.
Case 3: The Force acts in the opposite direction of motion of the body. Thus, work done is Negative.
Q. 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?
Ans: Yes, the acceleration in an object could be zero even when several forces are acting on it. This can happen when the forces balance each other.
Q. Find the energy in joules consumed in 10 hours by four devices of power 500 W each.
Ans: Given,
Power = 500 W
Time = 10 h i.e. 36000 seconds
Devices = 4
We Know that,
Energy = Power x Time
E = 500 x 36000
E = 18000000 Joules
For 4 devices:
Energy = 4 x 18000000
Energy consumed = 72000000 Joules.
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