Goyal Brothers Textbook Solutions: Work Power And Energy

 Page 1


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Work, Power And Energy 
 
Question 1. 
(a) Define work. 
(b) What are the conditions for doing work 
(c) State the mathematical expression for work. 
Answer: 
(a) “When force is applied on the body and body moves (covers , some distance) in the 
direction of force, work is said to be done.” 
Or 
“Work is said to be done, when a force or its component causes a displacement in its 
own direction.” 
(b) 
(1) Force should be applied. 
(2) Displacement of body should be there. 
(3) Work = Force x displacement 
W = F x S 
Question 2. 
In which case work is done and why ? 
(a) A man pushing a wall. 
(b) A girl climbing a stair case 
(c) A boy swimming in a tank. 
(d) A man standing at a place and holding a suitcase in hand. 
(e) A lady cooking food. 
(f) A porter carrying a load on his head walking along a level road. 
(g) A porter carrying a load and climbing upstairs. 
Answer: 
(a) A man pushing a wall does no work as wall does not move from its place and there 
is no displacement of wall in the direction of force. 
(b) A girl climbing a staircase does work as component of force is in the direction of 
displacement. 
(c) A boy swimming in a tank is doing work as force is applied in a direction opposite to 
the direction of displacement. 
(d) The man is not doing work as displacement is zero. 
(e)  A lady cooking food is doing no work as displacement of lady is zero. 
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Work, Power And Energy 
 
Question 1. 
(a) Define work. 
(b) What are the conditions for doing work 
(c) State the mathematical expression for work. 
Answer: 
(a) “When force is applied on the body and body moves (covers , some distance) in the 
direction of force, work is said to be done.” 
Or 
“Work is said to be done, when a force or its component causes a displacement in its 
own direction.” 
(b) 
(1) Force should be applied. 
(2) Displacement of body should be there. 
(3) Work = Force x displacement 
W = F x S 
Question 2. 
In which case work is done and why ? 
(a) A man pushing a wall. 
(b) A girl climbing a stair case 
(c) A boy swimming in a tank. 
(d) A man standing at a place and holding a suitcase in hand. 
(e) A lady cooking food. 
(f) A porter carrying a load on his head walking along a level road. 
(g) A porter carrying a load and climbing upstairs. 
Answer: 
(a) A man pushing a wall does no work as wall does not move from its place and there 
is no displacement of wall in the direction of force. 
(b) A girl climbing a staircase does work as component of force is in the direction of 
displacement. 
(c) A boy swimming in a tank is doing work as force is applied in a direction opposite to 
the direction of displacement. 
(d) The man is not doing work as displacement is zero. 
(e)  A lady cooking food is doing no work as displacement of lady is zero. 
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(f) A porter is doing no work as the direction of his displacement is at right angle (90°) 
and force of gravity is downward (vertically) 
(g) Porter does work as component of force is in direction of displacement. 
Question 3 
A man climbs a slope and another walks the same distance on a level road. Who does 
more work and why ? 
Answer: 
The man who walk on a level road does no work as he is walking at right angle to the 
direction of gravitational force. Hence the man climbs a slope is doing more work. 
Question 4. 
(a) State the CGS and SI units of work. 
(b) How is joule related to erg ? 
Answer: 
(a) CGS unit of work is erg or gcm
2
s
2 
S.I. unit of work is joule or kgm
2
s
2 
(b) 1 J = 10
7
 ergs or 1 erg = 10
-7
 g 
Question 5. 
Define power. State two mathematical expressions for power. 
Answer: 
Work : “Rate of doing work is called power.” 
Two mathematical expressions for power are : 
 
Question 6. 
(a) State the absolute unit of power in SI system. 
(b) What is horsepower ? What is its magnitude in SI unit ? 
Answer: 
(a) Absolute unit of power is watt. 
(b) Horse power is unit of power used in engineering 1 H.P = 746 w . 
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Work, Power And Energy 
 
Question 1. 
(a) Define work. 
(b) What are the conditions for doing work 
(c) State the mathematical expression for work. 
Answer: 
(a) “When force is applied on the body and body moves (covers , some distance) in the 
direction of force, work is said to be done.” 
Or 
“Work is said to be done, when a force or its component causes a displacement in its 
own direction.” 
(b) 
(1) Force should be applied. 
(2) Displacement of body should be there. 
(3) Work = Force x displacement 
W = F x S 
Question 2. 
In which case work is done and why ? 
(a) A man pushing a wall. 
(b) A girl climbing a stair case 
(c) A boy swimming in a tank. 
(d) A man standing at a place and holding a suitcase in hand. 
(e) A lady cooking food. 
(f) A porter carrying a load on his head walking along a level road. 
(g) A porter carrying a load and climbing upstairs. 
Answer: 
(a) A man pushing a wall does no work as wall does not move from its place and there 
is no displacement of wall in the direction of force. 
(b) A girl climbing a staircase does work as component of force is in the direction of 
displacement. 
(c) A boy swimming in a tank is doing work as force is applied in a direction opposite to 
the direction of displacement. 
(d) The man is not doing work as displacement is zero. 
(e)  A lady cooking food is doing no work as displacement of lady is zero. 
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(f) A porter is doing no work as the direction of his displacement is at right angle (90°) 
and force of gravity is downward (vertically) 
(g) Porter does work as component of force is in direction of displacement. 
Question 3 
A man climbs a slope and another walks the same distance on a level road. Who does 
more work and why ? 
Answer: 
The man who walk on a level road does no work as he is walking at right angle to the 
direction of gravitational force. Hence the man climbs a slope is doing more work. 
Question 4. 
(a) State the CGS and SI units of work. 
(b) How is joule related to erg ? 
Answer: 
(a) CGS unit of work is erg or gcm
2
s
2 
S.I. unit of work is joule or kgm
2
s
2 
(b) 1 J = 10
7
 ergs or 1 erg = 10
-7
 g 
Question 5. 
Define power. State two mathematical expressions for power. 
Answer: 
Work : “Rate of doing work is called power.” 
Two mathematical expressions for power are : 
 
Question 6. 
(a) State the absolute unit of power in SI system. 
(b) What is horsepower ? What is its magnitude in SI unit ? 
Answer: 
(a) Absolute unit of power is watt. 
(b) Horse power is unit of power used in engineering 1 H.P = 746 w . 
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Question 7. 
(a) What is energy ? State and define SI unit of energy. 
(b) Define potential energy. Give two examples of potential energy. 
(c) Define kinetic energy. Give four examples of kinetic energy. 
Answer: 
(a) Energy : “Capacity of doing work” 
S.I. unit of energy is Joule 
(b)Potential energy: “Energy posserred by a body by virtue of it position or configuration 
is called potential energy.” 
Example : 
1. A key to works on P.E. when we wind the key its shape changes and on unwinding 
this energy is used by to do work. 
2. A stone kept at a height, when drop can break a plate of glass because of P.E. 
possessed by it 
(c) Kinetic energy: “Energy possessed by a body by virtue of its motion.” 
Examples : 
1. A bullet though of very small mass but moving with high speed and hence kinetic 
energy can peneterate a body. 
2. Running water of the river due its kinetic energy can rotate a turbine to produce 
electricity. 
3. A trunk running at high speed possesses kinetic energy and when hits a body can 
damage it. 
4. A shooting arrow possesses kinetic energy. 
5. Blowing wind possesses K.E. 
Question 8. 
What kind of energy is possessed by a body in the following cases ? 
(a) A cocked-up spring and an air gun. 
(b) A shooting arrow. 
(c) A stone lying on the top of a housi. 
(d) Water stored in the dam. 
(e) An electron spining around the nucleus. 
(f) A fish moving in water. 
Answer: 
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Work, Power And Energy 
 
Question 1. 
(a) Define work. 
(b) What are the conditions for doing work 
(c) State the mathematical expression for work. 
Answer: 
(a) “When force is applied on the body and body moves (covers , some distance) in the 
direction of force, work is said to be done.” 
Or 
“Work is said to be done, when a force or its component causes a displacement in its 
own direction.” 
(b) 
(1) Force should be applied. 
(2) Displacement of body should be there. 
(3) Work = Force x displacement 
W = F x S 
Question 2. 
In which case work is done and why ? 
(a) A man pushing a wall. 
(b) A girl climbing a stair case 
(c) A boy swimming in a tank. 
(d) A man standing at a place and holding a suitcase in hand. 
(e) A lady cooking food. 
(f) A porter carrying a load on his head walking along a level road. 
(g) A porter carrying a load and climbing upstairs. 
Answer: 
(a) A man pushing a wall does no work as wall does not move from its place and there 
is no displacement of wall in the direction of force. 
(b) A girl climbing a staircase does work as component of force is in the direction of 
displacement. 
(c) A boy swimming in a tank is doing work as force is applied in a direction opposite to 
the direction of displacement. 
(d) The man is not doing work as displacement is zero. 
(e)  A lady cooking food is doing no work as displacement of lady is zero. 
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(f) A porter is doing no work as the direction of his displacement is at right angle (90°) 
and force of gravity is downward (vertically) 
(g) Porter does work as component of force is in direction of displacement. 
Question 3 
A man climbs a slope and another walks the same distance on a level road. Who does 
more work and why ? 
Answer: 
The man who walk on a level road does no work as he is walking at right angle to the 
direction of gravitational force. Hence the man climbs a slope is doing more work. 
Question 4. 
(a) State the CGS and SI units of work. 
(b) How is joule related to erg ? 
Answer: 
(a) CGS unit of work is erg or gcm
2
s
2 
S.I. unit of work is joule or kgm
2
s
2 
(b) 1 J = 10
7
 ergs or 1 erg = 10
-7
 g 
Question 5. 
Define power. State two mathematical expressions for power. 
Answer: 
Work : “Rate of doing work is called power.” 
Two mathematical expressions for power are : 
 
Question 6. 
(a) State the absolute unit of power in SI system. 
(b) What is horsepower ? What is its magnitude in SI unit ? 
Answer: 
(a) Absolute unit of power is watt. 
(b) Horse power is unit of power used in engineering 1 H.P = 746 w . 
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Question 7. 
(a) What is energy ? State and define SI unit of energy. 
(b) Define potential energy. Give two examples of potential energy. 
(c) Define kinetic energy. Give four examples of kinetic energy. 
Answer: 
(a) Energy : “Capacity of doing work” 
S.I. unit of energy is Joule 
(b)Potential energy: “Energy posserred by a body by virtue of it position or configuration 
is called potential energy.” 
Example : 
1. A key to works on P.E. when we wind the key its shape changes and on unwinding 
this energy is used by to do work. 
2. A stone kept at a height, when drop can break a plate of glass because of P.E. 
possessed by it 
(c) Kinetic energy: “Energy possessed by a body by virtue of its motion.” 
Examples : 
1. A bullet though of very small mass but moving with high speed and hence kinetic 
energy can peneterate a body. 
2. Running water of the river due its kinetic energy can rotate a turbine to produce 
electricity. 
3. A trunk running at high speed possesses kinetic energy and when hits a body can 
damage it. 
4. A shooting arrow possesses kinetic energy. 
5. Blowing wind possesses K.E. 
Question 8. 
What kind of energy is possessed by a body in the following cases ? 
(a) A cocked-up spring and an air gun. 
(b) A shooting arrow. 
(c) A stone lying on the top of a housi. 
(d) Water stored in the dam. 
(e) An electron spining around the nucleus. 
(f) A fish moving in water. 
Answer: 
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(a) Potential energy, (stretched spring) 
(b) Kinetic energy as arrow is in motion 
(c) A stone lying on the top of a house has potential energy due to its position above the 
ground level. 
(d) Water stored in dam has potential energy. 
(e) An electron spining around the nucleus has kinetic energy. 
(f) A fish moving in water has kinetic energy. 
Question 9. 
(a) State the law of conservation of energy. 
(b) Prove mathematically the law of conservation of energy. 
(c) Explain how a freely swinging pendulum obeys the law of conservation of energy. 
(d) Name six kinds of energy familiar to you. 
Answer: 
(a) Law of conservation of energy : “Energy can neither be created nor destroyed. 
Though it can be transformed from one form to other.” 
(b) Mathematical proof of law of conservation of energy : 
At A Consider a body of mass m at A at a height h above the ground level. 
P.E. = mgh K.E. = 0 at rest. 
 
P.E. + K.E. = mgh + O = mgh     …………….. (i) 
Let it fall from A to B covering a distance x and still at (h-x) above ground. 
In doing so 
v
2
 – u
2
 = 2 gx 
v
2
 – 0 = 2gx 
v
2
 = 2gx          …(a) 
At B P.E = mg (h – x) 
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Work, Power And Energy 
 
Question 1. 
(a) Define work. 
(b) What are the conditions for doing work 
(c) State the mathematical expression for work. 
Answer: 
(a) “When force is applied on the body and body moves (covers , some distance) in the 
direction of force, work is said to be done.” 
Or 
“Work is said to be done, when a force or its component causes a displacement in its 
own direction.” 
(b) 
(1) Force should be applied. 
(2) Displacement of body should be there. 
(3) Work = Force x displacement 
W = F x S 
Question 2. 
In which case work is done and why ? 
(a) A man pushing a wall. 
(b) A girl climbing a stair case 
(c) A boy swimming in a tank. 
(d) A man standing at a place and holding a suitcase in hand. 
(e) A lady cooking food. 
(f) A porter carrying a load on his head walking along a level road. 
(g) A porter carrying a load and climbing upstairs. 
Answer: 
(a) A man pushing a wall does no work as wall does not move from its place and there 
is no displacement of wall in the direction of force. 
(b) A girl climbing a staircase does work as component of force is in the direction of 
displacement. 
(c) A boy swimming in a tank is doing work as force is applied in a direction opposite to 
the direction of displacement. 
(d) The man is not doing work as displacement is zero. 
(e)  A lady cooking food is doing no work as displacement of lady is zero. 
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(f) A porter is doing no work as the direction of his displacement is at right angle (90°) 
and force of gravity is downward (vertically) 
(g) Porter does work as component of force is in direction of displacement. 
Question 3 
A man climbs a slope and another walks the same distance on a level road. Who does 
more work and why ? 
Answer: 
The man who walk on a level road does no work as he is walking at right angle to the 
direction of gravitational force. Hence the man climbs a slope is doing more work. 
Question 4. 
(a) State the CGS and SI units of work. 
(b) How is joule related to erg ? 
Answer: 
(a) CGS unit of work is erg or gcm
2
s
2 
S.I. unit of work is joule or kgm
2
s
2 
(b) 1 J = 10
7
 ergs or 1 erg = 10
-7
 g 
Question 5. 
Define power. State two mathematical expressions for power. 
Answer: 
Work : “Rate of doing work is called power.” 
Two mathematical expressions for power are : 
 
Question 6. 
(a) State the absolute unit of power in SI system. 
(b) What is horsepower ? What is its magnitude in SI unit ? 
Answer: 
(a) Absolute unit of power is watt. 
(b) Horse power is unit of power used in engineering 1 H.P = 746 w . 
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Question 7. 
(a) What is energy ? State and define SI unit of energy. 
(b) Define potential energy. Give two examples of potential energy. 
(c) Define kinetic energy. Give four examples of kinetic energy. 
Answer: 
(a) Energy : “Capacity of doing work” 
S.I. unit of energy is Joule 
(b)Potential energy: “Energy posserred by a body by virtue of it position or configuration 
is called potential energy.” 
Example : 
1. A key to works on P.E. when we wind the key its shape changes and on unwinding 
this energy is used by to do work. 
2. A stone kept at a height, when drop can break a plate of glass because of P.E. 
possessed by it 
(c) Kinetic energy: “Energy possessed by a body by virtue of its motion.” 
Examples : 
1. A bullet though of very small mass but moving with high speed and hence kinetic 
energy can peneterate a body. 
2. Running water of the river due its kinetic energy can rotate a turbine to produce 
electricity. 
3. A trunk running at high speed possesses kinetic energy and when hits a body can 
damage it. 
4. A shooting arrow possesses kinetic energy. 
5. Blowing wind possesses K.E. 
Question 8. 
What kind of energy is possessed by a body in the following cases ? 
(a) A cocked-up spring and an air gun. 
(b) A shooting arrow. 
(c) A stone lying on the top of a housi. 
(d) Water stored in the dam. 
(e) An electron spining around the nucleus. 
(f) A fish moving in water. 
Answer: 
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(a) Potential energy, (stretched spring) 
(b) Kinetic energy as arrow is in motion 
(c) A stone lying on the top of a house has potential energy due to its position above the 
ground level. 
(d) Water stored in dam has potential energy. 
(e) An electron spining around the nucleus has kinetic energy. 
(f) A fish moving in water has kinetic energy. 
Question 9. 
(a) State the law of conservation of energy. 
(b) Prove mathematically the law of conservation of energy. 
(c) Explain how a freely swinging pendulum obeys the law of conservation of energy. 
(d) Name six kinds of energy familiar to you. 
Answer: 
(a) Law of conservation of energy : “Energy can neither be created nor destroyed. 
Though it can be transformed from one form to other.” 
(b) Mathematical proof of law of conservation of energy : 
At A Consider a body of mass m at A at a height h above the ground level. 
P.E. = mgh K.E. = 0 at rest. 
 
P.E. + K.E. = mgh + O = mgh     …………….. (i) 
Let it fall from A to B covering a distance x and still at (h-x) above ground. 
In doing so 
v
2
 – u
2
 = 2 gx 
v
2
 – 0 = 2gx 
v
2
 = 2gx          …(a) 
At B P.E = mg (h – x) 
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AT C P.E + K.E = 0 + mgh 
P.E. + K.E. = mgh 
Thus we find sum of P.E. + K.E. at A, B, C remains same mgh 
Hence varifies the law of can servation of energy. 
(c) Energy changes at B K.E. = O 
at highest pt. vel. = O 
P.E. = mgh 
? the bob is at height h 
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