Kerala Syllabus Class 9 Physics - Chapter 6 Work and Energy - Questions and Answers
Questions and Answers for Class 9 Physics പ്രവൃത്തിയും ഊര്ജ്ജവും | Text Books Solution Physics (English Medium) Physics: Chapter 06 Work and Energy - Questions and Answers | SAMAGRA Question Bank
ഒമ്പതാം ക്ലാസ്സ് ഫിസിക്സിലെ Work and Energy എന്ന പാഠം ആസ്പദമാക്കി samagra തയ്യാറാക്കിയ ചോദ്യോത്തരങ്ങള്.
Class 9 Physics - Work and Energy - Question Bank
∎Physics (English Medium Notes)
1. In which scenario can it be said that a force has done work?
• When a force is applied but the object doesn't move.
• When a force is applied and the object moves in the opposite direction.
• When a force is applied and the object moves in the same direction.
• None of the above.
Answer: When a force is applied and the object moves in the same direction.
2. The work done is calculated as the product of force and _____.
• time
• distance
• displacement
• acceleration
Answer: displacement
3. When you lift something, what force are you working against?
• Against the force of Earth's gravity
• Against the force of friction
• Against the magnetic force
• Against the electric force
Answer: Against the force of Earth's gravity
4. If the forces acting on an object are _____, then those forces cannot do work on the object.
• Balanced
• Unbalanced
• External force
• Unbalanced external force
Answer: Balanced
5. The work done by gravity on a freely falling object is _____.
• Positive
• Negative
• Zero
• Infinite
Answer: Positive
6. The energy obtained due to the motion, position, or configuration of objects is called _____.
• Electrical energy
• Potential energy
• Kinetic energy
• Mechanical energy
Answer: Mechanical energy
7. The energy stored in a stretched bow is _____.
• Potential energy
• Kinetic energy
• Configuration energy
• Sound energy
Answer: Configuration energy
8. Equation for calculating Kinetic Energy is …….
• K=mgh
• KE= ½ mgh
• KE=mv²
• Ek= ½ mv²
Answer: Ek = ½ mv²
9. When the speed of a moving object is doubled, its kinetic energy becomes:
• Half
• 4 times
• Double
• ¼
Answer: 4 times
10. The original source of energy for petroleum is _________.
• The sun
• Rain
• Fish
• Trees
Answer: The sun
11. We can find the rate of work using the formula ..................
• w = ᵖ⁄ₜ
• P = ʷ⁄ₜ
• w = pt
• P = ˢ⁄ₜ
Answer: P = ʷ⁄ₜ
12. Unit of work is ……… (Watt, Joule, Ampere, Volt)
Answer: Joule
13. Unit of energy is …….. (Watt, Joule, Ampere, Volt)
Answer: Joule
14. If an object undergoes displacement in a direction opposite to the applied force, the work done is said to be _____.
(positive, negative, no work done, none of these)
Answer: negative
15. On what factors does the amount of work done depend? What is the unit of work?
Answer:
• Force, Distance
• Joule (J)
16. a) What is the product of force and displacement called?
b) When displacement increases work …………..
Answer:
a) Work
b) Increases
17. If a force of 200 N is applied to move an object 4 meters across a floor, how much work is done?
Against which force is the work done when moving across the floor?
Answer:
• W=Fs = 200 X 4 = 800 J
• Friction
18. How do we calculate the work done against gravity when lifting objects? Calculate the amount of work required to lift an object of mass 500 g to a height of 2 m (g=10 m/s²).
Answer :
W = mgh
= 0.5 x 10 x 2 = 10 J
19. When is work considered positive? When is work considered negative?
Answer:
• Positive work: Force and displacement are in the same direction.
• Negative work: Force and displacement are in opposite directions.
20. Classify the following situations as examples of positive or negative work:
a) Work done by gravity when a mango falls from a tree.
b) Work done by the buoyant force on a bucket when it is submerged in water while drawing water from a well.
c) Work done by gravity on a bucket when it is submerged in water while drawing water from a well.
d) Work done by the frictional force when objects are moved across a floor.
Answer:
a) Positive
b) Positive
c) Negative
d) Negative
21. List two forms of energy that are used to do work. In what unit is the amount of energy expressed?
Answer:
• Electrical Energy, Potential Energy
• Joule (J)
22. How many types of mechanical energy are there? What are they? How do objects acquire mechanical energy?
Answer:
• There are two types: kinetic energy and potential energy.
• Due to the motion of objects, position, configuration, etc.
23. Can the arrow in a stretched bow and a toy car placed at a height do work? What kind of energy is stored in them?
Answer: Yes, they can. The energy stored in them is potential energy.
24. Classify the following scenarios based on the type of potential energy involved:
• Energy stored in water held back by a dam
• Energy stored in the bend of a pole vault pole
• Energy stored in a stone held at a height
• Energy stored in a stretched bow
• Energy possessed by a coconut on a coconut tree
• Energy stored in a compressed spring
• Energy possessed by the water stored in a tank
• Energy in the pole due to its bending during the pole vault jump
Answer:
| Potential energy due to position | Potential energy due to configuration |
| • Energy stored in water held back by a dam | • Energy stored in the bend of a pole vault pole |
| • Energy stored in a stone held at a height | • Energy stored in a stretched bow |
| • Energy possessed by a coconut on a coconut tree | • Energy stored in a compressed spring |
| • Energy possessed by the water stored in a tank | • Energy in the pole due to its bending during the pole vault jump |
25. What is the name given to the work done in the direction opposite to the applied force?
Give two examples of such situations.
Answer: Negative work.
Example:
1. Work done by gravity on a bucket when it is submerged in water while drawing water from a well.
2. The work done by the frictional force when objects are moved across a floor.
26. What is meant by the term 'energy due to configuration'? Give two examples of such energy sources.
Answer: Energy due to configuration refers to the energy stored in an object due to its shape or position, often caused by stretching, compressing, or bending.
Examples: A stretched bow and a compressed spring.
27. Why do we need to wind the spring of a clock frequently?
Answer: Winding the clock spring stores potential energy in it.
28. What is the term used for work done per unit time? Its unit is equivalent to...........
( J x S, J/s, W/s, S/J )
Answer: Power, J/s
29. If a device does 4476 J of work in 3 seconds, what is the horsepower of the device?
Answer:
P = ʷ⁄ₜ = 4476/3 = 1492 W
Since 1 Hp = 746 W, 1492 W = 2 Hp
30. Why can we say that water stored behind a dam at a height has energy? Explain a situation where this energy is utilized.
Answer:
An object at a height has potential energy.
For the generation of electricity.
31. a) When drawing water from a well, against what force do we work?
b) What is the work done to raise an object of mass 'm' to a height 'h'?
c) What is the energy contained in it when it reaches this height?
Answer:
a) Gravitational Force
b) mgh
c) Potential Energy
32. A 5kg object is located 10m above the ground.
a) What is the energy contained in the object?
b) Just before hitting the ground, what type of energy does it have?
c) What is its value?
Answer:
a) mgh = 5 x 10 x 10 = 500 J
b) Kinetic energy
c) 500 J
33. When a stone is thrown upwards,
a) What kind of work is done by gravity?
b) Give an explanation for this type of work.
c) Give an example.
Answer:
a) Negative work
b) Work done in the direction opposite to the applied force
c) The work done by the frictional force when objects are moved across a floor.
34. a) State the law of conservation of energy.
b) Give an example to show that energy can be converted from one form to another.
Answer:
a) Law of conservation of energy.
b) Water stored in a dam flows downward, operating a generator to produce electricity.
35. a) What is meant by mechanical energy?
b) How many types of mechanical energy are there? What are they?
c) Calculate the mechanical energy of a 15 kg stone placed at a height of 3 meters.
d) Calculate the mechanical energy of the stone just before it touches the ground when it falls. What can be inferred from the energy conversion that occurred here?
Answer:
a) Mechanical energy is the energy that objects acquire by virtue of their motion, position or configuration.
b) Two types: potential and kinetic
c) mgh = 15 x 10 x 3 = 450 J
d) v2 = u2 + 2gh = 0 + 2 x 10 x 3 = 60
Ek = ½ mv2 = ½ x 15 x 60 = 450J
Potential Energy to Kinetic Energy
36. a) How is the energy obtained due to position calculated?
b) How is the energy obtained due to motion calculated?
Answer:
a) Ep = mgh
b) Ek = ½ mv2
37. a) What can be inferred from a 40 W rating on an electric bulb?
b) How much time will this bulb take to consume 1000 J or one kilojoule?
c) How many watts are there in one horsepower?
Answer:
a) The bulb consumes 40 joules of energy per second.
b) P = ʷ⁄ₜ
t = ʷ⁄ₚ = ¹⁰⁰⁰⁄₄₀ = 25 s
c) 1 HP = 746 W
39. A force of 2984 N is applied to lift an object to a height of 5 m in 20 seconds.
a) Calculate the work done.
b) Calculate the power used.
c) Convert the power to horsepower.
Answer:
a) W = mgh, mg = 2984 N, h = 5 m
W= 2984 x 5 = 14920 W
b) P = ʷ⁄ₜ = ¹⁴⁹²⁰⁄₂₀ = 746 W
c) 746 W = 1 HP
40. What is meant by Work?
Work is said to be done by a force if there is a displacement for the object in the direction of the applied force.
41. What is meant by Energy?
Energy is the ability to do work.
42. What is meant by Potential energy?
Potential energy is the energy possessed by objects due to their position or configuration.
43. What is meant by Kinetic energy?
Kinetic energy is the energy possessed by an object by virtue of its motion.
44. What is the Law of Conservation of Energy?
Energy can neither be created nor destroyed. One form of energy can be converted into other forms of energy without loss or gain of energy. This is the law of conservation of energy.
45. What is meant by Power?
Power is the quantity of work done per unit time or power is the rate of doing work.
Let's Assess
1. Which one of the following quantities has the same unit as that of work?
a) power
b) energy
c) force
d) displacement
Answer: b) energy
2. Which of the following pairs are scalar quantities?
a) time and energy
b) force and power
c) speed and acceleration
d) displacement and velocity
Answer: a) time and energy
3. Which of the following is the unit of power in terms of the fundamental units metre, kilogram and second?
a) kgm²/s³
b) kgm/s
c) kgm/s²
d) kgm²/s
Answer: a) kgm²/s³
4. An object of mass 2.4 kg is kept on a level surface. On applying a force of 50 N, the object moves 8 m in the direction of the force. Calculate the quantity of work done.
a) 40 J
b) 400 J
c) 50 J
d) 17.6 J
Answer: b) 400 J
5. The figure (Textbook Page: 125) depicts a body of mass 20 kg lifted to a height 5m using a pulley. If the work done is 1020 J, answer the following questions. (Acceleration due to gravity = 10m/s²)
a) What is the work done here if the force of friction of the pulley is not considered?
b) Calculate the work done against the friction of the pulley.
Answer:
a) mass, m = 20 kg
Height, h = 5 m
Work done = 1020J
Work done = F × s
The force required to lift the body is equal to the weight of the body.
Weight = m = 20 × 10 = 200 N
Work done to lift the body to a height of 5 meters
W = F × s = 200 × 5 = 1000 J
b) Work done against friction = Total work done – work done to lift without friction = 1020 – 100 = 20 J
6. An electric motor pumps 186.5 kg of water into a tank at a height of 8 m in 10 s. (g = 10 m/s²).
a) Calculate the power of the motor.
b) Express this in horsepower.
c) How much time will it take to fill the same quantity of water if the power of the motor is halved?
Answer:
a) m = 186.5 kg, h = 8 m, t = 10 s
Power = ʷ⁄ₜ
Work = F × s
Force is equal to the weight of the body
Weight= m × g = 186.5 × 10 = 1865
Work = 1865 × 8 = 14920J
P = ¹⁴⁹²⁰⁄₁₀ = 1492 W
b) 1 horsepower = 746 W
To convert 1492W to horsepower
Horsepower =1492W746W/hp = 2.00hp
Horsepower =
c) Total power = 1492 W
New power = 14922 = 746 W
We know, Power =worktime
Time =workpower = 14920746 = 20 s
We know, Power =
Time =
Thus, it will take approximately 20 seconds to fill the same quantity of water if the power of the motor is halved.
7. The mass of a body is 4 kg. When a continuous force of 3 N is applied to
this object towards the east, there is a displacement of 6 m in the
direction of the force. Then, the same force (3 N) is continuously
applied towards the south, and the displacement is 5 m in the direction
of this force.
a) Calculate the work done when the object is moved towards the south.
b) Calculate the total work done on the object.
Answer:
a) Force = 3 N
Work = F × s
= 3 × 5 = 15 J
b) Work done when moving east
Work = F × s
= 3 × 6 = 18 J
Total work done = 15 + 18 = 33 J
a) Calculate the work done when the object is moved towards the south.
b) Calculate the total work done on the object.
Answer:
a) Force = 3 N
Work = F × s
= 3 × 5 = 15 J
b) Work done when moving east
Work = F × s
= 3 × 6 = 18 J
Total work done = 15 + 18 = 33 J
8. If an object is thrown vertically upwards, its potential energy and kinetic energy will change. Which of the following is the correct graph related to kinetic energy? Justify your answer.
(a) As the object moves upwards, the gravitational force does work against the upward motion, reducing its speed. As the speed decreases, the object's kinetic energy also decreases. At the maximum height, the velocity becomes zero, so the kinetic energy is momentarily zero.
9. The Vikram lander module of Chandrayan III has a mass of 1752 kg.
a) What will be its potential energy when it reaches a height of 100 m from the lunar surface? (Acceleration due to gravity on the moon is 1.6 m/s²)
b) If the same Vikram module is located at a height of 100 m from the Earth’s surface, then what will be its potential energy? (g = 10 m/s²)
Answer:
a) m = 1752 kg
g = 1.6 m/s²
h = 100 m
P.E = m × g × h = 1752 × 1.6 × 100 = 280320 J
b) g = 10 m/s²
P.E = m × g × h = 1752 × 10 × 100 = 1725000 J
10. A person of mass 80 kg is standing on a suspended platform of mass 170 kg to paint the outer wall of a flat. The quantity of work done by the motor to raise him and the platform to this height is 150 kJ. How high is the platform? (g = 10 m/s²)
Answer:
Mass of person = 80 kg
Mass of suspended platform = 170kg
Work = 150 kJ = 150000J
Work = Force × displacement (Height)
In this case, the total force is the weight of both the person and the platform
Total mass = mass of person + mass of platform = 80 + 170 = 250 kg
Weight = Total mass × g = 250 × 10 = 2500 N
Height =workForce = 1500002500 = 60m
Work = 150 kJ = 150000J
Work = Force × displacement (Height)
In this case, the total force is the weight of both the person and the platform
Total mass = mass of person + mass of platform = 80 + 170 = 250 kg
Weight = Total mass × g = 250 × 10 = 2500 N
Height =
11. The figure 6.14 shows two cars of the same mass and a lorry of double the mass moving along a straight road. Observe the figure and answer the following questions.
b) Is the kinetic energy of the lorry and the car in front the same? Why?
c) Calculate the kinetic energy of the car at the back if its mass is 1800 kg.
Answer:
a) No.
We know K.E = ½ × mv²
Here, both cars have the same mass, but they are moving with different velocities. So, they will have different kinetic energy
b) No, because the lorry and car have different masses.
c) K.E = ½ × mv² = ½ × 1800 × 20² = 360000 J.
12. Analyse the figure and answer the following questions.
a) What is the kinetic energy at A?
b) What is the potential energy at A?
c) What is the total energy at A?
d) What is the kinetic energy at C?
e) What is the potential energy at C?
f) What is the total energy at C?
g) What change in energy has taken place as the ball falls? Describe.
Answer:
a) K.E = 0J
b) P.E = 20 J
c) Total energy at A = 20 J
d) Kinetic energy at C = 20 J
e) Potential energy at C = 0 J
f) Total energy at C = 20 J
g) The potential energy of the ball is converted into kinetic energy. According to the law of conservation of energy, the total energy remains constant.
13. Using a conveyor belt that transports construction material to the top of a building, 5 sacks of cement of mass 50 kg each are brought to a height of 8 m in 16 s. Calculate the power of the motor of the conveyor belt (g =10m/s²).
Answer:
mass of one cement sack = 50 kg
Mass of 5 sacks = 5 × 50 = 250 kg
We know, Power =WorkTime
Work = F × S
The force applied here is equal to the weight of the body
Weight W = m × g = 250 × 10 = 2500N
Then, W = 2500 × 8 = 20000 J
Therefore, P =2000016 = 1250 W
mass of one cement sack = 50 kg
Mass of 5 sacks = 5 × 50 = 250 kg
We know, Power =
Work = F × S
The force applied here is equal to the weight of the body
Weight W = m × g = 250 × 10 = 2500N
Then, W = 2500 × 8 = 20000 J
Therefore, P =
14. Babu and Raju have brought the materials for house construction to the top of a 15 m tall building. The details are given below.
b) If it is said that Babu has more power, will you agree with it? Explain.
Answer:
a) Height of the building = 15 m
• Babu
Weight = 600 N, Time = 300s
Work = F × s = 600 × 15 = 9000J
• Raju
Weight = 400 N, Time=200s
Work F × s = 400 × 15 = 6000J
Babu did more work.
b) No.
• Babu
Power = WorkTime
P = ⁹⁰⁰⁰⁄₃₀₀ = 30 W
• Raju
Power = WorkTime
P = ⁶⁰⁰⁰⁄₂₀₀ = 30 W
Both have the same power.
15. Observe the graph between the force and displacement of a body.
b) From the graph, find the maximum displacement caused by the applied force.
c) What is the method to find the quantity of work using graph? Find the area under the graph and check whether it is equal to the work done.
Answer:
a) 6 N
b) 0.4 m
c) Area under Force – displacement graph gives work.
Area of rectangle = length breadth = 0.4 × 6 = 2.4 J
Work done = F × s = 0.4 × 6 = 2.4 J
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