Case Based Questions : Work and Energy | Science Class 9 PDF Download

Q1: Read the following passage and answer the questions that follow:

In daily life, people often say they are "working hard", but in science, work is defined differently. Consider two situations:
A student reads a book for hours, using a lot of mental effort.
A person pushes a huge rock with all their strength, but the rock does not move.
In both cases, no displacement occurs, meaning no work is done according to the scientific definition. In contrast, if a girl pulls a trolley and it moves, work is done because a force is applied, and displacement occurs.

i. What are the two conditions necessary for work to be done? (1 mark)
ii. Why is no work done when a person pushes a rock with all their strength but it does not move? (2 marks)
iii. A porter holds a heavy load on his head but remains stationary. Is he doing work according to science? Why or why not? (1 mark)

Ans:
i. Work is done only when (i) a force is applied, and (ii) the object is displaced in the direction of the applied force.
ii. No work is done because work = force × displacement. Since the displacement is zero, the work done is also zero.
iii. No, the porter is not doing work according to science. Even though he applies a force to support the load, there is no displacement, so the work done is zero.

Q2: Read the following passage and answer the questions that follow:

When a force is applied to an object and it moves in the direction of the force, work is done. If a constant force F moves an object through a distance s, then work is calculated as:
W = F x s
For example, if a force of 5 N acts on an object and moves it 2 meters, the work done is:
W = 5 x 2 = 10J
If a force acts opposite to displacement, the work done is negative.

i. What is the SI unit of work? (1 mark)
ii. A force of 10 N moves an object 4 m in the direction of the force. Calculate the work done. (2 mark)
iii. In which case is work done negative—when lifting an object up or when friction slows it down? Explain. (1 mark)

Ans:
i. The SI unit of work is joule (J)
ii. Work done is:
W = F x s = 10 x 4 = 40 J
iii. Work is negative when friction slows down an object because the force of friction acts opposite to the displacement.

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Q3: Read the following passage and answer the questions that follow:

A moving cricket ball has energy because it can knock down the stumps. Similarly, a moving bullet, running water, and a flying airplane all have kinetic energy. The formula for kinetic energy is:
KE = (1/2)mv^{2
where m is mass and v is velocity. The faster an object moves, the more kinetic energy it has.}

i. What is kinetic energy? (1 mark)
ii. A ball of mass 2 kg moves with a velocity of 3 m/s. Calculate its kinetic energy. (2 mark)
iii. Why does a moving truck have more kinetic energy than a moving bicycle, even if both move at the same speed? (1 mark)

Ans:
i. Kinetic energy is the energy possessed by a body due to its motion.
ii. Using the formula:
KE = (1/2) m v² = (1/2) × 2 × 3² = 9J
iii. A truck has more kinetic energy than a bicycle because kinetic energy depends on mass. Since a truck has a larger mass, it has more kinetic energy even at the same speed.

Q4: Read the following passage and answer the questions that follow:

An object raised above the ground has gravitational potential energy. The formula for potential energy is:
PE = mgh
where m is mass, g is acceleration due to gravity (9.8 m/s²), and h is height. If a 10 kg object is raised 5 m, its potential energy is:
PE = 10 × 9.8 × 5 = 490J

i. What happens to an object's potential energy if it is lifted higher? (1 mark)
ii. A 12 kg object is raised 3 m above the ground. Calculate its potential energy. (2 mark)
iii. Why does a stretched bowstring have potential energy? (1 mark)

Ans:
i. The potential energy increases as the object is lifted higher because it is directly proportional to height.
ii. Using the formula
PE = mgh = 12 × 9.8 × 3 = 352.8J
iii. A stretched bowstring has elastic potential energy because work is done in stretching it, and this energy is released when the arrow is shot.

Q5: Read the following passage and answer the questions that follow:

The Law of Conservation of Energy states that energy cannot be created or destroyed, only transformed. For example, in a pendulum, energy constantly changes between kinetic and potential energy. When it swings to the highest point, it has maximum potential energy and zero kinetic energy. As it moves downward, potential energy changes into kinetic energy.

i. What does the Law of Conservation of Energy state? (1 mark)
ii. Explain how energy is transformed in a freely falling object. (2 mark)
iii. Why does a pendulum eventually stop swinging? (1 mark)

Ans:
i. The Law of Conservation of Energy states that energy can neither be created nor destroyed; it only changes from one form to another.
ii. In a freely falling object, potential energy at the top is converted into kinetic energy as it falls. Just before hitting the ground, all potential energy has changed into kinetic energy.
iii. A pendulum eventually stops due to air resistance and friction at the pivot, which convert its mechanical energy into heat and sound energy.

Q6: Read the following passage and answer the questions that follow:

In certain situations, the work done by a force can be positive or negative. When a force is applied in the same direction as the displacement, work done is positive. For example, when a person lifts a book, the force applied is upward, and the displacement is also upward, making the work positive.However, when a force is applied opposite to the direction of displacement, work done is negative. For example, when a ball is thrown upwards, gravity acts downward, but the displacement is upward, making the work done by gravity negative.

i. When is work done considered negative? (1 mark)
ii. Why is the work done by friction always negative? (2 mark)
iii. Give an example where both positive and negative work act on an object at the same time. (1 mark)

Ans:
i. Work is considered negative when the force applied is opposite to the displacement of the object.
ii. Friction always acts opposite to the motion of an object, so it removes energy from the system, making the work done by friction negative.
iii. When a car is moving forward, the engine applies a force in the direction of motion (positive work), but friction and air resistance act opposite to the motion (negative work).

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Q7: Read the following passage and answer the questions that follow:

Power is the rate at which work is done. It is given by:
P = W / t
where P is power, W is work, and t is time. The SI unit of power is the watt (W). If two people do the same amount of work but one does it faster, they have different power.
For example, a girl climbs a staircase in 10 seconds, while a boy climbs the same staircase in 5 seconds. Even though they do the same amount of work, the boy is more powerful because he does it in less time.

i. What is the SI unit of power? (1 mark)
ii. A person does 300 J of work in 6 seconds. Calculate their power. (2 mark)
iii. Why does a powerful car accelerate faster than a less powerful one? (1 mark)

Ans:
i. The SI unit of power is watt (W).
ii. Using the formula:
P = W / t = 300 / 6 = 50W
iii. A powerful car converts energy into motion faster, meaning it can apply more force in a short time, leading to faster acceleration.