Chapter 9: Gravitation Case Study Questions | science Class 9 CBSE

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Chapter 9: Gravitation Case Study Questions | science Class 9 CBSE | ByteNotes.in

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Case Set 1

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During a science demonstration, a teacher swings a stone tied to a string in a circular path and then releases it. The stone flies off in a straight line. Students wonder why it didn’t continue in a circle.

Question 1: Q1. Why does the stone move in a straight line when released?

While tied, the stone is kept in circular motion by centripetal force.
Upon release, this force no longer acts, and the stone follows a straight-line path due to inertia.
The straight path is tangent to the circular path at the release point.

Question 2: Q2. What is centripetal force?

It is a center-seeking force that keeps an object in circular motion.
It always acts perpendicular to the direction of motion.
Without it, the object would move in a straight line.

Question 3: Q3. How does this relate to planetary motion?

The gravitational force acts as centripetal force for planets.
It keeps the planets in orbit around the Sun.
Without gravity, planets would move in straight lines.

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Case Set 2

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A plastic bottle is pushed under water in a bucket. As it is released, the bottle pops up and floats. Students feel the bottle is being pushed upward by water.

Question 1: Q1. What is the force acting on the bottle?

It experiences an upward buoyant force from the water.
This force is called upthrust or buoyancy.
It acts opposite to gravity.

Question 2: Q2. Why does the bottle rise when released?

The upward buoyant force exceeds the weight of the bottle.
This net force causes upward acceleration.
Hence, it floats back to the surface.

Question 3: Q3. What determines the strength of the buoyant force?

It depends on the density of the fluid.
Also on the volume of fluid displaced by the object.
Greater density or displacement → stronger buoyancy.

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Case Set 3

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Ravi places a nail and a cork in a beaker filled with water. The nail sinks while the cork floats. He wonders why the two behave differently.

Question 1: Q1. Why does the cork float?

Its density is less than that of water.
Hence, the buoyant force is greater than its weight.
It floats as a result of net upward force.

Question 2: Q2. Why does the iron nail sink?

Its density is greater than that of water.
The weight is more than the buoyant force.
Thus, it sinks due to net downward force.

Question 3: Q3. What principle explains this behavior?

Archimedes’ Principle governs floating and sinking.
It states that the buoyant force equals the weight of fluid displaced.
Whether an object floats or sinks depends on this force relative to weight.

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Case Set 4

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In a physics lab, a 5 kg wooden block is placed on a table in two different ways. First on its narrow face, then on its broad face. Students calculate the pressure each time.

Question 1: Q1. Why does the pressure differ in both cases?

Pressure is inversely proportional to contact area.
Smaller area leads to higher pressure.
Wider area spreads the force, reducing pressure.

Question 2: Q2. What is the formula used?

Pressure = Force / Area.
Force is constant here (equal to weight).
Area changes based on orientation.

Question 3: Q3. How does this relate to daily life?

Knives have sharp edges to apply more pressure.
Camels have broad feet to reduce pressure on sand.
Foundations are broad to reduce pressure on soil.

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Case Set 5

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A student throws a stone upward. It slows down, stops at a height, and returns back. He is asked to calculate the velocity and height using motion equations.

Question 1: Q1. What is the acceleration during upward and downward motion?

It is acceleration due to gravity, g = 9.8 m/s².
During upward motion, it's taken as negative.
During downward fall, it's taken as positive.

Question 2: Q2. Why does the stone return to the thrower?

Gravity constantly pulls it down.
At highest point, its velocity becomes zero.
It then accelerates back downward.

Question 3: Q3. Which formulas are used in such problems?

v = u + at for final velocity.
s = ut + ½ at² for distance.
v² = u² + 2as for maximum height.

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Chapter sections

Chapter 9: Gravitation | Case Study Questions

Passage

Question 1: Q1. Why does the stone move in a straight line when released?

Question 2: Q2. What is centripetal force?

Question 3: Q3. How does this relate to planetary motion?

Passage

Question 1: Q1. What is the force acting on the bottle?

Question 2: Q2. Why does the bottle rise when released?

Question 3: Q3. What determines the strength of the buoyant force?

Passage

Question 1: Q1. Why does the cork float?

Question 2: Q2. Why does the iron nail sink?

Question 3: Q3. What principle explains this behavior?

Passage

Question 1: Q1. Why does the pressure differ in both cases?

Question 2: Q2. What is the formula used?

Question 3: Q3. How does this relate to daily life?

Passage

Question 1: Q1. What is the acceleration during upward and downward motion?

Question 2: Q2. Why does the stone return to the thrower?

Question 3: Q3. Which formulas are used in such problems?

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