Short Notes -2- Chapter 9-Gravitational -Science- Ncert -class 9

Short Notes -2- Chapter 9-Gravitational -Science- Ncert -class 9

9.5 Thrust and Pressure

Thrust and Pressure:

◆Thrust refers to the net force acting in a particular direction, while pressure is the force per unit area acting on an object.

◆Thrust is the force exerted perpendicular to the surface of an object, while pressure is the effect of thrust distributed over a given area.

Illustrative Situations:

◆Situation 1: Fixing a poster on a bulletin board using drawing pins. The force applied to the pin’s head is thrust, acting perpendicular to the surface of the board.

◆Situation 2: Standing on loose sand versus lying down on the sand. In both cases, the force exerted is the weight of the body acting downward. When standing, the force is distributed over the area of the feet, while when lying down, the force is distributed over the entire body’s contact area with the sand.

Relationship between Thrust and Pressure:

◆While the thrust remains constant in both situations, the effects differ due to the difference in the area over which the force is applied.

◆The effect of thrust on different areas is described as pressure, which is the thrust divided by the area it acts upon. Mathematically, Pressure=ThrustAreaPressure=AreaThrust.

Unit of Pressure:

◆The SI unit of pressure is the pascal (Pa), named after the scientist Blaise Pascal. It is equivalent to one newton per square meter (N/m²) or one newton meter per square meter (N m⁻²).

Example:

◆The passage suggests considering a numerical example to understand the effects of thrust acting on different areas, further illustrating the concept of pressure.

9.5.2 BUOYANCY

Introduction to Buoyancy:

◆Buoyancy is the upward force exerted by a fluid (such as water) on an object submerged in it.

◆The passage begins by posing questions related to experiences with water, such as feeling lighter while swimming and observing objects behaving differently when submerged.

Activity to Understand Buoyancy:

◆An activity is provided to demonstrate buoyancy:

◆A plastic bottle is placed in a bucket of water, where it floats due to buoyant force.

◆Pushing the bottle deeper into the water results in an increasing upward force exerted by the water.

◆Releasing the bottle causes it to bounce back to the surface, indicating that the buoyant force is greater than the gravitational force acting on the bottle.

Explanation of Buoyancy:

◆Buoyancy arises due to the upward force exerted by the fluid on the submerged object.

◆When an object is immersed in a fluid, such as water, the fluid exerts an upward force (buoyant force) on the object, which opposes the force of gravity pulling the object downward.

◆The buoyant force is greater than the weight of the object, causing it to float or rise to the surface when released.

Achieving Equilibrium:

◆To keep an object completely immersed in water, an externally applied force must balance the upward buoyant force exerted by the water.

◆This applied force must be at least equal to the difference between the buoyant force and the weight of the object.

Magnitude of Buoyant Force:

◆The magnitude of the buoyant force depends on the density of the fluid. Objects experience a greater buoyant force in denser fluids.

9.6 Archimedes’ Principle

Activity Setup:

◆A stone is tied to one end of a rubber string or a spring balance.

◆The stone is suspended in the air, and the elongation of the string or the reading on the spring balance is noted due to the weight of the stone.

Immersion in Water:

◆The stone is slowly dipped into a container of water.

◆Observations are made on the changes in the elongation of the string or the reading on the balance as the stone is immersed in water.

Observations:

◆It’s noted that the elongation of the string or the reading on the balance decreases as the stone is gradually lowered into the water.

◆Once the stone is fully immersed in water, no further change in elongation or reading is observed.

Inference:

◆The decrease in elongation or reading indicates that some force acts on the stone in the upward direction as it is lowered into the water.

◆This upward force exerted by water is identified as the force of buoyancy.

Archimedes’ Principle:

◆Archimedes’ principle is introduced, stating that when a body is fully or partially immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced by it.

◆This principle explains why objects experience buoyant force when submerged in a fluid.

Explanation of Observation:

◆The reason for not observing further decrease in elongation or reading once the stone is fully immersed in water is attributed to Archimedes’ principle.

◆The upward buoyant force exerted by the water on the stone becomes equal to the weight of the stone, leading to equilibrium and no further change in the system.

Applications of Archimedes’ Principle:

◆The passage mentions some applications of Archimedes’ principle, including its use in designing ships, submarines, lactometers, and hydrometers.