Class 9 Chapter 9 Gravitation

 Class 9 Chapter 9 Gravitation

• Q: What force is responsible for the planets going around the Sun and the moon going around the earth? A: The gravitational force is responsible for these motions.
• Q: What keeps a body moving along a circular path? A: A centre-seeking force called the centripetal force keeps a body moving along a circular path.
• Q: What is the universal law of gravitation? A: It states that every object attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
• Q: What is the accepted value and SI unit of the universal gravitation constant ($G$)? A: The accepted value of $G$ is $6.673 \times 10^{-11} \text{ N m}^2 \text{ kg}^{-2}$.
• Q: Who first discovered the value of the universal gravitation constant ($G$)? A: Henry Cavendish found the value of $G$ using a sensitive balance.
• Q: What does it mean for an object to be in "free fall"? A: An object is in free fall when it falls towards the earth under the influence of the earth's gravitational force alone.
• Q: What is the standard value of acceleration due to gravity ($g$) on the earth? A: The value of acceleration due to gravity is $9.8 \text{ m s}^{-2}$.
• Q: How does the value of $g$ change across the surface of the earth? A: The value of $g$ is greater at the poles than at the equator because the earth is not a perfect sphere and its radius increases from the poles to the equator.
• Q: What is the difference between mass and weight? A: Mass is a measure of an object's inertia and remains constant everywhere, whereas weight is the force with which the earth attracts the object and can vary depending on location.
• Q: How does an object's weight on the moon compare to its weight on earth? A: The weight of an object on the moon is one-sixth ($\frac{1}{6}$) of its weight on the earth.
• Q: What is thrust? A: Thrust is the force acting on an object perpendicular to its surface.
• Q: What is the SI unit of pressure? A: The SI unit of pressure is the pascal (Pa), which is equal to $\text{N m}^{-2}$.
• Q: What is buoyancy? A: Buoyancy, or upthrust, is the upward force exerted by a fluid on an object immersed in it.
• Q: Why do some objects float while others sink in water? A: Objects with a density less than that of the liquid will float, while objects with a density greater than the liquid will sink.
• Q: What does Archimedes' principle state? A: It states 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.
• Q: What are some practical applications of Archimedes' principle? A: It is used in designing ships and submarines, and is the operating principle behind lactometers (for testing milk purity) and hydrometers (for determining liquid density).