Class 9 Science
Question 1. State the universal law of gravitation.
Answer: Universal Law of Gravitation: Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centres of two objects.
Question 2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.
Answer: If M is the mass of earth, m is the mass of the object and R is the radius of earth then formula for the gravitational force between the earth and the object on the surface of earth can be given as follows:
F = G × M × m/R2
Question 3. What do you mean by free fall?
Answer: When an object is dropped from a certain height, it moves towards the earth under the influence of gravitational force of the earth. This is called free fall.
Question 4. What do you mean by acceleration due to gravity?
Answer: When an object is under free fall, its velocity changes due to gravitational force. The change in velocity brings about acceleration in the object. This is called acceleration due to gravity. The value of acceleration due to gravity or g = 9.8 m/s2.
Question 5. What are the differences between the mass of an object and its weight?
|It is the matter contained in the object.||It is the force acting on the object.|
|It gives the measure of inertia in the object.||It gives the measure of gravity.|
|It is a constant quantity.||Weight changes from one place to another.|
|It has magnitude only.||It has both magnitude and direction.|
|The SI unit of mass is Kg.||The SI unit of weight is Newton.|
Question 6. Why is the weight of an object on the moon 1/6th its weight on the earth?
Answer: The acceleration due to gravity on moon is 1/6th of that on earth. Hence, weight of an object on the moon is 1/6th its weight on the earth.
Question 7. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Answer: We know that pressure is inversely proportional to the area on which the force is acting. A thin strap has smaller area compared to a wide strap. Hence, a thin strap creates more pressure. So, it is difficult to hold a school bag having a strap made of a thin and strong string.
Question 8. What do you mean by buoyancy?
Answer: When an object is immersed in a liquid, the object experiences an upward force. This force is called buoyancy.
Question 9. Why does an object float or sink when placed on the surface of water?
Answer: When an object is immersed in a liquid, the object experiences an upward force. When this upward force is greater than the weight of the object the object floats on water. When this upward force is less than the weight of the object the object sinks in water.
Question 10. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Answer: When an object is put on a weighing machine, the object experiences an upward thrust because of the fluid (air) around it. Hence, the reading is slightly less than the actual mass. So, in this case; the mass is less than 42 kg.
Question 11. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
Answer: For the same mass; the bag of cotton has greater volume than the iron bar. So, the bag of cotton will experience a greater upward thrust in air. Due to this, the bag of cotton is heavier than the iron bar.
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