plss send me some extra questions with answers of NCERT class 9 ch 3 gravitation and ch 4 work and energy
Answers
Answer:
Short Answer Type Questions
Question 1. What is the S.I. unit of thrust?
Answer: Newton.
Question 2. What is the S.I. unit of pressure?
Answer: The S.I. unit of pressure = N/m2 = Pascal.
Question 3. Define thrust.
Answer: The net force exerted by a body in a particular direction is called thrust.
Question 4. Define pressure.
Answer: The force exerted per unit area is called pressure.
Question 5. Why is it easier to swim in sea water than in river water?
Answer: The density of sea water is more due to dissolved salts in it as compared to the density of river water. Hence the buoyant force exerted on the swimmer by the sea water is more which helps in floating and makes swimming easier.
Question 6. Why a truck or a motorbike has much wider tyres?
Answer: The pressure exerted by it can be distributed to more area, and avoid the wear and tear of tyres.
Question 7. Why are knives sharp?
Answer: To increase the pressure, area is reduced,
As pressure ∝ 1/Area hence the pressure or force exerted on a body increases.
Question 8. Why is the wall of dam reservoir thicker at the bottom?
Answer: The pressure of water in dams at the bottom is more, to withstand this pressure the dams have wider walls.
Question 9. Why do nails have pointed tips?
Answer: The force exerted when acts on a smaller area, it exerts larger pressure. So the nails have pointed tips.
Question 10. While swimming why do we feel light?
Answer: The swimmer is exerted by an upward force by water, this phenomenon is called buoyancy and it makes the swimmer feel light.
Question 11. Define density and give its unit.
Answer: The density of a substance is defined as mass per unit volume. Its unit is kg/m3.
Question 12. What is relative density?
Answer: The relative density of a substance is the ratio-of its density to that of water.
Answer Type Questions
Question 1. A ship made of iron does not sink but the iron rod sinks in water, why?
Answer: The iron rod sinks due to high density and less buoyant force exerted by the water on it, but in case of ship the surface area is increased, the upthrust experienced by the body is more. So it floats on water
Question 2. Camels can walk easily on desert sand but we are not comfortable walking on the sand. State reason.
Answer: Camels feet are broad and the larger area of the feet reduces the force/ pressure exerted by the body on the sand. But when we have to walk on the same sand, we sink because the pressure exerted by our body is not distributed but is directional.
Question 3. What is lactometer and hydrometer?
Answer: Lactometer is a device used to find the purity of a given sample of milk. Hydrometer is a device used to find the density of liquids.
Question 4. The relative density of silver is 10.8. What does this mean?
Answer: It means that the density of silver is 10.8 times more than that of water. T
Question 5. he relative density of gold is 19.3. The density of water is 103 kg/m3? What is the density of gold in S.I. unit?
Answer:
NCERT Solutions for Class 9 Science Chapter 10 Gravitation SAQ Q5
Question 6. State Archimedes’ principle.
Answer: Archimedes’ principle—When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
It is used in designing of ships and submarines.
Question 7. Two cork pieces of same size and mass are dipped in two beakers containing water and oil. One cork
floats on water but another sink in oil. Why?
Answer: The cork floats on water because the density of cork is less than the density of water, and another cork sinks in the oil because the density of cork is more than the oil.
Question 8. What are fluids? Why is Archimedes’ principle applicable only for fluids? Give the application of Archimedes’ principle.
Answer: Fluids are the substances which can flow e.g., gases and liquids are fluids. Archimedes’ principle is based on the upward force exerted by fluids on any object immersed in the fluid.
Hence it is applicable only for fluids.
Applications of Archimedes’ principle: