# Class 9 Science Thrust & Pressure (Questions from board papers)

Science

Chapter- 10 (Part-2)

Thrust & Pressure

(Questions from Board Papers)

Question 1: How is pressure related to thrust?

Answer 1: Pressure = Total thrust / Total surface area

Question 2: Name the SI unit of pressure.

Answer 2: Pascal (Pa) [ 1 Pa = 1 N/m2

Question 3: Why does a truck or a motor-bus has much wider tyres?

Answer 3: So that pressure acting on the road due to weight of truck or motor-bus may be small.

Question 4: The cutting edge of a knife should be as sharp as possible. Why?

Answer 4: To have greater pressure at the edge even for a smaller force applied so that its cutting action is sharp and quick.

Question 5: The edge of a drawing pin is sharp and pointed one. Why?

Answer 5: So as to increase the pressure of applied force. As a result the drawing pin easily pierces into the card board sheet.

Question 6: What is force of buoyancy?

Answer 6: Force acting vertically upward due to a fluid on a solid immersed completely or partly in that fluid.

Question 7: What is the SI unit of density?

Question 8: What is the value of density of water in SI system?

Question 9: What is the unit of relative density? Why?

Answer: Relative density is a unit-less quantity because it is a pure ratio (i.e., ratio of two densities)

Question 10: Relative density of steel is 7.8. What is the density?

Answer: Density of steel = relative density of steel x density of water = 7.8 x 103 Kg/m3

Question 11: Density of wood is 0.25 x 103 kg/m3. Will it sink or float in water? Why?

Answer: It will float in water because its density is less than the density of water.

Question 12: Arrange the following in increasing order of their relative densities: Iron, air, water

Question 13: The relative density of a substance is greater than 1. What does it signify?

Answer: The substance will sink when put on water surface.

Question 14: Lactometers are used to determine the purity of a sample of milk. On which principle is this instrument based?

Answer: Working of lactometer is based on Archimedes’ principle.

Short Answer Type Questions (Two Marks Each)

Question 15: Describe thrust and write its SI unit. Name one factor on which the effect of thrust depend.

Answer: The normal force acting on a surface die to an object is called thrust. SI unit of thrust is newton (N).

Thrust on a surface depends on weight of the object placed on the surface.

Question 16: Define pressure. Give its mathematical expression and its SI Unit.

Answer: The thrust on unit surface area is called pressure.

Pressure = Total thrust / Total surface area

SI unit: Pascal (Pa) [ 1 Pa = 1 N/m2]

Question 17: Why do you prefer a broad and thick handle for your suitcase?

Answer: Due to large surface area of broad and thick handle, the pressure exerted on the hand of person, carrying the suitcase, due to weight of suitcase will be less. Hence, it will be convenient to carry it.

Question 18: Why are railway tracks laid on large sized concrete (wooden) sleepers?

Answer: When a train passes over a railway track, whole weight of train acts on the railway line beneath it. Due to large sized concrete (wooden) sleepers the force of weight of train spreads over a large area and pressure is less. As a result, railway track can bear it.

Question 19: We cannot walk comfortably on sand but camel can run in a desert easily. Explain the reason in brief.

Answer: The surface area of feet of camel is much more than that of man. When a camel walks/ runs in a desert, the pressure due to his weight is much less. As a result, camel can run easily.

Question 20: Shoes with broad sole are made more comfortable to wear than high heel shoes. Why?

Answer: When a person wears shoes having broad sole, the weight of a person is shared uniformly by the total area of soles of the pair of shoes. As a result, pressure is less and the person wearing the shoes feels comfortable. For a high heel shoes the entire weight of person acts on small area of heel and pressure is very high. So these shoes are not comfortable.

Question 21: Give reasons of the following:

(i) Cutting and piercing tools are made sharp.

(ii) An egg sinks in fresh water but floats in highly salty water.

Answer: (i) Cutting and piercing tools are made sharp so as to have greater pressure at the edge of the tool even for a small applied force. As a result, its cutting/ piercing action is sharp and quick.

(ii) An egg sinks in fresh water because its density is more than that of fresh water. However, it floats in highly salty water because density of egg is less than that of highly salty water.

Question 22: A cork piece floats on water but an iron nail sinks in water. Why?

Answer: A cork piece floats on surface of water because its density is less than the density of water. On the other hand, an iron nail sinks because density of iron nail is greater than the density of water.

Question 23. Find the total thrust acting on the bottom surface of a tank 4 m long, 2 m broad and 2 m deep when fully filled with water. The density of water = 103 kg m -3.

Answer: Total thrust acting on the bottom surface of a tank (F) = mg =Vρg

Here, V = 4 x 2 x 2 = 16 m3 and ρ = 103 kg m -3

Therefore, F = 16 x 103 x 9.8 = 1.568 x 105 N

Question 24. State Archimedes’ principle. What does it signify?

Answer: Whenever an object 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.

Archimedes’ principle predicts the magnitude of buoyant force acting on a body immersed in a liquid.

Question 25. What is the difference between density of a substance and its relative density? Write SI units of both.

Answer: (i) The density of a substance is defined as its mass per unit volume. Relative density of a substance is the ratio of its density to the density of water.

(ii) The SI unit of density is kg/m3 . Relative density is a unitless quantity.

Question 26. Density of gold is 19300 kg m-3. Determine its relative density.

Question 27. Density of an object is 1.8 g/cm2. Express it in kg/m3.

Question 28. A body floats in kerosene of density 0.8 x 103 kg/m3 up to a certain mark. If the same body is placed in water of density 1.0x 103 kg/m3, will it sink more or less? Give reason for your answer.

Answer: The body will sink less in water. When a body of weight W floats in a liquid of density ρ with a volume V immersed in a liquid, the upthrust must balance the weight and hence

W= Vρg or, V =W/ρg ⇒ V α 1/ρ

As density of water (ρw = 1 x 103 kg/m3) is greater than density of kerosene (ρk = 1 x 103 kg/m3), hence volume of body immersed in water will be less than that in kerosene.

Question 29. A solid weighs 80g in air and 64g in water. Calculate the relative density of solid.

Answer: Here, weight of solid in air W1 = 80g and weight in water W2 = 64 g

Loss of weight of solid in water = W1 – W2 = 16 g

Weight of water displaced by solid= 16 g

Relative density of solid = Density of solid / Density of water

= Weight of solid in air / Weight of equal volume of water

= 80/16 = 5

Question 30. The volume of 100g of a solid substance is 25 cm3 and the density of water is 1 g cm-3. Find the density of the substance. Will it float or sink in water?

Answer: 4 g cm-3 (it will sink in water)

(i) Find the relative density of substance.

(ii) Ravi finds his mass to be 42 kg on a weighing machine. Is his true mass more or less than 42 kg?

Answer: On a weighing machine we actually find weight and from this we determine mass. When Ravi finds his weight 42 kg on weighing machine, a buoyant force due to air is acting on him. Hence the observed weight is slightly less than his true weight.So, the true mass of Ravi is greater than 42 kg.

(iii) Does the force of buoyancy act on the body if it sinks? If yes, then how much?

Answer: Yes, a force of buoyancy acts on a body even if it sinks in water. The force has a magnitude equal to the weight of water displaced by the body when immersed in water.

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