Fluids Book Back Questions 9th Science Lesson 3
9th Science Lesson 3
3] Fluids
Book Back Questions with Answer and Do You Know Box Content
Do You Know?
If a single nail pricks our body it is very painful. How is it possible for people to lie down on a bed of nails, still remain unhurt? It is because, area of contact is more.
Problem 1: A man whose mass is 90 kg stands on his feet on a floor. The total area of contact of his two feet with the floor is 0.036 m2 (Take, g = 10 ms-2). How much is the pressure exerted by him on the floor?
Solution: The weight of the man (thrust), F = mg = 90 kg x 10 ms-2 = 900 N
Pressure, P = F/A = 900 N/0.036 m2 = 25000 Pa
Problem 2: Calculate the pressure exerted by a column of water of height 0.85 m (density of water, ρw = 1000 kg m-3) and kerosene of same height (density of kerosene, ρk = 800 kg m-3)
Solution: Pressure due to water = hρwg = 0.085 m x 1000 kg m-3 x 10 ms-2 = 8500 Pa
Pressure due to kerosene = hρkg = 0.85 m x 800 kg m-3 x 10 ms-2 = 6800 Pa.
Human lung is well adapted to breathe at a pressure of sea level (101.3 k Pa). As the pressure falls at greater altitudes, mountain climbers need special breathing equipments with oxygen cylinders. Similar special equipments are used by people who work in mines where the pressure is greater than that of sea level.
Problem 3: A mercury barometer in a physical laboratory shows a 732 mm vertical column of mercury. Calculate the atmospheric pressure in pascal. [Given density of mercury, ρ = 1.36 x 104 kg m-3, g = 9.8 ms-2].
Solution: Atmospheric pressure in the laboratory, P = hρg = 732 x 10-3 x 1.36 x 104 x 9.8 = 9.76 x 104 Pa (or) 0.976 x 105 Pa.
In petrol bunks, the tyre pressure of vehicles is measured in a unit called psi. It stands for pascal per inch, an old system of unit for measuring pressure.
Problem 4: A hydraulic system is used to lift a 2000 kg vehicle in an auto garage. If the vehicle sits on a piston of area 05 m2, and a force is applied to a piston of area 0.03 m2, what is the minimum force that must be applied to lift the vehicle?
Given: Area covered by the vehicle on the piston A1 = 0.5 m2
Solution: P1 = P2; F1/A1 = F2/A2 and F2 = F1/A1 A2;
F2 = (2000 x 9.8) 0.03/0.5 = 1176 N
Problem 5: You have a block of a mystery material, 12 cm long, 11 cm wide and 3.5 cm thick. Its mass is 1155 grams. (a) What is its density? (b) Will it float in a tank of water, or sink?
Solution: (a) Density = Mass/Volume = 1155 g/12 cm x 11 cm x 3.5 cm = 1155 g/462 cm3 = 2.5 g cm-3
(b) The mystery material is denser than the water. So it sinks.
Salt water provides more buoyant force than fresh water, because, buoyant force depends as much on the density of fluids as on the volume displaced.
Flotation therapy uses water that contains Epsom salts rich in magnesium. As a floater relaxes, he or she is absorbing this magnesium through the skin. Magnesium helps the body to process insulin, which lowers a person’s risk of developing Type 2 Diabetes.
Choose the best answers:
1. The size of an air bubble rising up in water
(a) Decreases
(b) Increases
(c) Remains same
(d) May increase or decrease
2. Clouds float in atmosphere because of their low
(a) Density
(b) Pressure
(c) Velocity
(d) Mass
3. In a pressure cooker, the food is cooked faster because
(a) Increased pressure lowers the boiling point
(b) Increased pressure raises the boiling point
(c) Decreased pressure raises the boiling point
(d) Increased pressure lowers the melting point
4. An empty plastic bottle closed with an airtight stopper is pushed down into a bucket filled with water. As the bottle is pushed down, there is an increasing force on the bottom. This is because,
(a) More volume of liquid is displaced
(b) More weight of liquid is displaced
(c) Pressure increases with depth
(d) All the above
Fill in the blanks:
1. The weight of the body immersed in a liquid appears to be _______________ than its actual weight.
2. The instrument used to measure atmospheric pressure is ______________.
3. The magnitude of buoyant force acting on an object immersed in a liquid depends on ____________ of the liquid.
4. A drinking straw works on the existence of __________________.
State true or false. If false, correct the statement:
1. The weight of fluid displaced determines the buoyant force on an object.
2. The shape of an object helps to determine whether the object will float or not.
3. The foundations of high-rise buildings are kept wide so that they may exert more pressure on the ground.
4. Archimedes’ principle can also be applied to gases.
5. Hydraulic press is used in the extraction of oil from oil seeds.
Match the following:
1. Density – hρg
2. 1 gwt – milk
3. Pascal’s law – mass / volume
4. Pressure exerted by a fluid – pressure
5. Lactometer – 980 dyne
Consider the statements given below and choose the correct option:
(a) If both assertion and reason are true and reason is the correct explanation of assertion
(b) If both assertion and reason are true but reason is not the correct explanation of assertion
(c) If assertion is true but reason is false
(d) If assertion is false but reason is true
1. Assertion: To float, body must displace liquid whose weight is equal to the actual weight.
Reason: The body will experience no net downward force in that case.
2. Assertion: Pascal’s law is the working principle of a hydraulic lift.
Reason: Pressure is thrust per unit area.
Answers:
Choose the best answers:
1. decreases 2. Density 3. increased pressure lowers the boiling point 4. All the above
Fill in the blanks:
1. Less 2. Barometer 3. Density 4. Pressure
State true or false. If false, correct the statement:
1.True
2.True
3.True
4.False
Correct answer: Archimedes’ principle cannot be applied to gases
5.True
Match the following:
1.Density- Mass/ volume
2.gwt- 980 dyne
3.Pascal’s law- Pressure
4.Pressure exerted by a fluid- hρg
5.Lactometer- milk
Consider the statements given below and choose the correct option:
1.If both assertion and reason are true and reason is the correct explanation of assertion
2.If both assertion and reason are true but reason is not the correct explanation of assertion