A. Answer the following.<\/span> Question 2. Question 3. b. A long spanner is used to loosen a tight nut. This is because by increasing the length, of the spanner, the moment arm gets increased and even a small force can produce a large turning effect.<\/p>\n B. Solve the following numericals.<\/span><\/p>\n Question 1. Question 2. Question 3. Check Your Progress<\/span><\/p>\n A. Fill in the blanks.<\/span><\/p>\n Answer:<\/p>\n B. Answer the following.<\/span><\/p>\n Question 1. Question 2. A. Tick the most appropriate answer.<\/span><\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Question 6. Question 7. B. Fill in the blanks.<\/span><\/p>\n Answer:<\/p>\n C. Write true or false. Correct the false statements.<\/span><\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Question 6. Question 7. D. Answer the following in short.<\/span><\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Question 6. E. Answer the following in detail.<\/span><\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Question 6. F. Solve the following numerical problems.<\/span><\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Think your answer<\/span><\/p>\n Question 1. Question 2. 2. Moreover, the pressure of water increases with depth. So, it is greater at the base than at the top. So, the lase is broad to withstand the huge pressure of the water.<\/p>\n Question 3.
\nQuestion 1.
\nDefine moment of a force. Write its SI unit.
\nAnswer:
\nThe turning effect of a force about a fixed point (pivot) is called the moment of the force. Its SI unit is N m
\n(newton metre).<\/p>\n
\nIt is easier to open a door by applying the force at its handle than at the hinged end. Why?
\nAnswer:
\nIt is easier to open a door by applying the force at its handle than at the hinged end because the moment of force is large when the force is applied at the handle as the moment arm is larger as compared to when the force is applied at the hinged end.<\/p>\n
\nWrite any two applications of moment of force.
\nAnswer:
\nThe tw\u00f0 applications of moment of force are :
\na. The hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a. large turning effect.<\/p>\n
\nA nut is opened by a spanner of length 30 cm. If the applied force is 20 N, find the moment of force needed to loosen the nut.
\nAnswer:
\nHere,
\nForce (F) = 120 N
\nMoment arm (d) = 30 cm = \\(\\frac {30}{100}\\) = 0.3 m
\nMoment of force = F X d
\n= (120 N) x (0.3 m) = 36 N m
\nSo, the moment of force needed to loosen the nut is 36N m.<\/p>\n
\nA force of 250 N produces a moment of force of 500 N m. Calculate the moment arm.
\nAnswer:
\nHere,
\nForce (F) = 250 N
\nMoment of force 500 N m
\nMoment arm (d) = ?
\nBy using the formula,
\nMoment of force = F X d
\nwe get,
\n500 N m = 250 N x d
\nd = \\(\\frac {500Nm}{250N}\\) = 2m<\/p>\n
\nCalculate the force which will produce a moment of 60 N m when the perpendicular distance between the point of application of force and the fixed point is 60 cm.
\nAnswer:
\nHere,
\nMoment of force = 60 N m
\nMoment arm (a) = 60 cm = \\(\\frac {60}{100}\\)m = 0.6 m
\nBy using the formula,
\nMoment of force F x d
\nwe get,
\n60Nm = F x 0.6m
\nF= \\(\\frac {60 Nm}{0.6 m}\\) = 100N<\/p>\n\n
\n
\nIf the magnitude of force acting on a surface is doubled and area over which it acts is halved, then
\nwhat will be the effect on pressure?
\nAnswer:
\nWhen,
\nmagnitude-of force = F, and
\narea over which the force acts = A
\nthen, the pressure (P) is given by
\nP = \\(\\frac {F}{A}\\)
\nNow, if the force is doubled (2F) and area over is halved (A\/2), then the pressure is given by –
\nPressue =\\(\\frac {2F}{A\/2}\\)= \\(\\frac {4F}{A}\\) = AP
\nSo, when the magnitude of force acting on a surface is doubled and area over which it acts \u00a1s halved, then the pressure becomes 4 times.<\/p>\n
\nWhy do school bags have broad straps?
\nAnswer:
\nSchool bags have broad straps to reduce the pressure on our shoulders.<\/p>\n
\nThe turning effect of a force is known as
\na. work.
\nb. energy.
\nc. moment.
\nc. pressure.
\nAnswer:
\nc. moment<\/p>\n
\nWhat is the pressure exerted by a body of weight 200 N acting on an area of 5 m2<\/sup>?
\na. 50 Pa
\nb. 200 Pa
\nc. 40 Pa
\nd. 0.02 Pa
\nAnswer:
\nc. 40 Pa<\/p>\n
\nWhat is the value of one pascal?
\na. 1 N2<\/sup>\/m2<\/sup>
\nb. 1 N\/m2<\/sup>
\nc. 1 N2<\/sup>\/m
\nd. 1 N\/rn
\nAnswer:
\nb. 1 N\/m2<\/sup><\/p>\n
\nArmy tanks are provided with caterpillar tracks under their wheels in order to
\na. decrease pressure on the ground
\nb. increase pressure on the ground
\nc. make them look different
\nd. prevent them from damage.
\nAnswer:
\na. decrease pressure on the ground<\/p>\n
\nPressure \u00a1n liquid\u2019s is measured by an instrument called
\na. barometer
\nb. hydrometer
\nc. odometer
\nd. manometer
\nAnswer:
\nd. manometer<\/p>\n
\nThe value of atmospheric pressure at sea level in SI system is approximately equal to
\na. 1000 Pa.
\nb. 10000 Pa.
\nc. 100000 Pa.
\nd. 10000000 Pa.
\nAnswer:
\nc. 100000 Pa<\/p>\n
\nAt sea level, the height of the mercury column in a barometer is
\na. 72 cm.
\nb. 76 cm.
\nc. 82 cm.
\nd. 98 cm.
\nAnswer:
\nb. 76 cm.<\/p>\n\n
\n
\nThe perpendicular distance from the pivot of a rotating body to the line of action of the force is known as the moment arm.
\nAnswer:
\nTrue.<\/p>\n
\nPressure is inversely proportional to the force applied.
\nAnswer:
\nFalse.
\nCorrect : Pressure is directly proportional to the force applied.<\/p>\n
\nWooden sleepers are placed under railway tracks in order to increase pressure on the ground.
\nAnswer:
\nFalse.
\nCorrect : Wooden sleepers are placed under railway tracks to reduce the pressure on the ground.<\/p>\n
\nPressure is independent of the roughness of the surfaces in contact.
\nAnswer:
\nTrue.<\/p>\n
\nSkis have a large surface in order to reduce the pressure on snow.
\nAnswer:
\nTrue.<\/p>\n
\nLiquids seek their own level regardless of the shape and cross sectional area of the vessels they are kept
\nin.
\nAnswer:
\nTrue.<\/p>\n
\nThe standard value of atmospheric pressure is about 1,50,000 Pa.
\nAnswer:
\nFalse.
\nCorrect : The standard value of atmospheric pressure is about 101325 Pa.<\/p>\n
\nName the factors affecting the moment of a force.
\nAnswer:
\nThe factors affecting the moment of force are magnitude of the force applied and the moment arm.<\/p>\n
\nThe hand flour grinder \u00a1s provided with a handle near its rim. Give reasons why.
\nAnswer:
\nThe hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a large turning effect.<\/p>\n
\nHow are force, area and pressure related to each other?
\nAnswer:
\nForce, area and pressure are related by the following expression.
\n![\"Living](\"https:\/\/i1.wp.com\/icsesolutions.com\/wp-content\/uploads\/2021\/04\/Living-Science-Physics-Class-8-ICSE-Solutions-Chapter-3-Force-and-Pressure-2.png?resize=213%2C48&ssl=1\")
\nIf the force applied is doubled for the same area, then the pressure will also double.<\/p>\n
\nWhy do safety pins and drawing pins have narrow and sharp tips?
\nAnswer:
\nSafety pins and drawing pins have narrow and sharp tips because the smaller surface area of the tip causes a high pressure and, therefore, makes it easier for these objects to pierce through a given material.<\/p>\n
\nWhat do you understand by atmospheric pressure? How does it change with altitude?
\nAnswer:
\nThe force exerted by the atmosphere per unit area of the surface of earth is called atmospheric pressure. It decreases with increase in altitude.<\/p>\n
\nWhy is water not used as a barometric liquid?
\nAnswer:
\nWater is not used as a barometric fluid as it has less density due to which it will have to rise to a height of 10.5 m to balance the atmospheric pressure. Moreover, it vaporizes in vacuum and thus, never records the true atmospheric pressure.<\/p>\n
\nDifferentiate between the clockwise and anticlockwlse moments.
\nAnswer;
\nIf the force has a tendency to turn or rotate a body in clockwise direction, the moment of force is called clockwise moment. If the force has a tendency to turn or rotate a body in anticlockwise direction, the moment of force is called an anticlockwise moment.<\/p>\n
\nDefine pressure. Write the factors on which the pressure exerted by a body depends.
\nAnswer:
\nPressure is defined as the amount of force acting per unit area. It can be mathematically expressed as
\n
\nIt depends on the following two factors – magnitude of force and the area of contact.
\nPressure increases if the magnitude of force (thrust) increases, or the area of contact decreases.
\nPressure decreases if the magnitude of force (thrust) decreases, or the area of contact increases,<\/p>\n
\nWhere is pressure of the sea water greater : 5 m below the surface of the sea or 10 m below the surface of the sea? Why?
\nAnswer:
\nPressure \u00a1s greater at 10 m below the surface of the sea as compared to that at 5 m below the surface of the sea. This is because pressure exerted by a liquid
\nincreases with depth.<\/p>\n
\nWhy do the pillars of a bridge have a broader base than the top?
\nAnswer:
\nPillars of a bridge have a broader base than the top. This is done to reduce the pressure exerted on the ground by the enormous weight of the bridge. As pressure is inversely proportional to the area of contact, so if the base is broader, the area of the contact increases, reducing the pressure on the ground due to the weight of
\nthe bridge.<\/p>\n
\nList any three characteristics of liquid pressure.
\nAnswer:
\nThe characteristics of liquid pressure are given as follows.<\/p>\n\n
\nDescribe the function of a simple mercury barometer. State its disadvantages.
\nAnswer:
\nA simple mercury barometer is used to measure atmospheric pressure. Some of its disadvantages are listed below.<\/p>\n\n
\nA force of 200 N acts on a rigid body such that the perpendicular distance between the force and the turning point is 50 cm. Calculate the moment of force.
\nAnswer:
\nForce (F) = 200 N
\nMoment arm (d) = 50 cm = \\(\\frac {50}{100}\\)= 0.5 m
\nMoment of force = F x d
\n= (200 N) x (0.5 m) = l00 N m<\/p>\n
\nCalculate the force applied on a body producing a moment of force of 16 N m such that the perpendicular distance between the point of application of force and the turning point is 80 cm.
\nAnswer:
\nMoment of force = 16 N m
\nMoment arm (d) = 80 cm = \\(\\frac {80}{100}\\)m = 0.8 m
\nForce (F) = ?
\nFrom the formula,
\nMoment of force = F x d
\nwe get,
\nl6 N m = F x 0.8 m
\nF= \\(\\frac {16Nm}{0.8m}\\) = 20N<\/p>\n
\nA 40 kg boy sits 1.2 m from the fulcrum of a see saw. Where should another boy of 30 kg sit on the other side \u00a1n order to balance it ?
\nAnswer:
\nThe see-saw will be balanced when the moment of force of both the boys is equal.
\nFor 40 kg boy,
\nWeight, F = 40 kg x 10 m\/s2<\/sup> = 400 N
\nMoment arm, d = 1.2 m
\nMoment of force = F X d = 400 N x 1.2 m = 480 N m
\nFor 30 kg boy,
\nWeight,F = 30kg x 10m\/s2<\/sup>= 300N
\nMoment of force = 480 N m
\nMoment arm, ![\"Living](\"https:\/\/i0.wp.com\/icsesolutions.com\/wp-content\/uploads\/2021\/04\/Living-Science-Physics-Class-8-ICSE-Solutions-Chapter-3-Force-and-Pressure-1.png?resize=255%2C46&ssl=1\")
1.6 m
\nThe 30-kg boy should sit at a distance of 1.6 m from the fulcrum of the see-saw.<\/p>\n
\nThe tip of a drawing pin has an area of 0.01 mm2<\/sup> Find the pressure exerted by it if a force of 20 N is
\napplied by it.
\nAnswer:
\nForce exerted by the tip of a drawing pin, F = 20 N
\nArea of the tip of the drawing pin, A = 0.01 mm2<\/sup>
\n\\(\\frac{0.01}{1000 \\times 1000}\\)m2<\/sup>
\nPressure exerted by the tip of the pin,
\nP = \\(\\frac {F}{A}\\) = \\(\\frac{20 \\mathrm{~N}}{0.01 \/(1000 \\times 1000) \\mathrm{m}^{2}}\\)
\n= 2,00,00,00,000 Pa<\/p>\n
\nA motorcycle weighs 1200 N. If the pressure exerted by one of its types is 120 Pa, then find the area of the type in contact with the road.
\nAnswer:
\nWeight of the motorcycle = 1200 N
\nForce exerted by the motorcycle on the road, F = Weight
\nof the motorcycle = 1200 N
\nPressure exerted by one of its tyres, P = 120 Pa
\nArea of the tyre in contact with the road, A = ?
\nFrom the formula, P = \\(\\frac {F}{A}\\)
\nweget, 120Pa = \\(\\frac {1200 N}{A}\\)
\nor A = \\(\\frac {1200 N}{120Pa}\\) = 10 m2<\/sup><\/p>\n
\nExplain in brief why birds that walk on the leaves floating on the surface of water have wide feet.
\nAnswer:
\nBirds that walk on leaves floating on the surface of the water have wide feet. This is because wide feet increases the area of contact with the leaves. So, they exert less pressure on the leaves, allowing them to walk on these leaves.<\/p>\n
\nExplain why the wall of a dam is made thicker at the bottom.
\nAnswer:
\nThe wall of a dam is made thicker at the bottom because of the following reasons.
\n1. A dam has a massive weight and thus, applies a large force on the ground. In order to reduce the pressure exerted by it on the ground, its area of contact with the ground is increased by making its base broad. So, a broad base supports the weight of the dam.<\/p>\n
\nWhen the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the liquid pushes up into the syringe. Give reason.
\nAnswer:
\nWhen the nozzle of a syringe Is dipped in a liquid and its piston is withdrawn, a region of low pressure is created in the syringe. The atmospheric pressure acting on the surface of the liquid is greater and hence forces the liquid
\nto rise in the syringe.<\/p>\n