CSEC Physics MCQ Answer Key - Collins
CSEC Physics Answer KeySection A: MechanicsA1 Scientific method and measurementNo.Answers Further explanations1A2CT 3BTime for ¾ of an oscillation is 0.15 s.34 0.15 s T 0.15 sT 0.20 s4311f 5.0 Hz or s 1T 0.20 s4A5B6D(12.0 4.0 ) A 1Slope 2.0 A 1cm 1( 4.0 0 ) cm7AThe markings on the scale are separated by intervals of 1 mm. Sincethis is 0.001 m, the readings should be presented to 3 decimal placesof a metre.8BThe answer should be represented to 2 significant figures, since the leastnumber of significant figures of the items in the calculation is 2.9CSince the third figure is less than 5, the second figure is not changed.If the third figure were equal to or greater than 5, the second figurewould be incremented by 1.10CArea 200 cm 80 cm 2.00 m 0.80 m 1.6 m211CThe number is represented as a power of ten: M 10p. The ‘mantissa’M is a number in decimal form with only one non-zero digit beforethe decimal point and p is an integer. The value of p is obtained by thenumber of decimal places moved in representing the mantissa. A negativevalue of p indicates that the number being expressed is less than 1.40 2.0 s20f 11 0.50 HzT 2.01 HarperCollins Publishers 2017
No.12Answers Further explanationsA13D14C15CThe reading on the main scale at the mark just before the zero markon the smaller vernier scale is 0.5 cm. The number of the mark on thevernier scale that best aligns with a mark above it on the main scalegives a measure of the hundredths of a cm, which must be added tothe reading from the main scale. The number of this mark is 3 andrepresents 3 cm .100Main scale0.5 cmVernier scale 0.03 cmDiameter of S 0.53 cm16DThe reading on the sleeve at the mark just before the thimble is 6.0 mm.The number of the mark on the thimble that aligns with the centre lineon the sleeve gives the reading of the hundredths of a mm, which mustbe added to the reading from the sleeve. The number of this mark is 12and represents 12 mm.100Sleeve6.0 mmThimble0.12 mmDiameter of R 6.12 mm17B18C19AAn object floats in a fluid if its density (NOT its mass or weight) is lessthan that of the fluid.ρ m ( 200 140 ) g 2.0 g cm 3V ( 50 20 ) cm32 HarperCollins Publishers 2017
A2 Physical quantities, SI units and vectorsNo.Answers Further explanations1C2CThe seven fundamental quantities are: mass, length, time, current,temperature, luminous intensity and quantity of substance.3CEnergy is derived from the quantities force and displacement.4B5BQuantities: F ma Units: N kg ms 26CFrequency 7CEnergy per second power. The unit of power is the watt (W).8A9D10B11C11 Unit of frequency speriodvoltageVResistance Unit of resistance currentAenergyJPower Unit of power timesforceNPressure Unit of pressure 2areamDisplacement can be defined as distance IN A SPECIFIEDDIRECTION from some reference point. Since it has magnitude anddirection, it is a vector.Resultant force directed north 4 N 1 N 3 NResultant force directed west 5 N 1 N 3 N 3 NN3N/3Nθ tan 1SEW( 33 ) 45 The resultant force, and therefore the resultant acceleration, is directedtowards the north-west.3 HarperCollins Publishers 2017
No.12Answers Further explanationsCFV15 N30ºFHVertically: FV 15 N sin 30 7.5 NHorizontally: FH 15 N cos 30 13 N13BMagnitude of the resultant force FR 162 122 N 20 N14BUsing the polygon method for adding vectors, the vectors to be addedare represented in magnitude and direction by arrows drawn headto tail forming a chain. The resultant vector is then represented inmagnitude and direction by the arrow, originating from the beginningof the chain, which closes the polygon.15DThe following diagram illustrates the polygon method for finding theresultant of two vectors.5º16 NFR20 NMagnitude of resultant force FR 162 202 2 16 20 cos 5 4.3 N16BImportant vectors required by the CSEC syllabus are: displacement,velocity, acceleration, force and momentum.A3 StaticsNo.1Answers Further explanationsBSimilar electric charge will produce repulsion.Option A: Copper is not a magnetic substance.Option B: Two similarly charged spheres would repel each other.Option C: Gravitational forces would produce ATTRACTION, notrepulsion.Option D: Nuclear forces would only come into effect at much smallerdistances between the nucleons of the two spheres.4 HarperCollins Publishers 2017
No.2Answers Further explanationsCOption I: Not true. The density of a body is the ratio of its mass to itsvolume, neither of which depends on the acceleration due to gravity.Option II: True. The force exerted on the spring of a spring balancedepends on the force of gravity on the mass causing it. If the accelerationdue to gravity is lower, the reading produced by the associated force willalso be lower. The lever arm balance also depends on the accelerationdue to gravity since forces are necessary to produce the requiredmoments. However, the effect on the clockwise moment is the same ason the anticlockwise moment and therefore the net effect is zero.Option III: True. Since the density of the object is given relative to water,a value of 1 unit is assigned to the density of water. The object is lessdense than water since 0.9 is less than 1 and therefore the body floatsin water.3BThe terms acceleration due to gravity and gravitational field strength areequivalent. At the surface of the Earth the acceleration due to gravity is–2–110 m s and the gravitational field strength is 10 N kg .4CThe weight of a body is the product of its mass and the gravitationalfield strength at its location. The weight of a body is therefore greateron Jupiter.The mass of a body is the quantity of matter from which it is made andtherefore does not change when the body is transferred between theplanets.5BThe effort required to overcome the load occurs when the clockwise andanticlockwise moments are equal. Since each required moment is theproduct of the force and the perpendicular distance of its line of actionfrom the pivot, the larger this distance, the smaller is the necessary force.Diagram B is the only one in which the line of action of the effort has ashorter distance to the pivot than does the line of action of the load, andtherefore is the only one in which the effort is greater than the load.6C7D5 HarperCollins Publishers 2017
No.8Answers Further explanationsBThe body is in equilibrium and therefore the sum of the clockwisemoments about any point is equal to the sum of the anticlockwisemoments about that same point.W eight of hanging block 0.150 kg 10 m s 2 1.5 NTaking moments about the point of support of the rod:1.5 N 20 cm 1.0 N30 cmNote that since the rod is uniform its centre of gravity is at itsgeometric centre, which is 30 cm from the point of support.1.5 N 20 cm W 30 cm9CW Option I: True. The body is in equilibrium and therefore the sumof the upward forces must be equal to the sum of the downward forces:R T W. So W T R.Option III: True. The distances from the centre of the rod to the forcesT and R are the same. These forces must therefore also be the same inorder to provide the equal but oppositely directed moments about thecentre of the rod required for equilibrium. Therefore R T.Option II: Not true. Since R T and R T W, therefore W T.10AOption I: True. The body is in equilibrium and therefore the sumof the upward forces must be equal to the sum of the downward forces:R P Q. Therefore R P Q.Option II: Not true. Rx and Qy are moments taken about differentpoints and therefore are not necessarily equal.Option III: Not true. Px and Qz are moments taken about differentpoints and therefore are not necessarily equal.11CWhen a body in stable equilibrium is slightly displaced, its centre ofgravity rises and a moment is created which restores it to its base.When a body in unstable equilibrium is slightly displaced, its centre ofgravity falls and a moment is created which topples it.When a body in neutral equilibrium is slightly displaced, its centre ofgravity remains at the same level and no moment is created.6 HarperCollins Publishers 2017
No.12Answers Further explanationsCOption I: True. The force constant is the gradient of the straight-linesection of the graph.F90 Nk 6.0 N cm 1e 15 cmOption II: Not true. For Hooke’s law to be conformed to, the load mustbe proportional to the extension, and therefore the graph must be astraight line. The curve in the graph indicates that the law was notalways obeyed.Option III: True. The x-coordinate of 10 cm corresponds to they-coordinate of 60 N.Option IV: Not true. The proportional limit has been surpassed wherethe graph ceases to be a straight line.13DStability is increased by lowering the height of the centre of gravity and/or by increasing the width of the base of the object. Option D is the onlyone in which both of these changes are made and is therefore the onlyoption which DEFINITELY increases the stability of the object.14BWhen the load was increased by 40 N the length of the spring stretchedfrom 15 cm to 25 cm. The extension corresponding to this increase inload is therefore 10 cm.k 15BF40 N 4.0 N cm 1e 10 cmSince the force constant of the spring is 4.0 N cm 1, the first 20 Nstretched it by 5.0 cm.F20 NF ke e 5.0 cmk 4.0 N cm 1The length when the load is completely removed is therefore 15 cm 5.0cm 10 cm.7 HarperCollins Publishers 2017
A4 KinematicsNo.Answers Further explanations1Cv 2 v1 (19.0 4.0 ) m s 1a 3.0 m s 2t( 5.0 ) s2AThis problem involves velocities in two opposite directions. Taking thedirection of v2 as positive and the direction of v1 as negative:a v 2 v1 ( 24 24 ) m s 1 400 m s 2t0.12 s3A4C5D6A7DVelocity is represented by the gradient of the displacement–time graph.8C9DTaking upwards as positive displacement, an object shot into the air startswith an initial positive velocity. This will uniformly decrease with timedue to the opposing gravitational force. At the highest point the velocitywill have decreased to zero. It will then increase uniformly but in theopposite direction (downwards, i.e. negative) until it returns to its startingpoint. Since the acceleration due to gravity is always constant and directeddownwards, the gradient of the graph is always constant and negative.Distance D travelled is represented by the area between the graph lineand the time axis.speed distance 36 km36 000 m 5.0 m s 1time2.0 3600 s2.0 haverage speed average velocity v D 10B11A( 8.0 8.0 ) mdistance16 m 0.80 m s 1time20s( 2.0 12.0 6.0 ) sdisplacement( 8.0 8.0 ) m0m 0 m s 1time( 2.0 12.0 6.0 ) s 20 sΔs (120 30 ) m 30 m s 1Δt3.0 0s()20 m s 1 4 s 40 m2The object changes direction when its velocity switches from positive tonegative at t 4 s.Acceleration is obtained from the gradient of the velocity–time graph. 1Δv ( 48 24 ) m sa 3.0 m s 2Δt( 20.0 12.0 ) s8 HarperCollins Publishers 2017
No.12Answers Further explanationsCDistance D travelled is represented by the area between the graph lineand the time axis. 24 m s 1 8.0 s ( 24 m s 1 8.0 s ) 290 mD 2 ( 2 s.f . )13DThe only force on the object is the constant force of gravity. Since theresultant force is the product of the object’s mass and acceleration, theacceleration is therefore always constant.14BAs the object falls its acceleration is the constant acceleration due togravity. This is represented by the constant gradient of the velocity–timegraph Q.15CAs the object falls its velocity increases. This is represented by theincreasing gradient of the displacement–time graph R.16BSince the gradient of the displacement–time graph is initially zeroand then constant, the velocity of the object is initially zero and thenconstant.A5 DynamicsNo.Answers Further explanations1CAristotle believed that the force on a body was proportional to itsvelocity. He argued that the more horses pulling a carriage, the fasterit will go. He failed to consider all the forces acting on the bodyand did not consider, for example, friction. Newton later showedthat the resultant force on a body is PROPORTIONAL TO ITSACCELERATION (FR ma) and not to its velocity.2DThe resultant force on a body must be zero if it is not accelerating. FR ma(i.e. if it is at rest or is moving at constant velocity).3A4CFR ma a FR ( 40 8 ) N 64 m s 2m0.500 kgX: Law 3. If body A exerts a force on body B, then body B exerts anequal but oppositely directed force on body A.Y: Law 2. The resultant force on a body causes it to accelerate.Z: Law 1. Every body continues in its state of rest or uniform motion ina straight line unless compelled to act otherwise by a resultant force.9 HarperCollins Publishers 2017
No.5Answers Further explanationsATaking velocity to the right as positive:FR m ( v 2 v1 )t Time of impact t 6Cm ( v 2 v1 ) 0.400 kg (10 20 ) m s 1 0.020 sFR600 NOption I: True. A has a greater resistive force on it, due mainly togreater air resistance at its greater velocity.Options II, III: The resultant force on both cars is zero since they bothtravel at constant velocity.7DOption I: True. Note that a closed system is one in which external forcescan have no effect.Option II: Not true. It is the TOTAL momentum immediately before acollision which is equal to the TOTAL momentum immediately afterthe collision.Option III: True. Since momentum is a vector quantity, two bodieshaving momentum equal in magnitude but opposite in direction willhave a total momentum that cancels to zero.8COnly one of the masses is moving before the collision and only one ismoving after the collision. The total momentum immediately beforethe collision must be equal to the total momentum immediately afterthe collision. Since the masses are the same, the velocities of the movingmasses must therefore also be the same.9ATaking velocity to the right as positive:Initial momentum ( 2.0 kg 5.0 m s 1 ) ( 3.0 kg 10 m s 1 ) 20 kg m s 1The negative sign indicates that the momentum is directed to the left.10CTaking the initial velocity of the mass m as positive:Total momentum before the collision ( m v ) ( m2 2v ) 0By the law of conservation of linear momentum, the total momentumafter the collision is therefore also zero.10 HarperCollins Publishers 2017
A6 EnergyNo.Answers Further explanations1BChemical potential energy stored in the diesel releases thermal energyas it is burnt. The thermal energy is used to boil water and producesteam which then creates a pressure to turn a turbine, giving it kineticenergy. This produces a relative motion between a coil and magneticfield in a generator, which causes a transfer of kinetic energy toelectrical energy.2DGravitational potential energy of the water transfers to kinetic energy asit falls and turns a turbine. The kinetic energy of the turbine producesa relative motion between the coil and magnetic field in a generator,which causes a transfer of kinetic energy to electrical energy.3BCrude oil is a non-renewable source of energy because it cannot bereadily replaced.4CW Fs 800 N 5.0 m 4000 J5CSince the acceleration is zero, the resultant force is zero and the 800 Napplied by Kimran is used only to overcome friction. The 4000 J of workdone by Kimran is all used to overcome friction.6A7CA stretched elastic band stores elastic potential energy.Bread stores chemical potential energy.A mango hanging from a tree stores gravitational potential energy.8DAs the coconut falls its gravitational potential energy converts tokinetic energy.P W 4000 J 200 Wt20 sΔEP mg Δh 32 N 20 m 640 J9AEKA 1 2mv2EKB 112m ( 2v ) 4 mv 222 EKB 4 200 J 800 J10Bmgh 1 2mv2gh 1 2v220 m s 1 )(v2h 20 m2 g 2 10 m s 2211 HarperCollins Publishers 2017
No.Answers Further explanations11A12Aefficiency useful work output 1200 N 5.0 m 0.75 or 75%8000 Jwork input1 2mv mgh21 2v gh2v 2 gh 2 10 m s 2 4.0 m 8.9 m s 113B14CΔEP mg ΔhΔEP 2.0 kg 10 m s 2 4.0 m 80 JA7 HydrostaticsNo.1Answers Further explanationsAP FATo create maximum pressure the block must rest on the smallest area.1200 NP 10 000 Pa( 0.60 0.20 ) m 22BP PAtm PW PAtm ( hρ g )WP 1.0 105 Pa ( 20 m 1000 kg m 3 10 m s 2 ) 3.0 105 Pa3CP FATherefore by increasing the area of contact the pressure is reduced.4DP hρ gwhere h is the depth of water and ρ is its density.5CThe PRESSURE produced by the force at X is transmitted to Y. For thepressure to be the same, the force at Y must be greater than that at Xsince Y has a greater area.fFPX PY X YaX AY12 HarperCollins Publishers 2017
No.6Answers Further explanationsCPX is zero since above X is a vacuum.PY 15 cm Hg since 15 cm of mercury is above it, causing the pressureat Y.PZ 75 cm Hg since 75 cm of mercury is above it, causing the pressureat Z.7AP PAtm PL PAtm ( hρ g )LP 1.0 105 Pa ( 0.20 m 5.0 103 kg m 3 10 m s 2 ) 1.1 105 Pa8BIn accordance with the principle of Archimedes, an upthrust will act onthe small mass, equal to the weight of the water it displaces.9DUpthrust is always equal to the weight of the fluid displaced; if thebody floats, then it is also equal to the weight of the body itself. If theacceleration of a body is zero, then the resultant force on the body isalso zero (Since FR ma).10CSince the body accelerates upwards, the resultant force must be directedupwards and therefore the upthrust ( weight of air displaced) must begreater than the weight of the balloon and its contents.11AThe object floats and therefore:Weight of object weight of liquid displaced mass of object mass of liquid displaced200 g ρLVL (since mass density volume)200 g 1.60 g cm 3 VL200 g3VL 3 125 cm1.60 g cmSection B: Thermal Physics and Kinetic TheoryB1 Nature of heat, macroscopic properties and phenomenaNo.Answers Further explanations1A2BThe caloric theory was REPLACED BY the kinetic theory. CountRumford’s ‘cannon boring’ experiment provided evidence AGAINSTthe caloric theory.13 HarperCollins Publishers 2017
No.3Answers Further explanationsBOption B: The body temperature of a human is about 37 C andfluctuates just a few degrees below and above this. A suitable range of thethermometer is therefore from about 34 C to 43 C.Option A, C & D: When a liquid-in-glass mercury thermometer isremoved from a patient, the mercury in its stem rapidly contracts. Sincemercury has very strong cohesive forces between its particles, it snaps atthe narrow constriction in its bore, leaving the section of mercury abovethe constriction to be read. Alcohol is not capable of snapping at theconstriction and would all return to the bulb on removal from the patient.4BOption II: Not true. A small red hot steel pin is at a higher temperaturethan a bucket of warm water but contains less thermal energy since ithas a much lower heat capacity.Option I: True. Although the small pin has less thermal energy than thebucket of warm water, heat will flow from the pin to the water since thepin is at a higher temperature than the water.Option III: True. Heat is thermal energy which flows from places ofhigher temperature to places of lower temperature.5B6DThe body temperature of a healthy human is about 37 C and onlyfluctuates by a few degrees.7COption I: True. Thinner walls provide less material for the thermalenergy to conduct through and therefore the transfer process is quicker.Option II: Not true. Mercury is a metal and is a much better conductorthan is alcohol. It therefore responds faster to temperature changethan alcohol.Option III: True. A larger bulb and longer stem implies that a greatervolume of liquid can be contained and a correspondingly greater changein volume of liquid will be obtained on heating. The readings on the scalewill therefore be further separated and can be read with greater precision.Option IV: True. A thinner bore results in a longer stem for a givenvolume and therefore the readings wil
16 B Important vectors required by the CSEC syllabus are: displacement, velocity, acceleration, force and momentum. A3 Statics No. Answers Further explanations 1 B Similar electric charge will produce repulsion. Option A: Copper is not a magnetic substance. Option
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