Solutions to Forces Worksheet 1 Physics

7/22/2019 Solutions to Forces Worksheet 1 Physics

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ANSWERS TO FORCES _ WORKSHEET 1

QuestionL:Describe the typical effects of external forces on bodies ncluding:- friction between surfaces- air resistanceLet us considerhow external orces affect the motion of cars. When dealingwith friction andcars here aredifferent types of friction to consider.(i) Static friction: This is the friction between he tyre and the road when the car isstationary or when the tyre rolls without slipping. It is this static friction force thatprovidesthe reaction force that causes hewheel to roll and the car to move. Whenit is removed e.9.a carbogged n mud or trying to drive on ice) the wheels otatebut because here is no reaction force between he tyres and the surface, he car

will not move. Static riction is also he force that holds a car in placewhen it isparkedon a slope.(ii) Kinetic friction: This is the friction between wo surfaces hat are moving withrespect o eachother. It applies, or instance,when hard-brakingstops he wheelsfrom turning and the car skids to a stop.(iiD Rolling friction: There s some oss of energyand somedeceleration rom frictionfor any real wheel in motion, and his is sometimes eferred o as rolling friction.It is partly friction at the axle and can bepartly due to flexing of the wheel whichwill dissipatesomeenergy.(iv) Internal friction: this is the friction that occursbetweenmoving partsof a car suchas he pistonsand cylinders.When an objectmoves hrougha fluid (liquid or gas) t has o push he particlesof fluid outof the way. When the fluid is air this is known as air resistance. Air resistance pposeshemotion of a car. Streamlining, .e. sloping he shapeof the car, so that air flows over itsmoothly, greatly educesair resistance. ir resistancencludes he movementof a carthrough still air aswell as he movementof air againstthemotion of the vehicle (wind).

Question2:

Outline the forces involved in causing a change n the velocity of a vehicle when:- coastingwith no pressureon the accelerator- pressingon the accelerator- pressingon the brakes- passingover an icy patch on the road- climbing and descendinghills- following a curve in the roadNote thatbecause ou areonly asked o outline the forces,your answersshould be very brief.You needmaybe a sentenceor two and an appropriatediagram. I have suppliedmore of adescription& explanationof the above orces n order o help your understanding.


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Inthe diagrams hat follow:.Ep: force of engine,fe: air resistance,Jr: force due o friction,jr : force dueto gravityJ. : centripetalforce andy - velocity.(a) Coasting with no pressure on the accelerator: When there s no pressure n theaccelerator here areno forward forces. If there were no backward forces then the car

would keep moving forever at the same speed.However there is rolling friction, airresistanceandfriction between he moving partsof the car acting to oppose hemotion of the car. Consequently he car will gradually slow down and stop.

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(b) Pressing on the accelerator: Pressingon the acceleratorncreaseshe rateat whichfuel is fed to the cylinders of the car. This in turn increaseshe rate of rotation of thewheels and increasesstatic friction. The reaction force to this increases he forwardforce on the car allowing it to overcome he forces that retard the motion. If theaccelerators held in a position where it balances he forces opposing motion, the carwill maintain constantspeed. If the accelerators presseda little harder he car willincrease ts speed.

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(c) Pressing on the brakes: The hydraulic system ransferspressure o the pads hat gripthe discs or drums. Thepadsexert a frictional force on the discs or drums that is in theopposite direction to the motion of the wheel. This slows the rate at which the wheelsarerotating. This causesa forward force between he wheels andthe road and thereactionforce to this forward force causes he car to slow down and stop. If too muchpressures applied o the brakes hey completely stop he rotation of the wheels.Thiscauseshe wheels o slide (skid) over the roadand since sliding (kinetic) friction isless han static friction the car travels a much greaterdistancebefore stopping.

- - v - +

n /r / / ///-d

/ 1r // / F /r //// /*J

//r/ /1 / 1r// / tt

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(d) Passing over an icy patch on the road: Ice reduces riction to a very low value sopressingon the acceleratorwould speedup the rate of rotation of the driving wheelsbut they would not be able o grip the road.The wheelswould spin on the ice but thecar would have no traction andwould not increase ts speed. The loss of frictionwould be disastrousor braking and steering.The brakeswould stop he wheels romrotatingbut according o Newton's first law) a car n motion continues n uniformmotion unlessactedon by a force. Hence he car would skid with liule change nspeeduntil it wasclear of the icy patchor until it met an opposing orce suchas hebackof the car in front. Also because f the lack of friction, turning thewheelswouldnot be able o producea centripetal orce. Consequently he driver would not be ableto steerand he car would continue n a straight ine even f the roaddidn't.

Climbing and descendinghills:All objectson Earth havea gravitational orce on them pulling them towards the centre ofthe Earth.This is true whether he object s on level groundor on a slope suchas a hill. Acar on a hill has a gravitational orce acting vertically down. This applieswhether he caris climbing (ascending), escending r parkedon the hill. The gravitational orce mg, canbe divided into components erpendicularo the plane andparallel o the plane.Theparallelcomponentappliesa force of mg sinOdownhill.

The parallel componentappliesa forcemg sinOdownhill.(e) Climbinga hill: f,

lg

/ / t /1/,/ / / / / / / /


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When a car s climbing a hill the driver has o pressharderon the accelerator oovercome he gravitational orce downhill.

(f) Descending hill:

When the car s descending he hill the driver doesnot have o press he accelerator shard sincesomeof the downhill force is suppliedby gravity. On steephills the enginemay have o supplyno force at all and he car will coastdown the hill. In thesecasesthe carhas o be slowed by the brakes.The problem s that constanthardbraking cancause he brakes o overheatand become esseffective. An alternative o constantbraking s to choosea lower gear o descendhe hill (eg "SteepDescent Trucks UseLow Gear" signson the road). Using a lower gearensureshat the wheels urn moreslowly and are subject o less static riction.

(g) Following a curve in the road: A body will travel with uniform velocity unlessactedon by a force (Newton's first law). Thereforea force is required o change hedirection of a car. When following a curve n the road, he necessaryorce is a"centripetal orce". This force always acts owards he centreof the motion i.e.towards he centreof the road's curve.The force is suppliedby the friction betweenthe tyres and he roadandhas a valueof: F, : mu'lr,where Fr: centripetal orce, m:massof car"v : linearvelocitv of car.r : radiusof curve.

E '' C"*lrnof cur,reThe centripetal force is alwaystowards the centre.

f,


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Question3:Interpret NewtonosSecondLaw of Motion and relate it to the equationXF = maNewton's second aw states hat the net force on an object s equal o the productof its massand ts acceleration nd hat the accelerations in the direction of the force.Thus, f a force of a given size s applied o severaldifferent masseswe will find that thelarger he mass he smaller he acceleration f the object.The term "net force" refers o the sumof all the forces acting on the object. Hence heX (capital sigma) n front of the F in the formula. E stands or "the summationof'.Examples: i) At the school athleticscarnival, athletesprefer to use a light shotput ratherthan a heavy one because hey areable to give the light shotput a larger acceleration.Thismeans hat it will attain a largervelocity as t leaves he hand and so travel further before ithits the ground. (ii) In cars, f the sameengine s installed in a heavy and a light car, the lightcar will attain the higher acceleration or the sameapplied force from the engine.

Question4:Identify the net force in a wide variety of situations involving modes of transport andexplain the consequencesf the application of that net force in terms of NewtontsSecondLaw of MotionNote thewords "identifytt and"explain"Consider he forcesacting n planes, rainsandcars.Planes: The physicsof flight is extremely complex. Let us considera plane ust at take-off.Therearetwo componentsof the net force that we haveto consider. First there are thehorizontal componentsof the motion that provide the planes orward motion and secondlythere arethe vertical forces acting that cause he planeto lift from the ground.The vertical motion of the plane s causedby dynamic ift (asopposed o static ift suchas aballoonwhere he object rises n a more dense luid). The dynamic ift of an aircraft s causedby two factors, both of which rely on the planemoving forward through the air.

The first is causedby the wing sloping upward towardsthe direction of forward motion sothat the air is deflecteddown. The seconds due to the shapeof the wing. The wing is curved


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so that air travelling over the top of the wing travels a greaterdistance han air travellingunderneathhe bottom of the wing. This results n the air travelling over the top occupyingagreatervolume in the same ime and so being ess dense.While the difference n densitywould createvery little lift for a stationaryobject, here s substantialift for a moving object.This is due o the Bernoulli principle that is beyond he scopeof the currentHSC physicscourse. It used o be done n a Fluid Dynamics Half Elective someyearsago.The forward thrustof a et plane s causedby the reaction orce to the exhaustgases(Newton's third law). A bigger engineor more enginesgivesa greater orward force. There salso he force of air resistance pposing he motion of the airqaft. The net force is forwardand he accelerations givenby a: Fn.1m.Note that as aplanetakes off, the net force acting on the planeat anypoint in time is the sumof the vertical &horizontal forcesacting on the plane atthatpoint in time. This wouldrequire a vector diagram o determine he net force on the planeat anypoint in time.Trains: There are severaldifferent types(steam,diesel,electric) but all rely on the principleof a force causing he wheels to turn and the static friction between he wheels and the railproviding the forward (reaction) orce to cause he train to move. There s friction betweenthe moving partsof the train as well as air resistance ut the overall result s a net forceforward which results n the forward accelerationof the train. The train will attain a constantvelocity when the forward force s equal o the sum of the backward orces.Cars: Theseexperiencemuch the same orces as rains except hat there s friction betweenthe tyres and he road rather han between he wheels andrail. Tyres have atreadpattern,sothe friction between he tyres and he road s usually greater han the correspondingrictionbetween rain wheels and he rails. There s friction between he moving partsof the car aswell as air resistancebut the overall result is a net force forward which results n the forwardacceleration f the car. The car will attaina constantvelocitv when the forward force is equalto the sum of the backward orces.

A Note on Friction for the TeacherTwo types of friction can occur between a car yre and the ground,a) static friction, and b)kinetic friction.Static friction is when the tyre maintainsgrip or traction on the road surface,whereaskineticfriction (as he name mplies) is when the tyre is moving relative o the ground.To illustratestatic riction, consider or a moment a dot on the tyre's surface.Your vehicle s movingforward (let'ssay t's moving very slowly), andyour tyre rotatesso that the dot comes ntocontactwith the groundat a certainpoint. Sincea car yre compresses bit on the road'ssurface, here'sapproximately Ocm of tyre flat againsthe road at anygiven time. As yourcar moves orward, the tyre rotates,andonce he dot touches he groundat a given point, thetyre and the groundmove at the same ate relative to the car. That is, the dot on the tyre andthe point on the groundremain n contactuntil the tyre reaches he end of that 10cm strip ofcontact,when it is pulled upward from the ground to rotate around top and backto theground.


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An exampleof kinetic friction from the illustration abovewould be that the dot on the tyrereachesa point on the ground,but the dot andpoint move away from one another. n real lifethis would be if you hit the brakes and skid, or if you hit the acceleratorand burn out or spinyour tires (e.g. n the snow or mud). The problem with kinetic friction is that it is weakerthanstatic friction. Thus, when you hit the brakes, f your tyres lock up (you will hear thesquealing yres against he road) you are now in kinetic friction andyour car will slow downlessquickly compared o when your tyres were in static friction with the ground. That is whyyou pump your brakes...alsowhy anti-lock braking systems ABS) were developed.Read more: http://wiki.answers.com/Q/Flow_does_friction act on_a_car#ixzzlyl iQAOKu

Question5:The mass of an object is a measureof the amount of matter contained n the object. Mass is ascalarquantity. The weight of an object is the force due to gravity acting on the object.Weightis avectorquantity. W-mg, whereW-weightforce, m:masS, g: accelerationdue to gravity.

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Solutions to Forces Worksheet 1 Physics