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Year 11 GCSE Physics Unit 1 W o r k , Power and E f f i c i e n c y 1.2.4 recall, understand and use t h e equation useful output energy Efficieny total input energy 1.2.5 describe and explain various ways o f making b e t t e r use o f energy; 1.2.6 review primary and secondary sources relating t o t h e e f f i c i e n c y o f domestic appliances (w - (ii)b); and 1.2.7 recall and use t h e equation work = force x distance and t h a t t h e work done equals t h e amount o f energy t r a n s f e r r e d . 1.2.8 recall and use t h e equations energy transferred work done and Power = time taken Power = time taken t o calculate power, work done, t i m e taken or energy t r a n s f e r r e d ; 1.2.9 plan and carry out experiments t o measure personal power and t h e output power o f an electric motor, and evaluate t h e validity and reliability o f t h e i r data (w - (i)a): and Work When energy is changed f r o m one f o r m t o another, work is done. Work Done = Energy W o r k depends on t w o things: Transfered rli^nn^ ^woJgri pare -r-o dh Work = Force x Dis tan ce Complete Where: W o r k is in Joules ( J ) t h e memory triangle Force is in Newton's ( N ) Distance is in meters (m) Example: I f John l i f t s up a box o f apples weighing 120N a distance o f 1.25m, how much work does he do? 91 = 110x1.25) Year 11 pages 107-9 GCSE Physics Unit 1 Work and energy Extension .Sheet When a force moves an object, energy is transferred and work is done. Work done = energy transferred Work done = (in joules) force (in N) x It is measured in joules. distance moved (in metres) Example A man lifts a parcel, mass 4 kg, from the floor to a shelf 2 m high. a) What is the weight of the parcel? b) How much work is done on it? c) Where does this energy come from? Answer a) Weight = 4 kg x 10 = 40 N (see page 75) b) Work done = force x distance moved = 40N x 2m = 80 joules (from his food) Questions For each question show all your working clearly. 1. How much work is done in these situations: a) A man pushes a van against a friction force of 300 N for 10 m. b) A mother pushes a pram with a force of 30 N for a distance of 100 m. 500 N ^ 2m 3cxz=> S c) A weight-lifter lifts a weight of 500 N through a height of 2 m. I O O O J 2. A worker pushes a barrow at a steady speed of 2 m/s for 10 s, using a force of 100 N. a) How far did he travel? b) How much work is done? 2 o o o T c) Where does the energy come from? chsrr->icaS c-vfcjg-j mSLcka. i_>-x^"t_c_/ 3. A boy with a mass of 60 kg climbs 10 m vertically up a ladder. a) What is his weight? b) How much work is done? t y c o o n c) What are the energy changes here? ICeS -» Ci^E 4. An archer pulls back the arrow in his bow a distance of 0.5 m against an average force of 200 N. a) How much work is done? l o c t l b) What are the energy changes here? Kj= S^el A car is travelling along the road with 40 000 J of kinetic energy. The brakes are applied and it comes to rest in 20 m. a) Calculate the average braking force. b) What happens to the kinetic energy? ^ _ cr^ 92 20 m GCSE Physics Year 11 Unit 1 Power Power is I f you move a f o r c e o f I N (which is t h e same as a mass o f 1kg on E a r t h ) a distance o f l m you will have done 1J o f work. I f you do t h a t work in Is, you will have done i t with a power o f 1W. Power = Complete work done t h e memory time taken triangle W h e r e : Power is in W a t t s ( W ) W o r k is in Joules ( J ) Time is in seconds (s) Remember: Work Done = Energy Transferee! Examples: 1. A crane does 5 0 0 0 J o f work in 2 seconds. W h a t is t h e output power? p-. &**>A 2. Find t h e power o f t h e man who pushes t h e box 8m w i t h a f o r c e o f 15N in a 6seconds. . p. p . ^ iSxS. 93 GCSE Physics Year 11 WORK AND Unit 1 POWER 1. A cyclist moves along a flat road against a resistive force of 100K. If the cyclist travels 1000m calculate the work done by the cyclist. ICO. QQqTJ. Z. Ponna lifts a parcel of weight 100N onto a shelf that is 2m above the ground. a) Calculate the work done in lifting the parcel onto the shelf. b) What type of energy does the parcel gain? . QP£........ 3. fit ear is driven up a mountain pass. Ft gains a vertical height of 300w. The weight of the car and its passengers is 10,000N. a) Calculate the work done by the car against gravity. 3 CCCs. OCG.ZF.. b) What is the gain in potential energy of the car? 3 o c o .coo. .d Matt cycles a distance of 2000m against a resistance force of 150N. He travels this distance in 400s. a) Calculate the work done by Matt. (5r% 3co..cxx>.J. .. b) What is Matt's power output? . ."^tSauJ 5. The diagram shows a pumped storage system used to store water in a dam. a) Calculate the work done in pumping 10,000N of water from the lower to the upper reservoir. o . c o..cm."J. 100m b) If it takes 10s to move 10000N of water from the top to the bottom calculate the power output of the pump. lOO. ...CCO. LO. 6. The output power of a crane Is 1.6kW. Calculate how long tt will take to lift a load of 5000N through a distance of 8m. b. s • f...2S: 94 GCSE Year 11 page 118 Physics Unitl Power Extension Sheet work done (in joules) Power = time taken (in seconds) (in watts) or Power = energy transferred (in J) time taken (in s) 1 watt = 1 joule per second Example A force of 100 N moves a distance of 5 m in 2 seconds. a) What is the work done? b) What is the power? Answer a) Work done f force x distance moved (see p. 107) - 100N x 5m = 500 J ,„ work done 500 J b) Power = = = 250 W time taken 2s Questions For each question show all your working clearly. 1. A boy does 500 J of work in 10 seconds, ^q^^ What is his power output? 2. A mother pushes a pram with a force of 30 N for a distance of 100 m in 50 s. toco What is her power output? An electric lamp is marked 60 W. How much energy does it transfer a) in 1 second? b) in 100 seconds? i&oooZ What are the energy transfers here? eveeWicc^ -=> W«j*v 4. An athlete runs a 100 m race in 10 s against a friction force (drag) of 100 N. What is his power output? 5. A weightlifter lifts an object of mass 30 kg through a height of 2 m in 3 seconds. a) What is the weight of the object? 3 c o « o (Hint: see p. 75.) b) What is the work done on the object? t o o t l c) What is his power output? '2oo<—> 6. A boy weighing 600 N runs up the stairs, a) in 3 seconds, and then Soo«-~> b) in 4 seconds. boo L-O The vertical height of the stairs is 4 m. What is his power output in each case? 7. A lift containing 6 people is raised through a height of 20 m in 10 s. The total weight of the lift and passengers is 6000 N. What is the power of the lift motor, a) in watts? '^ooci—> b) in kilowatts? 12jc.ua 95 60 w GCSE Physics Year 11 Unit 1 Personal Power Plan and carry out an experiment t o measure t h e amount o f power you can develop when running up a f l i g h t o f stairs. Method: Results: H e ^ * c u r b e d (uS) W h a t a r e t h e sources o f e r r o r in t h i s experiment and how can you make t h e measurement more reliable? r\n> 96 Year 11 GCSE Physics Unit 1 Power o f an Electric M o t o r Plan an experiment t o determine t h e output power o f an electric motor by plotting a graph o f work done against t i m e taken. Method: Results: we. Graph: 97 Year 11 GCSE Physics Unit 1 Efficiency A machine changes energy f r o m one type t o another, but not all t h e energy produced is useful - usually some is lost as heat. When building a machine engineers want i t t o produce as much useful energy as possible - t h e y want i t t o be e f f i c i e n t . The e f f i c i e n c y can be calculated f r o m e i t h e r o f these equations: Efficieny = useful output energy — • total input energy gr useful output Efficieny power = total input power Why will t h e efficiency o f a machine always be less than 100%? W h a t does an e f f i c i e n c y o f 0.8 mean? ill L - Example: Find t h e e f f i c i e n c y o f t h e ramp shown. -• 1 X 0 0 3 cx-><- nod Weight = SOON Physics f o r CCEA Questions 17 - 24, Page 51 Physics f o r CCEA Questions 3c, 4 - 6c + 8, Pages 5 6 Physics f o r You Questions 18b, 19b, 20b, 23 - 26c 2 7 - 2 9 , Pages 150 + 151 98 Year 11 GCSE Physics pages 112, 122 Energy transfer and efficiency An Energy Transfer Diagram (Sankey diagram) shows what happens during an energy transfer: Efficiency = useful energy transferred total energy input Unit 1 for a torch chemical energy stored in * e battery X 100% Questions For each question show all your working clearly. K light energy r (useful energy) energy heating u| torch + room (wasted energy) 10O3 1. A lot of energy is wasted in a car. For every 100 J of chemical energy in the petrol, only 25 J are transferred to useful kinetic energy. The rest just heats up the engine and the air. a) Draw an Energy Transfer Diagram for this, to scale. b) Calculate die efficiency. 167. energy heating up the room 600 J 2. The diagram shows the energy transfers for a Bunsen burner heating a beaker of water. What is its efficiency as a water heater? L\(fl i Useful energy > heating water Y 400 J 0 In a solar cell, for every 80 J of solar energy shining on it, only 4 J is transferred to useful energy (as electricity). b Wr a) What happens to the oflier 76 J? U3qab}jJ QS koctVb) What is its efficiency? S°/ c) Draw a Sankey diagram of this, to scale. p £ > ^ T eWcW>cc\ 3 0 4. A pulley system lifts a load and gives it 6000 J of potential energy. The person pulling on the rope gives it 8000 J of energy. "^-tSTo What is the efficiency? 5. An electric kettle has a power rating of 2 kW and is switched on for 100 seconds. While heating up, it loses 60 000 J to the surroundings. a) How much energy is supplied to the kettle? cccrX (1 kW = 1000 W = 1000 joules per second) b) How much is given to the water? ' ^ o 0 0 0 3 c) What is the efficiency of heating water? 6. An electric motor on a building site has a power rating of 400 W and lifts a load of bricks weighing 600 N through a height of 10 m in 20 seconds. a) How much energy is needed to lift the bricks? (See page 107 or page 116.) ° b) How much energy is supplied to the motor in 20 seconds? S^cotJ c) What is the efficiency of the motor in doing this job? "}SY too 3 0 99 motor Year 11 GCSE Physics Unit 1 Making Good Use o f Energy List as many ways as you can t h a t we unnecessarily use energy in our homes and some ways in which we could r e c t i f y t h i s waste: e.g. leaving TV on standby - t u r n o f f completely uoa^M>^vg rK-ievcK..—JL. fori <S^s_v-e cy«a.op u j d L £r«Le.-2_«_v.3 rv./e loo^ci.s p._>U i^sVoA T h e E f f i c i e n c y o f Domestic Appliances All electrical appliance in t h e UK must display information regarding i t s e f f i c i e n c y . There are various ways o f displaying t h i s information: Task: Outline how each o f these systems work and list t h e info t h e y contain. EU Label - Energy €3 A rv>osf -e^p / £ - lensr e f f 3' info ^o.sc le_oeJ p= XYZ ^3 !abe- v po-^c* c v ~ e-Ln-a>V\.Ocy.\ .spaces 0 SEDBUK &<^&>cy^ SEDBUK Rating 90.0% 91.3% 86.0% 90.0% 82.0% 96.0% 78.0% 82.0% 74.0% 7BJO% 70.0% Lc^oeA ^:or b>e*_Lejs 710% 70% I O O ^ V J J J S . or-« 100 . oVc- ^) Oo-'gC- ^podb) GCSE Physics Year 11 Unit 1 Kinetic and Gravitational Potential Energy 1.2.10 recall and use t h e equations kinetic energy = j mass x velocity = potential energy 2 imv* = mass * acceleration due to gravity x height = mgh Kinetic Energy KE is t h e energy t h a t a body possesses because i t is moving. KE = i m v 2 Complete t h e memory W h e r e KE = k l , ^ g h ( r triangle ^ Rearranging KE= £ mv To g e t m: - 1 l £ X/ To g e t v: 1 W o r k done and Kinetic energy A car is moves along a road a t 60mph what has t o be done t o bring i t t o a stop? Explain t h e connection between t h e KE o f t h e moving car and t h e work done t o Stop i t . 1 leg o£>- = (>-3<^ V=x^ lp^o\Co_S, -V UgiQr- losk >K-> W - Q V I ^ S Example: A bullet o f mass 10g travels a t a speed o f 200m/s. Calculate i t s kinetic energy. = 2. v O.OV Sj xloo V ZooT 101 Year 11 GCSE Physics KINETIC L ENERGY a) What is kinetic energy? Unit 1 # Energy , g r ^ ^ . . . \ c ^ o 3 j j ^ . . . . v ^ . . . Q f € ooo^^c^ b) A truck of mass ZOOOkg and a car of mass 1000kg are travelling down a motorway at the same speed. (i) Which one has the greatest kinetic energy? (ii) Explain your answer. 1 p/Qjgdr:.. rv^aso c) Two cars of the same mass are travelling down a road. Explain how one car could have more kinetic energy than the other. one. brc9s^k\n.c\.... £Q,s*c-ey ... fa\o.o.... .-^WrA,... .c^Us-a- 1. For the following pairs of objects state which has the most kinetic energy, al A car of mass 1000kg or a lorry of mass SZOOkg, both moving at lOm/s. b) A car of mass 1000kg moving at lOm/s or a car of mass 1000kg moving at ZOm/s. 3. A car of mass 1000kg moves along a road at a constant speed of ZOm/s. Calculate it's kinetic energy. 2QO.. .000.3. 4. A -truck of mass 3Z,000kg moves along a road with a speed of lOm/s. Calculate the kinetic energy of the truek lbcx> ODD. T. 5. A skier of mass 90kg is skiing down a hill at a speed of 15m/s. What is the kinetic energy of the skier? 6. The kinetic energy of a cyclist moving along a road is 5000J. If the mass of the cyclist is 100kg calculate the speed of the cyclist. 7. A motorcyclist and motorcycle have a combined mass of 900kg. If they have 140,000J of kinetic energy calculate their speed. • 102 GCSE Physics Year 11 Unit 1 Gravitational Potential Energy GPE is t h e energy t h a t an o b j e c t has gained because o f a change in i t s vertical position. GPE = m g h Complete t h e memory \ h h ^ a \ gnO^(T) ' W h e r e GPE = Cm^\nY\nrA m= h - r/Yl^S t r i Q n 9 e (V-r^ IA^IOWV ( rv\) Potential Energy and W o r k bone When you l i f t your school bag onto t h e table i t gains PE. W h e r e did t h i s energy come f r o m ? JQj ' rJ^pnrMml ( rrv^ri) Explain t h e connection between t h e work you did l i f t i n g your bag and t h e energy i t gained. , v i -to hpl- » f.P£ ggnrxpd Conservation o f Energy A ball is dropped f r o m a height, h. ? PE = mgh KE = 0 PE a t t h e Top = KE a t t h e b o t t o m A t t h e t o t a l amount o f energy must remain t h e same. PE = KE o When t h e ball h i t s t h e ground t h e KE will be c o n v e r t e d into h e a t and sound energy. PE = 0 KE = £mv 2 Physics f o r CCEA Questions 25 - 3 3 , Pages 53 + 54 Physics f o r CCEA Questions 3b, 6 d • 7, Pages 5 5 - 5 7 103 GCSE Physics Year 11 page 117 o r Potential energy kinetic energy Change in gravitational PE = weight x change in gravitational PE — = „„„„ mass (in kg) (in J) Kinetic energy (inJ) Unit 1 = v change in height since weight = mass x g (see p. 75) x „g ~x change . •» in height (N) 6 (N/kg) (in m) VJ x mass x speed squared (kg) (kg) (N/kg) g = 10 here on Earth (m/s) J See the examples on page 117. Questions For each question show all your working clearly. 1. A diver, of mass 40 kg, climbs up to a diving platform 1.25 m high, a) What is his weight, in N? te-o b) What is his change in P.E.? SooJ c) Where does this energy come from? <^Ke»v>.c d) He walks off the platform and falls down. f What is his K.E. as he hits the water? S o o t J e) What is his speed as he hits the water? o o d 2. The same diver now climbs to the 5 m platform, four times as high. a) What is his change in P.E. now? ^ o c o j b) What is his speed as he hits the water? t o i l s c) What do you notice about this answer? 5 m 3. Another diver, of mass 80 kg, climbs to the 5 m platform. a) What is her speed as she hits the water? /cvnte b) What do you notice about your answer? $ ,wa as ^ojcg dnje/ (sps=d readied *\ (yee. fed I 4. A stone is dropped from a window 5 m high. Of. m a s s ) At what speed does it hit the ground? iorr»\s a 5. A tennis player hits a ball vertically with a speed of 10 m/s. How high does it go? 10 m/s 6. A car of mass 600 kg is travelling at 10 m/s. When the brakes are applied, it comes to rest in 10 m. What is the average force exerted by the brakes? 3CCCM 7. A car of mass 800 kg is at rest The engine exerts a resultant force of 2000 N for a distance of 5 m. t o d - fd a) What is then its K.E.? ooooT . S b) What is then its speed? . iooco3 104 fo^eookg^ 10 m 3c»oco = f * ' 0 Year 11 GCSE Physics Unit 1 Kinetic Energy and Potential Energy 1. A tourist's Fiat is driving along a mountain road. The combined mass of the car and luggage is 2920kg. The car is powering uphill at 23m/s. a) How much kinetic energy does the car have? '^'wlSuO'J At the top of the road, the car has gained a total height of 1200m. b) Calculate the potential energy the car has gained. 3 5 ^ 0 0 0 3 ~ 3 As the car rounds a bend at the top of the mountain, a suitcase falls from the roof into the valley below. The suitcase has a mass of 20kg. c) Work out the potential energy the suitcase lost when it had fallen a distance of 60m. 12 OCOj d) If all of this potential energy of the suitcase is converted into kinetic energy, how fast will it be travelling when it has fallen 60m? 3U.lo rV>ls e) Explain why it will not actually be travelling as fast as this. • f ^ ' , c r l £ r ( Some workmen are using a rope to lower a bucket full of bricks from a window. They tie off the rope when the bucket is just above the ground. As they are making their way downstairs to unload the bucket, a strong wind sets the bucket swinging. Draw a diagram of the path of the swinging bucket. On your diagram: mark with the letter A — where the potential energy is greatest. mark with 'he letter B — where the kinetic energy is greatest. mark with the letter C — where the bucket is travelling fastest. mark with the letter D — where the bucket's velocity is zero. A bouncy ball has a mass of 0.3kg. It is dropped from a height of 3.0m. a) How much potential energy has the ball lost when it hits the ground? ^ 3 Ignoring air resistance, how fast will the ball be travelling?.-) The ball rebounds vertically at a speed of 7.0m/s. What kinetic energy does it now have? " ^ . j S J What height will it reach on the rebound? , 2.^5^ Explain what has happened to the energy that the ball has lost. (Yichcry Three students carry out an experiment to compare their own personal power. They measure their mass, then time how long it takes them to run up a flight of stairs 12m high. Their results are shown in the table below. Copy and complete the table. Name Weigh! (N) Time (s) Alex 520 Billie 450 16 Jack 600 15 Potential Energy Power (W) Gained (J) 14 oL'OO 105 Year 11 Unit 1 Assessed Homework Karl takes Katie on a date t o a theme park. They decide t o have a go on t h e roller coaster. The roller coaster car weights 9 0 0 0 N and is brought t o a height of 45m above i t s s t a r t i n g point in 30 seconds. a) W h a t is t h e work done in raising t h e car t o t h i s height (point B)? [4] b) W h a t is t h e power of t h e motor used t o pull t h e car up t h e t r a c k ? [4] c) W h a t is t h e gravitational potential energy gained by t h e car a t point B? [4] d) I f t h e roller coaster is let run down f r o m point B t o a point 15m above i t s initial s t a r t i n g position (point C), what kinetic energy will i t have gained assuming t h a t t h e roller coaster is 100% e f f i c i e n t ? e) W h a t is t h e velocity o f t h e roller coaster car a t point C? [4] [5] f ) I f t h e roller coaster is actually only 8 2 % e f f i c i e n t what velocity will t h e car have a t point C? [4] Total [ 2 5 ] 106 .Assessed - UCS Hfc~o OOOj" b *l^o ceo 3" CD • -4 ) e(T = o-^l = /Jo coc s 22.1 ml3 <£, GCSE Physics Year 11 Unit 1 Momentum By t h e end o f t h i s section you should be able t o : 1.1.18 recall t h a t Momentum = mass x velocity 1.1.19 recall and understand t h a t Change in momentum - force x time and apply t h i s t o t h e solution o f mathematical problems; 1.1.20 apply t h e principles o f momentum, f o r c e s and time t o an analysis o f s a f e t y f e a t u r e s o f modern cars, t o include car air bags, car seat belts, car crumple zones and crash b a r r i e r s ; 1.1.21 investigate, using data loggers or computer simulations, one-dimensional inelastic collisions and, through mathematical modelling, use t h e data obtained t o show t h a t t h e momentum is conserved in such collisions; and 1.1.22 recall and use t h e principle o f conservation o f momentum t o solve simple problems involving one-dimensional inelastic collisions. Momentum Momentum is a useful quantity t o consider when o b j e c t s collide. Think about a Year 8 pupil colliding w i t h a Year 14 pupil. Who comes o f f t h e worst? W h a t f a c t o r s does t h i s depend on? Momentum is product o f t h e mass o f a body and i t s velocity. P=m v Complete t h e memory triangle m - Example: An o b j e c t has a momentum o f 15.90kgm/s and a velocity o f 9.04m/s. W h a t is t h e mass o f t h e o b j e c t ? IIS.^ ' Y~Y^ 107 K<=\ ,OU Year 11 GCSE Physics Unit 1 Impulse Consider a f o r c e F acting on a mass m f o r a time t j so t h a t i t accelerates f r o m initial velocity u t o final velocity v. The f o r c e x t i m e is called t h e impulse. As a = Then v — u F = m Impulse = F t And F = ma (v-u^ v t Force mv - mu I . e . Force change time taken J x time = change in momentum in momentum = = Impulse Questions: 1. While playing basketball in PE class, Logan lost his balance a f t e r making a lay-up and colliding w i t h t h e padded wall behind t h e basket. His 74kg body decelerated f r o m 7.6m/s t o Om/s in 0.16 seconds. a) Determine t h e f o r c e acting upon Logan's body. b) I f Logan had h i t t h e concrete wall moving a t t h e same speed, his momentum would have been reduced t o zero in 0.008 seconds. Determine what t h e f o r c e on his body would have been f o r such an abrupt collision. 2. A boy kicks a stone o f mass 1kg, accelerating i t f r o m r e s t t o lOm/s. The stone is rigid so t h e f o r c e acts f o r only 1/100 o f a second. He t h e n kicks a football o f t h e same mass t o give i t t h e same final speed. The football is s o f t in comparison t o t h e stone so t h e f o r c e acts f o r 1/10 o f a second t h i s t i m e . Which kick h u r t s less? Physics f o r CCEA Question 11, Page 20 108 GCSE Physics Year 11 Unit 1 Conservation o f Momentum Udr»' The law o f conservation o f momentum states t h a t : ^ m o m = n h -tdrol Cr?lll,1\Qp CC Vyzfrr-e. o ^-yfdriSlno There are no exceptions t o t h i s law. When t w o o b j e c t s collide t h e i r changes in momenta will be equal in size but opposite in direction. The momentum gained by one is equal t o t h e momentum lost by t h e other. Tackling Momentum Problems: 1. Choose a positive direction. 2. Draw b e f o r e and a f t e r sketches o f t h e o b j e c t s involved. 3. Calculate every momentum you can 4. Apply t h e law o f conservation o f momentum f o r collisions involving two bodieS. Pbefore = Pafter m i u i + m2U2 = m i V i + m2V2 Questions: 1. A railway wagon o f mass 800kg moves a t a steady speed o f 2.5m/s. I t collides w i t h another wagon o f mass 1000 kg. The two wagons couple t o g e t h e r a f t e r t h e collision. Calculate t h e final speed and t h e loss in KE i f t h e second wagon was stationary initially. 2. Rex (86kg) and Tex (92kg) board t h e bumper cars a t t h e local carnival. Rex is moving a t a full speed o f 2.05m/s when he rear-ends Tex who is a t r e s t in his path. Tex and his 125kg car lunge f o r w a r d a t 1.40m/s. Determine t h e post-collision speed o f Rex and his 125kg car. 109 Year 11 GCSE Physics Unit 1 3. A candy-filled pinata is hung f r o m a t r e e f o r Matthew's birthday. During an unsuccessful a t t e m p t t o break t h e 4.4kg pinata, Hayden cracks i t with a 0.54kg stick moving a t 4.8 m/s. The stick stops and t h e pinata undergoes a gentle swinging motion. Determine t h e swing speed o f t h e pinata immediately a f t e r being cracked by t h e stick i f i t initially was a t r e s t . 4. During an in-class demonstration o f momentum change and impulse, M r . H asks Jerome (102kg) and Michael (98kg) t o s i t on a large 14kg skate c a r t . M r . H asks Suzie (44kg) t o s i t on a second 14kg skate c a r t . The two carts are placed on low f r i c t i o n boards in t h e hallway. Both carts are initially a t r e s t and Jerome pushes o f f o f Suzie's c a r t . Measurements are made t o determine t h a t Suzie's c a r t acquired a post-impulse speed of 9.6m/s. Determine t h e expected recoil speed of Jerome and Michael's c a r t . 5. Jaclyn plays singles f o r South's varsity tennis team. During t h e match against N o r t h , Jaclyn won t h e sudden death tiebreaker point w i t h a crosscourt passing shot. The 57.5g ball h i t her racket with a velocity of 26.7m/s. Upon impact with her 331g racket, t h e ball rebounded in t h e exact opposite direction (and along t h e same general t r a j e c t o r y ) with a speed o f 29.5m/s. a. Determine t h e pre-collision momentum of t h e ball. b. Determine t h e post-collision momentum o f t h e ball. c. Determine t h e momentum change of t h e ball. d. Determine t h e velocity change of t h e racket. 6. To M r . H's disgust, a 450g black crow is raiding t h e recently-filled bird f e e d e r . As M r . H runs out t h e back door with his broom in an e f f o r t t o scare t h e crow away, i t pushes o f f t h e 670gram f e e d e r with a t a k e o f f speed of 1.5m/s. Determine t h e speed a t which t h e f e e d e r initially recoils backwards assuming t h e crow and f e e d e r were stationary initally. 110 Aw o/ V- \.\\ m\2> ] s v >^_x l o o o x ( o Y - /x. f5?Qo + t o c ^ x (\,\\ ) <? 3 Li) KV"»A 1. -1- l Y ) - i J - - r W V / . -»- rv->, \ A k^) P =- r r w / - o.oST-5 x 7Sa^ * icy)rwi J . -t"m.iJL - r V i . o , -*-rv-v>y. O-uSxO + o.b>xO * o.L(5x\.5 - r O . ^ x V x GCSE Physics Year 11 Unit 1 Elastic and I n e l a s t i c Collisions I n some collisions kinetic energy as well as momentum is conserved. The t o t a l kinetic energy o f t h e bodies b e f o r e t h e collision is equal t o t h e t o t a l kinetic energy o f t h e bodies a f t e r t h e collision. No energy will be lost in t h e collision as heat or sound o r in t h e permanent deformation o f t h e colliding bodies. T 2 TuUi + i 1TI2U2 2 2 = £ mivi + | m2V22 I f t h e collision is elastic t h e r e is no loss in kinetic energy. Collisions in which t h e kinetic energy is not t h e same b e f o r e and a f t e r are called inelastic collisions. Although kinetic energy may or may not be conserved in a collision, momentum is always conserved and so is t o t a l energy. Total momentum b e f o r e is equal t o t o t a l momentum a f t e r , providing no external f o r c e is applied. Types o f collision; • Elastic collision No loss o f kinetic energy on impact • Inelastic collision Some kinetic energy lost on impact • Completely O b j e c t s stick t o g e t h e r on impact inelastic collision NB: Remember t h a t momentum is always a vector quantity and so magnitude and direction are important. Hence i f t w o o b j e c t s are travelling towards each o t h e r t h e t o t a l momentum is found by subtracting t h e individual momentum o f t h e objects. Physics f o r You Questions 1 - 7 , Page 147 111 a -3. « c i . l D f (Ck x \via o ' ' i. - 4. I f - /*•> V, - (ci.~40^n\S> ua • r XO.OV XHOC. 1 - I y z(o.ono.3flw (£ c l) u o e d i c e . o^xrJ a o p . .o S^lcr r o e c 3 <dP> (jorl = P e l " >'5o ; • -ro H e — ~ P x o-o-» * ' H.iu<vXUS - o Year 11 GCSE Physics Car S a f e t y Make a poster presentation detailing how one o f t h e following car s a f e t y f e a t u r e s keeps passengers safer during a collision. Crumple zones Seat be Its I I I I « Revision Questions Phys cs f o r CCEA Questions 1 - 4 , Pages 11-12 Phys cs f o r CCEA Questions 12 - 1 6 , Page 22 Phys cs f o r You Questions 12 - 2 2 , Page 121 Phys cs f o r You Questions 1 - 6, Page 128 Phys cs f o r You Questions 8 - 1 7 , Page 141 Phys cs f o r You Question 3 9 , Page 153 113 Unit 1 Car S a f e t y All of t h e s a f e t y f e a t u r e s have one aim in mind, t o increase t h e time t o stop and t o increase • t h e distance over which t h e passengers stop. By increasing t h e time t o stop, you are decreasing t h e r a t e o f change o f velocity, i.e. decreasing t h e acceleration, so decreasing t h e f o r c e experienced. mv - mu F = m V • By increasing t h e distance over which t h e f o r c e is acting, t h e average f o r c e is less. Other information Crumple Zones Crumple zones are part of a car designed t o collapse during a collision - usually t h e f r o n t end. The f r o n t o f t h e car crumples and stops, but t h e passengers continue t o move a crucial half m e t r e or so. S e a t belts I f you were not wearing a seat belt and t h e car came t o a sudden stop, you would continue t o move due your inertia. Your body would most likely be stopped as a result of t h e f o r c e o f t h e windscreen or other rigid part of t h e car. Airbags Airbags need t o be used in conjunction with seat belts. They p e r f o r m t h e same function as a seat belt and should be fully inflated b e f o r e you h i t them. I f you were not wearing a seat belt then t h e r e would not be enough time f o r t h e airbag t o i n f l a t e b e f o r e you h i t i t . Airbags are designed t o inflate in 0.05s, and t o d e f l a t e within 0.3s. The quarter o f a second between these two times is s u f f i c i e n t t o slow you down. A f l e x i b l e nylon bag is f o l d e d into t h e steering wheel or dashboard. When t h e f r o n t end o f t h e spring is suddenly stopped, t h e mass continues f o r w a r d s t o make contact w i t h t h e switch, s t a r t i n g a chemical reaction. This occurs when t h e acceleration is around -lOg. An inflation system in which a spark ignites a violent chemical reaction between sodium n i t r i d e (NalSb) and potassium n i t r a t e (KNO3) producing nitrogen gas t o inflate t h e airbag.