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Written by M J Bradley P3:Higher Physics of Transport 400 300 Thinking distance is the distance you go while reacting. It is affected by alcohol, drugs and speed Braking distance is the distance travelled once the foot is on the brake and is affected by how hard the brakes are pressed (the harder the pedal is pressed the more the friction between the brake pad and wheel), speed, weather, road and tyre conditions. Anything which reduces friction will increase braking distance. Distance-Time graph for two cars 500 distance (m) stopping = thinking + braking distance distance distance Stopping Distance Speed(or velocity) = distance (m) Example: What is the distance travelled (m/s) time (s) in 30 seconds by a car travelling with a velocity of 30m/s? d answer: d = s x t = 30 x 30 t s = 900m Typical Questions 1) What is the distance travelled by car A? 500m stationary constant car A speed 2) What is the speed of car A? s =d/t = 500/20 = 25m/s 200 stationary 100 3) What is the distance travelled by car B? 200m constant speed car B 10 20 4) What is the speed of car B? s = d/t = 200/25 = 8m/s 30 time in seconds Acceleration This is the change in speed per second. The units of acceleration are m/s/s acceleration = change in speed (m/s) time (s) Example: What is the acceleration of a rocket if it accelerates from 20m/s to 100m/s in 2 seconds? Answer a = change in speed time = 100-20 = 80/2 = 40m/s/s 2 chs a t Calculating distance from a Speed-time graph Speed - Time graph for a car 30 speed (m/s) constant speed constant acceleration This is done by calculating the area under the graph. The distance travelled by the car while it is accelerating is the area of a triangle. area of triangle = 1/2 b x h =1/2 x 10 x 30 = 150m Typical Questions 20 1) What is the maximum speed of the car? 30m/s 2) What is the car's acceleration? acceleration = change in speed time =30/10 = 3m/s/s 10 5 10 The Link between Force, Mass and Acceleration acceleration = force(N) (m/s2) mass(Kg) F a m a=F m A big force on a small mass gives a very big acceleration Example: What force is needed to make a 100000 Kg boat accelerate at 0.5m/s2? answer F = ma = 100000 x 0.5 = 50000N Terminal Speed time in seconds Objects fall to the ground because of the pull of gravity. When an object falls on Earth it first gets faster (accelerates) then it reaches a steady speed just like when a cyclist reaches top speed on a bike. This maximum speed is called the terminal speed and it happens when the forces on an object become balanced The forces become balanced because the air drag increases. Strength of Earth's Gravity a = f/m so g = W/m So weight, W = mg g is the strength of Earth's gravity. It is about 10N/kg. On the moon it is 1.6N/kg Example How heavy is a 100kg man on Earth? answer W = mg = 100 x 10 = 1000N terminal velocity (or speed) velocity m/s air drag weight The weight and air drag forces are balanced at the terminal speed time (s) Work Work is done when a force moves an object through a distance. Work is a transfer of energy. E.g. lifting a book chemical potential energy in muscles is changed to gravitational potential energy Work Done = force x distance (J) (N) (m) Example The work done by a car pulling a trailer with a force of 200N through a distance of 1000m is WD = F x d = 200 x 1000 = 200000J Power = work done (J) (W) time (s) WD F Power Power is how quickly work is done. If a machine does a lot of work in a short space of time it has a high power. It is measured in joules per second which scientists call watts (W) d Example: The power of a machine that does 2000J of work in 2s is P = WD = 2000 = 500W t 2 WD P t Gravitational Potential Energy Potential Energy = mass x g x height This is the energy something has due to it height (J) (kg) (m) above the ground g is the acceleration due to gravity = 10 m/s2 PE Example What is the potential energy of a steel girder of mass 1000kg when a crane lifts it to a height of 100m.? PE = m x g x h = 1000 x 10 x 100 = 1000000J mxgxh kinetic energy = 1/2 x mass x velocity2 (J) (kg) (m/s) Anything which moves has kinetic energy. If Kinetic Energy an object has a big mass and a high speed it has a very big kinetic energy Examples: (1) What is the Kinetic energy of a 100 000kg train moving with a velocity of 30m/s? answer: KE = 1/2 x m x v2 = 1/2 x 100 000 x 302 = 1/2 x 100 000 x 900 = 45000000J Transfer of Potential and Kinetic Energy B A C Imagine you are towed up to the top of a high slope and the cable is released from the carriage Crashes In any collision kinetic energy needs to be absorbed and The law of energy conservation is that energy is transferred to other forms of energy. Car brakes transfer never destroyed but can be transferred kinetic energy to heat. A - The ball has kinetic and potential energy Crumple zones, car bumpers, air bags absorb energy B- The ball has maximum potential and minislowly and reduce the impact force mum kinetic C - On the point of impact the ball has maximum Seat belts keep the occupants in their seat. In a crash kinetic energy and zero potential energy they are best replaced because their is a danger of them being permanently stretched and weakened - dangerous in a further crash All gravitational potential energy, no kinetic The kinetic energy is transferred back to potential energy WEDNESFIELD HIGH SCHOOL EXTREME RIDES If the brakes are never applied will this ride last forever? What will happen? kinetic energy is maximum and the potential energy is zero UNIT P3 FORCES AND MOTION FOLD 1) distance/time 2) d = sxt = 20 x 20 = 400m 3) acceleration = speed change/time 4) a = chs/t = 20/4 = 5 m/s2 5) unbalanced (resultant) 6) bigger bigger small big 7) a = f/m 8) a = f/m = 8000/1000 = 8m/s2? 9) f = ma = 1000 x 9 = 9000N 10) balanced speed 1) speed = ? 2) What is the distance travelled by a car in 20s if travelling at 20m/s? 3) acceleration = ? 4) What is the acceleration of a car if it accelerates from 10m/s to 30m/s in 4s? 5) To make something accelerate there must be an ______ force acting upon it 6) The bigger the resultant force acting on an object the _______ the acceleration. The smaller the mass the ______ the acceleration. For a racing car the best combination for high acceleration is ______ mass and _____ thrust from the engine. 7) What equation links force, mass and acceleration? 8) What is the acceleration of a car if the force from the engine is 8000N and the mass is 1000 Kg? 9) What is the force required to make a 1000 kg car accelerate at 9m/s2? 10) When a falling object reaches its terminal speed the forces on it are ________ 11) Sketch a graph of speed against time for a falling object starting from rest (zero speed). time speed (m/s) 12) seat belts, air bags, crumple zones 13) increase the time for the collision, 12) Three safety features of cars are....... 13) An air bag reduces the force on a driver during a crash because reducing the deceleration and reducing the force because f = ma ................[3] 14) thinking distance + braking distance 14) Stopping distance = ________ + _________ 15) distance travelled with foot on brake 15) What is meant by braking distance? 16) wet weather, poor brake condition, 16) What things increase braking distance? 17) distance travelled while reacting 17) What is meant by thinking distance? 18) alcohol, drugs, tiredness 18) What things increase thinking distance? 19) What is the thinking distance of a driver if the reaction time is 0.5s and 19) d = sxt = 40 x 0.5 = 20m 20) sd = td + bd = 15 + 75 = 90m the speed is 40m/s? 20) What is the stopping distance of a car if the thinking distance is 15m and the braking distance is 75m? 21) Use the graph to describe the motion of a bus in terms of speed and 21) accelerates then steady speed then acceleration rapid deceleration 20 4 16 18 time (s) distance (m) 22) a = chs/t = 20/4 = 5 m/s2 22) What is the acceleration of the bus? 22b) area under graph =0.5bh = 22b) What is the distance travelled while accelerating 0.5 x 4 x 20 = 40m 23) For how long was it at steady speed? 23) 12s 24) What was the deceleration? 24) a = chs/t = 20/2 = 10m/s2 25) Use the graph below to describe the motion of a cyclist in terms of 25) steady speed for 5s then stationary speed 30 5 time (s) 26) What was the cyclists' speed in the first 5 seconds? 26) s = d/t = 30/5 = 6m/s 27) Work is a measure of _______ transfer 27) energy 28) Work = _______ x _______ or wd = _____ 28) force x distance , wd= fd 29) How much work is done by a 1000N man climbing stairs 10m high? 29) wd = fd = 1000x10 = 10000J 30) Power is the rate at which ________ is done 30) work 31) If a machine does a lot of work in a small amount of time it has a ________ power. If it does little work in a long time (like some of you) it has a ______ power. One joule per second is one _______ 31) high , low, watt 32) Power = work done /time, p = wd/t 32) Power = ______ or p = 33) What is the power of a machine if it does 10000J of work in 10s? 34) Gravitational potential energy is the__________ energy of an object which is ________ up and is equal to the amount of ______ done to get it there 35) Potential energy = mass x _____ x ______ or PE = ________ 36) What is the potential energy of a 10Kg mass 20m above the ground? 33) p = wd/t = 10000/10 = 1000w 34) stored, high, work 35) = mass x gravity strength x height PE = mgh 36) PE=mgh = 10x10x20 = 2000J ( take g = 10m/s2) 37) Moving objects have ________ energy 37) Kinetic 38) Kinetic energy = 0.5 x mass x _________ or KE = ________ 38) 0.5 x mass x velocity2 39) What is the kinetic energy of a 1000Kg car travelling at 20m/s? 39) KE = 0.5mv2 = 0.5x1000x 202 = 0.5x1000x400 = 200000J 40) Energy cannot be _________ but can be __________ to other forms 40 ) created or destroyed, transferred 41) A playground slide converts _________ energy to ________ energy 41) potential, kinetic 42) The brakes on a car do work to stop it. _______ energy is transferred to ________ energy. 42) kinetic, heat 43) A ball has 50J of potential energy. If it is dropped how much kinetic energy does it have the moment before it hits the ground? 44) Work done to stop = ke of car 43) 50 J 44) If a car has 200000J of kinetic energy what distance will it take to stop if the braking force is 4000N? fd = KE so d = KE/f = 200000/4000 45) At the top of the ride below Mr Bradley's _______ energy is at a ______ and his kinetic energy is _______. As he falls potential energy is transferred to _______. His kinetic energy is at a maximum at the ______ of the ride. This kinetic energy is then transferred back t o _____ _ energy. 45) potential, zero, kinetic , bottom, potential WEDNESFIELD HIGH SCHOOL EXTREME RIDES = 50m P3 TEST: Transport Physics Name ___________________________ Form:___________ 1(a) Nick thinks cars travel to fast near his school. He calculates the speeds of cars as they move past the school. Write about the experiment Nick did to calculate the cars speed In your answer include (i) what equipment he used (ii) what measurements he took __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ ________________________________________________________________________________________[4] 2) New cars are tested in crashes. 15_ car A mass 1000kg Scientists study the results. Two cars crash speed in m/s into a wall. Look at these graphs 10_ (a) Car A takes 0.2 s to stop during the crash. Calculate the acceleration of car A during 5__ the crash. 15_ car B mass 1500kg __________________________________ 0 __________________________________ __________________________________ __________________________________ Answer______________m/s2 [3] 0 time in seconds 0.2 s speed in m/s 10_ 5__ time in seconds 0.1 s (b) Calculate the distance car A travels during the crash. ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Answer___________________m [3] (c) After the crash car B is more damaged. Car B must have had much greater forces on it during the crash. Explain why. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _______________________________________________________________________________[3] (d) Elaine wants to buy a car. She has a choice of three engines. All the cars have the same mass. Each engine produces a different maximum force. Engine capacity Maximum Look at the information in the table. force (i) The GSi model has the highest acceleration. Explain Mass S model GS model GSi model 1.3 litres 1.6 litres 2.0 litres 2500N 3000N 4000N 1000Kg 1000Kg 1000Kg why. ____________________________________________________________________________________ _________________________________________________________________________________[2] (ii) The GSi model now pulls a caravan. Its maximum acceleration is now 2.5m/s2. Calculate the mass of the caravan. _______________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ _________________________________________________________________________________[4] ____________________________________________________________________________________ ____________________________________________________________________________________ _____________________________________________________________________________ (3) Look at the diagram on stopping distances for cars. car moving at 30 km/h thinking distance braking distance car has stopped Describe the factors that will increase stopping distance___________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ ________________________________________________________________________________[5] 4) Five cars travel along the motorway. They go through two tunnels. Both tunnels are the same length. They go through tunnel A first. Look at the results. Car Astra Focus Golf Mini Saxo Time to pass through tunnel A Time to pass through tunnel B in seconds in seconds 6 8 6 3 7 3 5 5 10 11 (a) Which cars increase their speeds? __________________________________________________________________________________[1] (b) Which car has the highest acceleration? __________________________________________________________________________________[1] (e) Tunnel A is 200m long. Calculate the speed of the Mini through tunnel A ____________________________________________________________________________________ ____________________________________________________________________________________ __________________________________________________________________________________[2] 5) Barry must lift some heavy weights onto a lorry. Each weight is 600N and he must lift them 1.1m onto the lorry. (a) How much work is done lifting one weight onto the lorry? 1.1m ________________________ _______________________[2] ____________________________________________________________________________________ __________________________________________________________________________________[4] b) How much work is done lifting all 3 weights onto the lorry? _________________________________ _________________________________________________________________________________[1] c) If it takes Barry 100 seconds to load the lorry with all three weights what is his average power? ____________________________________________________________________________________ ____________________________________________________________________________________ ________________________________________________________________________________[4] 6) Look at the picture of the ball. It is stationary on the edge of a table i) What is the gravitational potential energy of the ball? (take g as 10m/s2) mass = 0.1Kg __________________________________________________________ ________________________________________________________[4] 1.5m (ii) If the ball is stationary what is its kinetic energy? _______________________________________________________[1] (iii) Describe what happens to the potential and kinetic energy of the ball as it falls to the ground _____________________________________________________________________________________ ___________________________________________________________________________________[2] (iv) What is the kinetic energy of the ball just as it hits the ground? ___________________________________________________________________________________[1] 7a (i) A car has a mass of 1000kg and a speed of 20m/s. What is its kinetic energy? _____________________________________________________________________________________ ___________________________________________________________________________________[4] (ii) The brakes provide the force to stop the car. As the car stops what type of energy is the kinetic energy converted to? __________________________________________________________ [1] (iii) If the brakes provide a force of 5000N how far does the car travel while stopping? [4] ___________________________________________________________________________________ ___________________________________________________________________________________ (b) Look at this data of the braking distance of a car Describe in detail how braking distance changes with speed. Use the kinetic energy equation to explain your description ____________________________________________ ____________________________________________ ____________________________________________ __________________________________________[3] speed braking distance 0 0 1 2 2 8 3 18 4 32 5 50