Transcript Document
P3 Physics Forces for transport Speed Miles per hour Kilometres per hour Metres per second Centimetres per second Kilometres per second So what does the “per” mean? “Per” means “divided by” So kilometres per hour is the miles you did divided by the time it took. There is a rule: Speed = distance ÷ time Or “average speed is distance over time” Sometimes SIDOT Has to be “average” because most things can’t keep to exactly the same speed all of the time. Examples: Travel 100 km in 2 hours, average speed is therefore: 100 ÷ 2 = 50 The unit for this is km/h Travel 1000 km in 25 hours, average speed is therefore: 1000 ÷ 25 = 40 The unit for this is km/h Travel 1 m in 5 hours, average speed is therefore: 1 ÷ 5 = 0.2 The unit for this is m/h but you wouldn’t usually use that Travel 200 m in 20 seconds, average speed is therefore: 200 ÷ 20 = 10 The unit for this is m/s Try these: What is the average speed of an object that travels: 10 metres in 5 seconds 100 metres in 5 seconds 200 metres in 5 seconds 10 metres in 5 hours 300 metres in 50 seconds Don’t forget the units The Triangles Dist If you cover the one you Speed x t don’t know, the calculation is shown by the other two This changes the subject of the equation Acceleration This is how much an object’s speed changes in a certain time The units are always metres per second per second, written as m/s/s or m/s2 As an equation: acceleration = final speed – start speed time taken to change v a If the final t speed is less than the start speed, then the object has decelerated – negative acceleration Force and acceleration It is our experience that a heavy object needs more force to get it moving than a light one. Stopping a heavy object, at the same speed, takes more force. We also know that a lighter object will accelerate (and then move) faster than a heavy one. This comes as one equation: F = ma - force is mass times acceleration What do we know so far? Dist Speed x t Speed = distance time Distance – time graphs Acceleration = (change in speed) time Force = mass x acceleration Speed – time graphs Work Work = force x distance Pxt Power = work time Force Work mxa Fxd Change in speed Axt e.g. 200km in 4 hours is a speed of 50 km/h 0-100 m/s in 5 seconds is an acceleration of 20 m/s/s (m/s2) Moving an object 5m with a force of 5N is 25 J of work (energy) Doing 100 J of work in 4 seconds is 25 Watts. Terminal velocity When an object is accelerated by a force, it gets faster. (depends on the force, of course) But as it goes faster, the friction and (often) air drag get bigger So its speed reaches a limit, called terminal velocity. Continued… When an object has reached terminal velocity, the forces pushing it forward are equal and opposite to those pushing backwards. We call this balanced forces. If forces are unbalanced, then the object will accelerate until they do balance. New bits Kinetic energy = ½mV2 M is mass, V is speed (velocity) Notice, this is a square law, double the speed is four times the energy. Stopping distances Stopping a car or other vehicle takes time, during which it will travel a certain distance. This stopping distance is made up of: Thinking distance – the distance it takes for the driver to react and start braking Braking distance – the distance it covers while the brakes and tyres stop the vehicle. What affects……? Thinking distance: Poor reactions – drink, drugs, tiredness, inexperience Failing to recognise hazards – inexperience, old age, poor visibility Distractions – noisy mates, ‘phone, stereo, smoking What affects……? Braking distance: Poor brakes – too little friction Worn or faulty tyres – too little friction Weather – ice, snow, rain – too little friction What happens to stopping distance when you go faster? Thinking distance increases in direct proportion to speed – double the speed, double the thinking distance, because it took the same time. But the stopping distance is a square law (Because your brakes take out the same amount of energy, but the energy is kinetic, so follows the law E = ½mV2) Therefore double the speed gives four times the stopping distance. From the Highway Code See how doubling from 20mph to 40 gives braking distance going from 6m to 24m. (6 x 4 = 24) Doubling from 30mph to 60 gives braking distance going from 14m to 55m. (14 x 4 = 56) Brake effectiveness can vary depending on the speed (not part of this exam).