Physics P3

Ocr Gateway, Physics P3 Forces for transport

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Speed cameras measure the speed of cars

  • take 2 photos, one 0.5 seconds later than the first
  • white lines on the road are painted 1.5m apart showing how far the vechicle has travelled

Distance-Time Graphs

  • Gradient= Speed
  • Flat sections are where its stopped
  • steeper the graph faster its going
  • 'downhill' sections represent coming back towards starting point
  • curves represent acceleration or deceleration
  • steepening curve= speeding up,  levelling off curve= slowing down

Calculating speed from Distance-Time graph= the gradient

= vertical divided by horizontal

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Acceleration is how quickly you're speeding up

  • how quickly speed is changing
  • accelerate when you change direction without changing speed

Speed-Time Graphs

  • Gradient= Acceleration
  • Flat sections= steady speed
  • steeper the graph, the greater the acceleration or deceleration
  • uphill sections= acceleration
  • downhill sections= deceleration
  • the area under any section of the graph is equal to the distance travelled in that time
  • a curve= changing acceleration

calculating acceleration, speed and distance off a graph, A= gradient, S= on axis, D= area

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  • Gravity or weight- acts straight downwards
  • Reaction force- from a surface acts straight upwards
  • Thrust or Push or Pull due to an engine or rocket speeding something up
  • Drag, Air resistance or friction slows the thing down
  • Lift- aeroplane wing
  • Tension- in a rop or cable

Stationary object- gravity acts downwards, reactions forces pushes up- balances must be equal

Steady horizontal speed- thrust, drag, reaction and weight all balance

Steady vertical Speed- weight and drag balance

Horizontal acceleration- unbalanced forces- acceleration (thrust) greater

Vertical acceleration- unbalanced forces- acceleration (weight) greater

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Friction is always there to slow things down

  • Friction between solid surfaces ae gripping (static friction)
  • Friction between solid surfaces which are sliding past eachother
    • reduce by using lubricant like oil and grease
  • Resistance or 'Drag' from Fluids
    • keep object streamlined

Friction always increases as the speed increase

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Terminal Speed

Free fallers reach a terminal speed

1. have more accelerating force than resistance

2. as speed increases, resistance builds up

3. reduces the acceleration

4. the acceleration and resistance force end up equal- terminal speed

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Forces and Acceleration

If the forces on an object are all BALANCED, then it'll keep moving at the SAME SPEED in the SAME DIRECTION

If there is an UNBALANCED FORCE then the object will ACCELERATE in the direction of the force. The size of the acceleration is decided by the formula f=ma

The overall unbalanced force is sometimes called the resultant force

F=ma (f=force, m= mass, a= acceleration)

  • greater the force, greater the acceleration
  • bigger the mass, smaller the acceleration
  • to get a big mass to accelerate as fast as a small mass it needs a bigger force
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Forces and Acceleration

Resultant force

  • in most situations 2 forces act on an object
  • the overall effect of these forces will decide the motion of the object
  • the 'overall effect' can be found by adding or subtracting the overall force you get is the resultant force.
  • when you use the formula f=ma, F is always the resultant force

Reaction Forces

  • If an object EXERTS A FORCE on object A then object B exerts the EXACT OPPOSITE FORCE on object A
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Stopping Distances

Stopping distance= Thinking distance + Braking distance

Thinking distance is distance driver travels before brakes are applied affected by:

  • how fast your going
  • whether your affected by tiredness, drugs, alcohol, old age
  • whether your concentrating or being distracted

Braking distance is distance travelled when brakes are applied affected by:

  • how fast your going
  • how heavily loaded the car is- heavy car takes longer to stop
  • how good the brakes are
  • how good the grip is- depends on 3 things
    • road surface, weather and tyres

wet or icy roads are more slippy than dry roads friction decreases

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Car Safety

A moving car has a lot of kinetic energy, to slow a car down the kinetic energy must be converted into other types of energy

Cars are designed to convert kinetic energy safely in a crash

  • crumple zones are parts of the car at front and back which crumple in a collision, some of the kinetic energy is converted into other forms of energy as the car body changes shape
  • seat belts and air bags slow down the passengers safely converting kinetic energy into other forms over a longer period of time

Safety features reduce the forces acting in accidents

  • in a collision the force on the object can be lowered by slowing down the object over a longer time (f=ma)
    • safety feature on cars increase collision time
    • roads are made safer- crash barriers, escape lanes
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Active features

Active features take control in an emergency

  • they interact with the way the driver drives to help avoid a crash e.g. power assisted steering, traction control
  • ABS brakes are an active feature that prevent skidding and help the driver stay in control of the car when braking sharply, can also give shorter stopping distance
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Passive safety features

are non interactive features of a car that helps to keep the occupants of the car safe- seat belts, air bags, headrests

  • A safety cage is a feature that surrounds the people in the car- it protects passengers because it doesn't easily change shape
  • features in a car are designed to stop the driver getting distracted
  • most cars have controls on or near steering wheel or on control paddles- so stay in control of the vechicle whilst operating the stereo, electric windows, cruise control etc
  • cars are designed to keep drivers comfortable in the correct driving position e.g adjustable seats, ventilation

Safety features save lives

  • crash tests have shown wearing a seatbelt reduces number of fatalities
  • safety features are rigorously tested by car manufacturers to see how effective they are
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Work and Potential energy

When a force moves an object, ENERGY IS TRANSFERRED and work is done

work done= Force * distance

Potential energy is due to height

Potential energy= mass * g * height

=gravitational potential energy

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Kinetic Energy

Kinetic Energy is energy of movement

KE= 1/2 * mass * v (speed) squared

the kinetic energy of something depends on mass and speed, the more it weighs and the faster its going the bigger its kinetic energy will be

Stopping distances increase with extra speed mainly because of the velocity squared

If you double the speed, you double the value of v but the V squared means that the K.E is then increased by a factor of 4

However if F is the maximum possible braking force which can't be decreased so d must increase by a factor of 4 to make it balance

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Gravity and roller coasters

when something falls potential energy is converted into kinetic energy

kinetic energy gained= potential energy lost

weight= mass * gravity (10N/kg)

Roller coasters transfer energy

  • at the top the carriage has maximum potential energy
  • as the carriage descends P.E is transferred to K.E so it speeds up
  • the carraige keeps accelerating
  • at the lowest point the carriage has minimum P.E but maximum K.E
  • When the carraige goes back up again it has slowed down
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Power is the 'Rate of work done'

it is not the same as force or energy, a powerful machine is one that transfers alot of energy in a short space of time

Power= Work done

     Time taken

Power is measured in watts or J/s

Calculating your power output

power= energy transferred

Time Taken

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Power ratings of cars

  • the size and design of car engines determine how powerful they are
  • the more powerful the engine, the more energy it transfers from its fuel every second and usually the higher the fuel consumption
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Fuels for cars

Usually petrol or diesel which are made from fossil fuels

  • they cause pollution when they are burnt which can cause environmental problems like acid rain and climate change
  • the are non renewable

to avoid these problems scientists are developing alternative types of fuel such as alcohol, LPG, hydrogen and bio-diesel

  • hydrogen can be produced by electrolysis of very diliute acid but this requires energy from fossil fuels
  • some use batteries but they need to be charged
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Fuel Consumption

fuel consumption is usually stated as the distance travelled using a certain amount of fuel

measured in mpg- indicates how many miles a car can travel tom 1 gallon

depends on different things- type and size of engine, how the car is driven and the weight and shape of the car

A car will have a high fuel consumption if a large force is needed to move it, using f=ma you can see that the force needed depends on the mass and its acceleration, a heavier car will need a greater force

driving style also affects fuel consumption- frequent braking and acceleration will increase it

opening the windows increase fuel consumption as more energy is needed to overcome air resistance

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