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Physics AQA

Exam Date-30/05/12

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Speed

- Speed is measured in m/s

- speed is just how fast your are going with no regard to direction

Speed=  Distance/Time

- Example:

A cat sulks 20m in 35s. Find a)its speed b)how long it takes to sulk 75m

- Answer: a) s= d/t = 20/35= 0.57m/s

-b) t= d/s = 75/0.57 = 131s = 2 minutes 11 seconds

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Acceleration

- Measured in m/s2

- acceleration is how quickly you're speeding up

- Acceleration = Change in velocity/Time Taken

- Example: A sulking cat accelerates steadily from 2m/s to 6m/s in 5.6s. Find acceleration

-Answer: (6-2) / 5.6 = 4 / 5.6 = 0.71 m/s2

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Distance-Time graphs

- If its directly proportionate, its speed is constant

- flat sections are where it's stopped

- the steeper the graph, the faster it's going

- Downhill sections mean its going back toward its starting point

- curves represent acceleration or deceleration

- A steepening curve means it's speeding up

- A levelling off curve means its slowing down

- calculating speed from a d-t graph:

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Velocity-Time Graphs

- Flat sections represent a steady speed

- The steeper the graph, the greater the acceleration or deceleration

- Uphill sections are acceleration

- Downhill sections are deceleration

- The area under the graph is equal to the distance travelled in that time interval

- A curve means changing acceleration

Calculating acceleration, speed and distance from a V-T graph:

- acceleration = gradient = Vertical/Horizontal

-Speed at any point can be found by reading the value of the velocity axis

- The distance travelled in any time interval is equal to the area under the graph

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Mass, Weight and Gravity

-Gravity is the force of attraction between all masses

- There are three important effects of gravity on plantets:

• On the surface, makes all objects accelerate towards the ground
• It gives everything weight
• it keeps planets, moons and satellites in their orbits

Mass is just the amount of stuff in an object

weight is caused by the pull of gravity

- an object has the same mass whether its on the moon or on earth, but its weight is different because the force of gravity acting on it is different

weight is a force measured in newtons (n)

-mass is measured in kilograms (Kg)

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Friction forces

- if an object has no force propelling it along, it will always slow down and stop because of friction

- to travel at a steady speed, things always need a driving force to counteract the friction

-Friction occurs in three main ways:

• Friction between solid surfaces which are gripping
• Friction between solid surfaces which are sliding past each other
• Resistance or 'drag' from fluids

- the most important way in reducing drag in fluids is by keeping it in shape of the object streamlined

- lorries and caravans have deflectors on them to make them more streamlined and reduce drag

- For a given thrust, the higher the drag the lower the top speed of the car

- the opposite extreme to a sports is a parachute which is about as high drag as you can get

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Terminal speed

- When a free falling object first sets off they have much more force accelerating them than resistance slowing them down

- As the speed increases the resistance builds up

- this gradually reduces the acceleration until eventually the resistance force is equal to the accelerating force and then it won't be able to accelerate anymore

- It will have reached its maximum speed or terminal speed

- the terminal speed of falling objects depends on their shape and area

- Most important example is a skydiver, without his parachute open he has quite a small area and a force equal to his weight pulling him down

- he reaches terminal speed at about 120mph but with the parachute open, theres much more air resistance and still only the same force pulling him down

- This means that is terminal speed comes right down to about 15mph which is a safe speed to land at.

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The three laws of motion

- 1) Balanced forces mean no change in velocity- So long as the forces on an object are all balanced, then it'll just stay still, or else if its already moving it'll just carry on at the same velocity- so long as the forces are all balanced

- When a train, car or bus is moving at a constant velocity then the forces on it must be balanced, To keep them at a steady speed, there must be zero resultant force

-2) A resultant force means acceleration- If there is an unbalanced force, then the object will accelerate in that direction

- an unbalanced force will always produce acceleration, this acceleration can take five different forms: Starting, Stopping, Speeding up, Slowing down and Changing direction. The greater the mass, the smaller the acceleration. Resultant force on a diagram is the biggest arrow

- F = ma or a= F/m

-3) Reaction forces- If object A exerts a force on object B then object B exerts the exact opposite force on Object A

-I.e if you push a trolley, the trolley will push back against you. As soon as you stop pushing, so does the trolley. The forces are always equal.

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Stopping Distances

- many factors affect your total stopping distance.

- The stopping distance of a car is the distance covered in the time between the driver spotting a hazard and the car coming to a complete stop.

- Thinking Distance: Its affected by two main factors: How fast you are going (the faster you are going the further you'll go) and how dopey you are (i.e Tiredness, drugs, alcohol, old age and careless attitude)

- Braking Distance: its affected by How fast you are going (Faster you are going, further it takes to stop), How heavily loaded the vehicle is (with the same brakes, a heavily loaded vehicle takes longer to stop), How good your brakes are (all brakes must be checked, worn or faulty brakes will let you down catastrophically in an emergency) and How good your grip is (depends on road surface, weather conditions and tyres)

- so even at 30mph you should drive no closer than 6 or 7 car lengths away from the car in front

-This is why speed limits are so important

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Momentum and collisions

- measured in Kg m/s

- Momentum(Kg m/s) = Mass(Kg) x Velocity(m/s)

-momentum is conserved when no external forces act i.e the total momentum after is the same as it was before

- forces cause changes in momentum: When a force acts on an object, it causes a change in momentum

- Force acting(N)= Change in momentum(Kg m/s)/Time taken for change to happen (s)

- A larger force means a faster change of momentum and so a greater acceleration

- likewise, if someone's momentum changes very quickly (e.g car crash) the forces on the body will be very large, and more likely to cause injury

- Cars are designed to slow people down over a longer time when they have a crash. The longer it takes for a change in momentum, the smaller the force.

- Crumple zones crumple on impact, increasing the time taken for the car to stop, seat belts stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest. Air bags also slow you down more gradually

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Car Safety

- if a car crashes, it will slow down very quickly, this means a lot of kinetic energy is converted into other forms of energy in a short amount of time, which can be dangerous for the people inside

- cars are designed to convert this energy in the safest way possible.

- Crumple zones are parts of the car at the front and back that crumple up in a collision

- Seat belts and airbags slow the passengers down safely by converting their kinetic energy into other forms or energy over a longer period of time. These safety features also prevent the passengers from hitting hard surfaces inside the car

- roads have features such as crash barriers and escape lanes in dangerous locations. These increase the time of any collision which means the collision force is reduced.

-cars have safety features such as power assisted steering and traction control which interact with the way the car drives

- ABS brakes prevent skidding and help the driver to stay in control when braking sharply.

- A passive safety feature is any non-interactive feature on the car that helps keep occupants of the car safe. E.g air bags, seat belts, headrests.

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Victoria Smith wrote:

They aren't finished yet, I should have the whole syllabus done soon:) x

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You know what you did for Additional Physics, could you do it for Biology and Chem to please :) xox

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You know what you did for Additional Physics, could you do it for Biology and Chem to please :) xox

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you have some revision cards on your "archive" thing, but when I click on them, they say not yet published? have you deleted them? ***

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thankssssssssssssssss XD

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• wghg albjl ak swdfozsrt,.l;uo;' cv;xpgior/ eio5rdf    gp epg'ierhkzr

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i found this very useful thanks, just hope i now get at least a c in my physics. :) x

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