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GCSE AQA Physics:Module Two

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Lizzy Corbett

on 27 December 2012

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Transcript of GCSE AQA Physics:Module Two

Physics - Module Two Forces and Their Effects Electricity and The Atom Velocity and Distance-Time Graphs Acceleration and Velocity-Time graph Weight Mass and Gravity Resultant Forces Forces and Acceleration Frictional Force and Terminal Velocity Stopping Distance Work and Potential Energy Kinetic Energy Forces and Elasticity Power Momentum and Collisions Car Design and Safety Static Electricity Current and Potential Difference Circuits Resistance and V=IR Mains Electricity Fuses and Earthing Power and Energy change Atomic Structure Atoms and Ionising Radiation Half Life Uses of Radiation Radioactivity Safety Nuclear Fission and Fusion Life Cycle of Stars Speed and velocity are both how fast you are going. Speed is how fast your going with no regard to direction and velocity is how fast your going in a specified direction. They are both measured in m/s Distance-Time Graph The gradient of a distance time graph tells you the speed (how fast its going) The gradient is the change in distance - y axis - divided by the change in time - x axis. Flat sections are where its stationary
Straight uphill or downhill sections means its a steady speed
The steeper the graph, the faster it's going
Downhill parts mean it's going backwards toward the starting point
Curves represent acceleration and deceleration
A steepening curve means its speeding up
A leveling off curve means its slowing down. Acceleration is how quickly velocity is changing. This can be a change in speed or distance or both. The formula is...

Acceleration = change in velocity
time taken v-u a t v= final velocity
u=initial velocity
t=time Velocity-Time graphs Gradient is the acceleration
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 a section is equal to the distance travelled
A curve means changing acceleration. The acceleration is the gradient, vertical change divided by the horizontal change
The velocity at any point is found by reading the value off the graph
The distance travelled is the area under the graph. Gravitational Force is the force of attraction between all the masses. Gravity attracts all masses but it's only noticeable on large scales such as the gravity of planets. The gravity on Earth is 10m/s^2

Weight and Mass are NOT the same
Mass is the amount of stuff in an object. This will have the same value anywhere in the universe. It is measured in kilograms
Weight is caused by the pull of gravitational force. This changes depending what planet you are on. It is measured in newtons

Weight = mass x gravity This is the overall force on a point or object
There are at least two forces acting on an object. The overall effect of these two forces will decide the motion of the object (accelerates, decelerates, steady speed)
A resultant force means a change in velocity.

For Example
If you had a car. The driving force was 1000N to the left and the air resistance is 600N to the right. This means the resultant force would be 400N to the left.
You simply subtract the two. An object needs force to start moving. If the resultant force on a stationary object is zero, the object will remain stationary.

No resultant force means no change in velocity. If there is no resultant force on a MOVING object it carries on moving at the same velocity. To move at constant velocity the forces must be balanced.

A resultant force means ACCELERATION. If there is a resultant force it means the object will accelerate in the direction of the resultant force. It will either start, stop, speed up, slow down and change direction.

Force=Mass x Acceleration <--------- the force is the resultant force

Reaction forces are equal and opposite. When two objects interact, the forces they exert on each other are equal and opposite. This means if you push something, it pushes back at you just as hard. Friction is always there to slow things down
If an object has no force it will always slow down and stop because of friction
Friction always acts in the opposite direction to the movement
To travel at a steady speed, the driving force needs to balance the frictional force
You get friction when two surfaces make contact or an object passes through a liquid (drag)

Resistance or Drag from fluids
Most of the resistive forces are caused by air resistance or drag. To reduce the drag in fluids, you have to make the object streamline.
Drag increases as the speed increases. Frictional forces from fluids always increase with speed.

Objects falling through fluids reach terminal velocity. When an object first sets off, the force of gravity is more than the frictional force(air resistance or drag) slowing them down, so they accelerate. s the speed increases, the friction builds up/increases. This gradually reduces the acceleration until all the forces are equal and gravity = frictional force of air resistance of drag.

The terminal velocity depends on the shape and area. It depends on the force of gravity and it's weight compared to the drag. The frictional force depends on the shape and area. A good example is the human skydiver. Without the parachute he has a small surface area and a force pulling him down. He reaches a terminal velocity of about 120mph. When he opens the parachute, there is more air resistance because the area has increased but still the same force. This means his terminal velocity reduces to a safe speed to hit the ground at. Many factors affect your stopping distance
the speed of the vehicle - if its travelling fast its takes longer to stop
The total stopping distance is the distance covered in the time between first seeing a hazard to the vehicle completly stopped. The stopping distance is the thinking distance + braking distance Thinking Distance
Effected by two main factors
How fast your going
How dopey you are - this is caused by drugs, alcohol, tiredness, and a careless attitude/ distractions.
Bad visibility and distractions can also be a major factor in accidents - rain, messing with the radio, bright oncoming lights... all means the driver may not notice a hazard until they are very close to it. Braking Distance
Affected by four main factors
How fast your going
How good your brakes are - all brakes must be checked and maintained regularly. Warn or faulty brakes wont work as well and can cause crashes :(
How good your tyres are - Should have a minimum depth of 1.6mm in order to get rid of water in wet conditions. Leaves, diesel spills and muck can cause tryes to skid.
How good the grip is - this depends on road surface, weather conditions and the tyres. When a force moves an object over a certain distance ENERGY is transferred and WORK is done.
Work Done = Force x Distance
Work done is measured in joules because it is an energy!

Gravitational Potential energy is energy due to height and is measured in joules. It is the energy of an object due to its vertical position in a gravitational field. When an object is raised above the ground, work is done against the effect of gravity.
(change of)Gravitational Potential energy = mass x gravity x height (vertical distance moved)
This equation can be shorted. As mass x gravity is your WEIGHT, the equation can be
Gravitational Potential energy = WEIGHT x height

Ep = m x g x h Kinetic energy is all to do with movement, if something moves, it has kinetic energy
Kinetic energy = 1/2 x mass x speed

When kinetic energy is transferred it is work done. A moving car has alot of kinetic energy. To slow a car down, the kinetic energy needs to be transferred to a different type of energy. To stop the car, the kinetic energy is converted to heat energy as friction between the wheels and brakes so they heat up.
Kinetic energy transferred = maximum braking force x braking distance

When something falls its potential energy is converted into kinetic energy. The further it falls the faster it goes. The kinetic energy gained equals the potential energy lost.

When meteors and space shuttles enter the atmosphere they have very high kinetic energy. Friction due to collisions converts this kinetic energy to heat energy, work is done and the temperature rises. 2 Work done to an elastic object is stored as elastic potential energy. This is converted to kinetic energy when the force is removed.

Extension of and Elastic object is directly proportional to force. The extension of a stretched spring is how much it grows by when you put the weight on (not the whole length of the stretched spring)
F = k x e
F is the force, k is the spring constant and e is the extension.
BUT this stops becoming true when the force applied is too much. There is an elastic limit on any object which is the point where you stretch is so much it doesn't return to it's original shape. This is the limit of proportionality. Power is the rate of doing work - how much energy is transferred per second.

Power = Work done (or energy transferred)
Time taken
Power is measured in watts or joules per second. One watt = One joule. You can measure the power output of a person in a few different ways. The first one is calculating the potential energy gained (running up stairs) or the kinetic energy gained (running on the ground/ acceleration) Momentum is the property of a moving object and is basically how hard it is to stop a moving object. The greater the mass and velocity of an object, the more momentum they have. It is a vector quantity as it has size and direction. The sign for momentum is p
P=m x v This means Momentum = mass x velocity

Conservation of Momentum
In a closed system (no external forces acting on the object) the momentum of something before an event is the same as the momentum after the event. This is the conservation of momentum.

Forces in momentum can change. When a force acts on an object is causes it to change momentum. A larger force means a faster change in momentum so there is a greater acceleration. In a car crash this causes alot of force to be exerted on the people in the car, most likely causing injury. This is why cars have a safe design that slows down the change in momentum because the longer it takes to change, the weaker/smaller the force. ADD IMAGE AND STUFF FROM BLUE BOOK ADD FROM BLUE BOOK ADD IMAGES TO ALL OF THEM When you apply the brakes to slow down a car, work is done. The brakes reduce kinetic energy by transferring it to heat and sound energy. In traditional braking systems that would be it but new regenerative braking systems used in some electronic/hybrid cars make use of the energy instead of converting it all to heat energy.
Regenerative brakes use the system that drives the vehicle to do most of the braking.
Instead of converting kinetic to heat the brakes put the vehicles motor into reverse. With the motor running backwards the wheels slow down.
At the same time the motor acts as an electric generator converting kinetic energy into electrical energy that is stored as chemical energy in the battery.
In a crash the car slows down quickly so lots of kinetic of energy is transferred in a short period of time which can be dangerous for the people inside. There is a big change of momentum which means they feel huge forces acting upon them. Cars are designed to make it safer for people.
Crumple zones at the front and the back crumple on impact. The kinetic energy is transferred to other forms of energy that change the shape of the car. They increase the impact time and decrease the force.
Side impact bars are strong metal tubes fitted into the door panels. The direct the kinetic energy of the car away from the passengers to area's like the crumple zone.
Seat belts stretch slightly increasing the time taken the wearer to stop, so there is less force on the chest. The kinetic energy of the person is absorbed by the stretching belt.
Air bags also slow you down more gradually and prevent you hitting a hard surface. Cars have different power ratings
The size and design determines how powerful they are
The more powerful the engine, the more energy transferred and the faster the car goes.
They are designed to be aerodynamic. Hooke's Law
The extension of a spring is directly proportional to the force applied, provided it's limit of proportionality is not exceeded.
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