# Physics P2a

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• Created by: TheFame
• Created on: 14-04-16 18:29

## Velocity and Acceleration

VELOCITY = distance / time

• Velocity is speed in a given direction. Speed is just how fast you're going, regardless of direction.
• Measured in m/skm/h or mph.
• Graphs of velocity are distance-time.
• Gradient is the speed. Flat sections are stationary. Downhill means it is going back.

ACCELERATION = change in velocity / time taken

• Acceleration is how quickly velocity is changing (in speed, direction or both).
• Measured in m/s^2.
• Graphs of acceleration are velocity-time.
• Gradient is the acceleration. Flat sections are steady speed. Uphill / downhill is acceleration / deceleration.
• The area under a section of the line is the distance travelled.
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## Weight, Mass, Gravity and Resultant Forces

WEIGHT, MASS AND GRAVITY

• Weight is mass x gravity strength.
• Mass is always the same.
• Gravitational Field Strength is dependent upon the size of the planet / larger object.
• Earth's GFS is 10.

RESULTANT FORCE is the overall force on an object.

• The overall force will decide whether the object accelerates, decelrates or stays steady.
• Forces on the same line can be added or subtracted.
• The answer (overall force) is the resultant force.
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## Forces and Acceleration

• An object needs a resultant force to start moving.
• If the resultant force is 0, there is no change in velocity.
• If the resultant force is NOT 0, the object will accelerate.

An acceleration can take FIVE forms:

1) Starting, 2) Stopping, 3) Speeding Up, 4) Slowing Down, 5) Changing Direction

FORCE = mass x acceleration

ACCELERATION = force / mass

When two objects interact, the forces they exert on each other are equal and opposite.

When you push something, it pushes back equally hard.

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## Frictional Force and Terminal Velocity

FRICTION is resistance (or drag) from fluids.

• Drag increases as speed increases.
• An object will always slow down and stop, due to friction.
• Friction acts opposite to the direction of movement.

TERMINAL VELOCITY is the maximum speed an object can reach.

• The accelerating force on all falling objects is gravity.
• One example is a skydiver:
• He has a relatively small area and reaches terminal velocity of 120mph.
• With his parachute open, he increases his area. There is more air resistance and his terminal velocity becomes 15mph.

When you fall through a fluid, there is frictional force (drag).

Frictional forces increase with speed.

You eventually reach terminal velocity.

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## Stopping Distances

THINKING DISTANCE is the time it takes your brain to tell you to brake.

• Affected by:
• Tiredness; drugs; alcohol; carelessness; bad weather; other distractions.

BRAKING DISTANCE is the distance it takes the car to stop once brakes are applied.

• Affected by:
• Speed (the faster you're going, the further it takes to stop).
• Effectiveness of brakes.
• Effectiveness of tyres (appropriate tread depth).
• Grip on the road (depends on road surface; weather conditions; tyres).
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## Work and Potential Energy

When a force moves an object, energy is transferred and work is done.

WORK DONE = force x distance

• When something moves, it requires 'effort'.
• This 'effort' requires fuel (food, petrol, electricity).
• It then does 'work' by moving the object and transfers the fuel into other forms of energy.
• Energy can be either be:
• Used usefully (by lifting a load)
• Used wastefully (lost as heat).
• Either way, you can still say 'work is done'.

GRAVITATIONAL POTENTIAL ENERGY is energy due to height.

• Gravitational Potential Energy mass gravity height
• Measured in joules.
• Occurs when work is done against the force of gravity.
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## Kinetic Energy

KINETIC ENERGY = 1/2 x mass x speed^2

• Energy of movement.
• Dependent upon mass and speed.
• Cars have lots of kinetic energy. In order to slow it down, this energy needs to be converted into other types of energy.

KINETIC ENERGY TRANSFERRED is work done.

• 1/2 x mass x speed^2 = force x distance
• Falling objects convert gravitational potential energy into kinetic energy.
• When something falls, its GPE is converted into KE.
• The further it falls, the faster it goes.
• Kinetic energy GAINED = Potential energy LOST
• When meteors and shuttles fall to earth, they have a very high kinetic energy. They become hot, because of collisions with other atoms. Most meteors burn out before hitting earth. Shuttles are made of special materials that cause them to lose heat quickly.
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## Forces and Elasticity

• Any object that returns to its original shape is an elastic object.
• Work is done and is stored as elastic potential energy.
• The EPE is converted to kinetic energy as the objects springs back to shape.

FORCE = 'constant' x extension

• The extension is directly proportional to the force applied.
• The extension is measured in metres; the force is measures in newtons.
• 'k' is the spring's constant and depends upon the material that is being stretched.
• Measurd in newtons per metre (N/m).

LIMIT OF PROPORTIONALITY

• For small forces, force and extension are proportional (straight-line relationship).
• There is a maximum force the object can take before it become unproportional.
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## Power

POWER = work done (energy transferred) / time taken

• Power is the rate of work done per second.
• Measured in watts or J/s (joules per second).

EXAMPLE ONE: Running Upstairs

• Energy transferred is potential energy gained (mgh), in this case.
• Power = mgh / t = (mass x gravity strength x height) / time
• (62 x 10 x 12) / 14 = 531W

EXAMPLE TWO: Timed Acceleration

• Energy transferred is kinetic energy gained (1/2 x m x v^2), in this case.
• Power = (1/2 x m x v^2) / t = (1/2 x mass x velocity^2) / time
• (1/2 x 62 x 8^2) / 4 = 496W
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## Momentum and Collisions

MOMENTUM = mass x velocity

• All moving objects have momentum.
• The greater the mass and velocity of an object, the more momentum it has.
• Momentum has size and speed - it's a vector quantity.

CONSERVATION OF MOMENTUM

• The total momentum before a collision is equal to the total momentum after the collision.
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## Car Design and Safety

BRAKES do work against the kinetic energy of the car.

• Brakes convert kinetic energy into heat and sound.
• Regenerative brakes generate elecrticity in the car's battery.
• These brakes put the car into reverse when pressed.
• The motor then acts as a generator and stores energy in the battery.
• CRUMPLE ZONES: at the front and back of the car crumple on impact.
• SIDE IMPACT BARS: fitted in the door panels, direct kinetic energy away from passengers.
• SEAT BELTS: strech slightly, increasing time taken to stop (lowers momentum). Reduces forces on person's chest.

POWER RATINGS

• Size and design of car engines determine power (the amount of energy transfered / second).
• Cars are designed to be aerodynamic to minimise air resistance.
• Cars reach their top speed when resitive forces equal driving force of the engine.
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## Static Electricity

STATIC build-up is caused by friction.

• When some insulating materials are rubbed, they transfer electrons.
• One will have a positive charge, the other will have a negative charge.
• Electrically charged objects attract some objects to them (e.g: paper, fibres).
• One example: polythene / acetate rods rubbed with cloth.

ONLY ELECTRONS MOVE

• Positive and negative charges are caused by the movement of electrons.
• Positive charge: electrons moving away. Negative charge: electrons gained.
• Same charges repel, opposite charges attract.
• These forces are weake the further apart they are.

CHARGES MOVE EASILY THROUGH CONDUCTORS, such as metal.

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## Current and Potential Difference

• CURRENT is the flow of electric charge around a circuit. Ampere, A.
• POTENTIAL DIFFERENCE is the driving force that pushes the current around. Volt, V.
• RESISTANCE anything in the circuit that slows the current down. Ohm, Ω.
• The greater the resistance across a component, the smaller the current that flows.

CURRENT = charge / time

• Current is the rate of flow of charge. When current flows past a point for a length of time, then the charge can be shown as current x time.
• More charge passes around when the current is bigger.

POTENTIAL DIFFERENCE = work done / charge

• PD (voltage) is the work done between two points. The work done across an electrical component is the energy amount of energy that is transferred by it.
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## Test Circuits

This is a standard test circuit

THE AMMETER: 1) Measures the current (in amps). 2) Must be placed in series. 3) Can be put anywhere in the main curcuit, but never in parallel.

THE VOLTMETER: 1) Measures the potential difference (voltage). 2) Must be placed in parallel around the component being measured.

IMPORTANT:

• Use this for getting voltage / current graphs.
• The ammeter, component and variable resistor are all in series. The voltmeter is in parallel.
• Variable resistor controls the current, which allows you to take several pairs of readings and plot them on a graph.
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## Resistance

RESISTANCE increases with temperature.

• When a charge flows through a resistor, some of the energy is transferred to heat.
• This causes ions in the conductor to vibrate more. This makes it more difficult for charge-carrying electrons to get through the resistor. Current cannot flow, thus resistance increases.

If the graph curves, resistance is changing. Resistance can be found on a curved graph by taking a pair of values and substituting into 'R=VI'. If the graph is a straight line, the resistance is steady. The steeper a SL graph, the lower the resistance.

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## Circuit Devices

DIODE (current flows in one direction through it)

• A diode is a semiconductor made from silicon.
• Used to regulate potential difference.
• Current can flow freely in ONE direction. There is a very high resistance in the reverse direction.

LED (emits light when current is running through it)

LIGHT-DEPENDENT DIODE (depends on intensity of light)

• In bright light, resistance falls. In darkness, resistance is highest.

THERMISTOR (resistance decreases as temperature increases)

• Useful for temperature detectors in car engines.
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## Series Circuits

SERIES CIRCUITS have components that run one after the other.

• Each component is connected end-to-end.
• If one component is removed, the circuit is broken.

POTENTIAL DIFFERENCE is shared.

• The total PD is shared between each component. Voltages measured in the circuit always add up to the total voltage.

CURRENT is the same in all parts.

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## Parallel Circuits

PARALLEL CIRCUITS have components that are connected separately.

• Each component has a separate connection to the battery.
• If one component is removed, the circuit is fine.
• Components do not run one after the other.

POTENTIAL DIFFERENCE is the same.

• The total PD is the same across all components.

CURRENT is shared between branches.

• The total current flowing is equal to the amount of current across all separate components.

VOLTMETERS are always connected in parallel with a component.

AMMETERS are always connected in series with a component.

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