PHYSICS P2A- FORCES AND THEIR EFFECTS

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  • Created by: Ragitha
  • Created on: 12-03-17 20:40

VELOCITY AND DISTANCE TIME GRAPHS

SPEED is how fast you're going but VELOCITY is the same but it has a direction specified.

GRADIENT = SPEED

FLAT SECTIONS= STATIONARY

STRAIGHT UPHILL= STEADY SPEED

STEEPER THE GRAPH= GOING FAST

DOWNHILL= GOING BACK TOWARDS ITS STARTING POINT

CURVES= ACCELERATION/ DECELERATION

STEEPENING CURVE= SPEEDING UP

LEVELLING OFF= SLOWING DOWN

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ACCELERATION AND VELOCITY TIME GRAPHS

ACCELERATION: HOW QUICKLY VELOCITY IS CHANGING

GRADIENT= ACCELERATION

FLAT SECTIONS= STEADY SPEED

STEEPER THE GRAPH= THE GREATER THE ACCELERATION OR DECELERATION

UPHILL SECTIONS= ACCELERATION

DOWNHILL SECTIONS= DECELERATION

AREA UNDER ANY SECTION IS = DISTANCE TRAVELLED IN THAT TIME INTERVAL

CURVE= CHANGING ACCELERATION

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WEIGHT, MASS AND GRAVITY

GRAVITATIONAL FORCE IS THE FORCE OF ATTRACTION BETWEEN ALL MASSES:

2 Important effects:

  • it makes things accelerate towards the ground- 10m/s^2 on earth
  • it gives everything a weight

WEIGHT AND MASS ARE NOT THE SAME:

  • mass: "amount of stuff in an object"
  • Weight is caused by the pull of the gravitational force.
  • an object will have the same whereever in universe but it will have a different weight.

weight= mass x gravitation field strength

GRAVITATIONAL FIELD STRENGTH ON EARTH: 10N/ kg

GRAVITATIONAL FIELD STRENGTH ON THE MOON: 1.6N/kg

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RESULTANT FORCES

RESULTANT FORCE IS THE OVERALL FORCE ON A POINT OR OBJECT:

  • if you have a number of forces acting at a single point, you can replace them with a single force.
  • if the forces all act along the same line, the overall effect is found by adding or subtracting them.

A RESULTANT FORCE MEANS CHANGE IN VELOCITY:

  • if a resultant force is acting on an object, the object will change its state of rest or motion.
  • it basically causes a change in the object's velocity
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FORCES AND ACCELERATION

AN OBJECTS NEEDS A FORCE TO START MOVING:

If the resultant force on a stationary object is 0, the object will remain stationary.

NO RESULTANT FORCE MEANS NO CHANGE IN VELOCITY:

If there is no resultant force on a moving object it'll just carry on moving at the same velocity.

A RESULTANT FORCE MEANS ACCELERATION:

If there is a non-zero resultant force, then the object will accelerate in the direction of the force.

  • the 'acceleration' can take up 5 forms: starting, speeding, stopping, slowing down, changing direction.
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FORCES AND ACCELERATION

A NON ZERO RESULTANT FORCE PRODUCES AN ACCELERATION:

F= MA

m= mass in KG

a= acceleration in m/s^2

F= the resultant force in newtons (N)

REACTION FORCES ARE EQUAL AND OPPOSITE:

WHEN TWO OBJECTS INTERACT, THE FORCES THEY EXERT ON EACH OTHER ARE EQUAL AND OPPOSITE

  • if you push something, the object will push back
  • two forces are acting on different objects.
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FRICTIONAL FORCE

FRICTION IS ALWAYS THERE TO SLOW THINGS DOWN:

  • if an object has no force acting along, it will always slow down and stop because of friction
  • friction always acts in the opposite direction to movement
  • to travel at a steady speed, the driving force needs to balance the frictional force
  • you get friction between two surfaces in contact or when an object passes through a fluid.

RESISTANCE OR 'DRAG' FROM FLUIDS:

  • to reduce drag in fluids, the shape of the object has to be STREAMLINED

DRAG INCREASES AS THE SPEED INCREASES:

  • Frictioncal forces from fluids always increase with speed.
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TERMINAL VELOCITY

OBJECTS FALLING THROUGH FLUIDS REACH A TERMINAL VELOCITY:

  • the FORCE OF GRAVITY is much more than the FRICTIONAL FORCE slowing them down, so they accelerate.
  • as the SPEED INCREASES the friction BUILDS UP.
  • this gradually REDUCES THE ACCELERATION until eventually the FRICTION FORCE IS EQUAL to the ACCELERATING FORCE.
  • then it won't accelerate anymore, it has reached terminal velocity.

THE TERMINAL VELOCITY OF FALLING OBJECTS DEPENDS ON THEIR SHAPE AND AREA:

  • the accelerating force acting on all faling objects is gravity, it would fall at the same rate if it wasn't for air resistance.
  • terminal velocity of any object is determined by its drag in comparison to its weight. FRICTIONAL FORCE DEPENDS ON ITS SHAPE AND AREA.

EXAMPLE: Without the parachute, the man has a small area and a force of '?' pulling him down. When the parachute opens, there's much more air resistance and still only the same force pulling him down. Which reduces the terminal velocity to a safer speed.

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STOPPING DISTANCES

MANY FACTORS AFFECT YOUR TOTAL STOPPING DISTANCE:

  • If you need to stop in a given distance, then the faster a vehicle's going, the bigger braking force it'll need.
  • The faster you're going, the greater your stopping distance.
  • The total stopping distance of a vehicle is the distance covered in the time between the driver first spotting a hazard and the vehicle coming to a complete stop.
  • STOPPING DISTANCE: Is the sum of the thinking distance and the braking distance.

1) THINKING DISTANCE:

  • TWO MAIN FACTORS:
    • how fast you're going
    • tiredness, drugs, alcohol

2) BRAKING DISTANCE:

  • FOUR MAIN FACTORS:
    • your speed
    • how good your brakes are
    • how good your tyres are- minimum tread depth of 1.6mm
    • how good the grip is- road surface, weather conditions, tyres
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WORK AND POTENTIAL ENERGY

WHEN A FORCE MOVES AN OBJECT THROUGH A DISTANCE, ENERGY IS TRANSFERRED AND WORK IS DONE.

  • Whenever something moves, something else is providing some sort of 'effort' to move it.
  • The thing putting the effort in needs a supply of energy
  • It then does 'work' by moving the object- and one way or another it transfers the energy it receives into other forms.
  • Whether this energy is transferred 'usefully' or 'wasted', you can still say that 'work is done'

WORK DONE= FORCE X DISTANCE

GRAVITATIONAL POTENTIAL ENERGY= MASS X G X HEIGHT

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KINETIC ENERGY

KINETIC ENERGY IS ENERGY OF MOVEMENT:

KINETIC ENERGY= 1/2 X MASS X SPEED SQUARED

  • Kinetic energy of something depends on both speed and mass.
  • The more it weighs and the faster it's going, the bigger its kinetic energy will be

KINETIC ENERGY TRANSFERRED IS WORK DONE:

  • When a car is moving it has kinetic energy. To slow a car down this kinetic energy needs to be converted into other types of energy.
  • To stop a car, the kinetic energy has to be converted to heat energy as friction between the wheels and the brake pads, causing the temperature of the breaks to increas

KINETIC ENERGY TRANSFERRED= WORK DONE BY BRAKES

1/2mv^2 = F x d

  • When something falls, its potential energy is converted into kinetic energy.
  • Kinetic energy gained = Potential energy lost
  • Space shuttles have heat shields made from special materials which lose heat quickly, allowing the shuttle to re-enter the atmosphere without burning up.
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FORCES AND ELASTICITY 1

WORK DONE TO AN ELASTIC OBJECT IS STORED AS ELASTIC POTENTIAL ENERGY:

  • when you apply force to an object you may cause it to stretch and change in shape.
  • any object that can go back to its original shape after the force has been removed is an elastic object
  • work is done to an elastic object to change its shape. This energy is not lost but is stored by the object as elastic potential energy.
  • the elastic potential energy is then converted to kinetic energy when the force is removed and the object returns to its original shape

EXTENSION OF AN ELASTIC OBJECT IS DIRECTLY PROPORTIONAL TO FORCE:

  • F = K X e
  • K is the spring constant. Its value depends on the material that you are stretching and it's measured in newtons per metre
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FORCES AND ELASTICITY 2

...BUT THIS STOPS WORKING WHEN THE FORCE IS GREAT ENOUGH:

There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally.

  • On a graph a straight line represents force and extension being in direct proportion
  • There is a maximum force that the elastic object can take and still extend proportionally
    • This is known as the LIMIT OF PROPORTIONALITY- when the line on the graph begins to curve
  • If you increase the force past the limit of proportionality, the material will be permanently stretched.
  • When the force is removed, the material will be longer than at the start.
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POWER

POWER IS THE 'RATE OF DOING WORK'

  • power = work done or energy transferred / time take

POWER IS MEASURED IN WATTS

  • One watt= 1 joule of energy transferred per second
  • Power means how much energy per second, so watts are the same as 'joules per second'

CALCULATING YOUR POWER OUTPUT:

  • 1) The timed run upstairs- the energy transferred is the potential energy you gain
  • 2) The timed acceleration- the energy transferred is the kinetic energy you gain

To get accurate results you have to keep doing them several times and find an average.

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MOMENTUM AND COLLISIONS 1

MOMENTUM= MASS X VELOCITY

  • Momentum is a property of moving objects
  • The greater the mass of an object and the greater its velocity, the more momentum the object has.
  • Momentium is a vector quantity- it has size and direction

MOMENTUM BEFORE = MOMENTUM AFTER (CONSERVATION OF MASS)

FORCES CAUSE CHANGES IN MOMENTUM:

  • When a force acts on an object, it causes a change in momentum
  • A larger force means a faster change of momentum and so greater acceleration
  • If someone's momentum changes very quickly, the forces on the body will be very large, and so more likely to cause injury
  • This is why cars are designed with safety features that slow people down over a longer time when they have a crash- the longer in change of momentum, the smaller the force.
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CAR DESIGN AND SAFTEY 1

BRAKES DO WORK AGAINST THE KINETIC ENERGY OF THE CAR:

  • When you apply the brakes to slow down a car, work is done
  • The brakes reduce the kinetic energy of the car by transferring it into heat energy
  • New REGENERATIVE BREAKING SYSTEMS used in some electric or hybrid cars make use of the energy, instead of converting it all into heat during breaking.

REGENERATIVE BREAKING SYSTEM:

  • Uses the system that drives the vehicle to do the majority of the breaking
  • The brakes put the vehicle's motor into reverse- with the motor running backwards, the wheels are slowed
  • The motor acts as a electric generator converting kinetic energy into electrical energy that is stored as chemical energy in the vehicle's battery.
  • Advantage is that they stored the energy of braking rather than wasting it
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CAR DESIGN AND SAFETY 2

CARS ARE DESIGNED TO CONVERT KINETIC ENERGY SAFELY IN A CRASH

  • If a car crashes it will slow down very quickly- this means that a lot of kinetic energy is converted into other forms of energy in a short amount of time, which can be dangerous to the passengers.
  • In a crash, there will be a big change in momentum over a very short time so the people inside experience huge forces that could be fatal.
  • Cars are designed to convert the kinetic energy of the car and its passengers in a way that is safest for the car's occupants
  • They often do this by increasing the time over which momentum changes happen, which lessens the forces on the passengers.

CRUMPLE ZONES:

  • The car's kinetic energy is converted into other forms of energy by the car body as it changes shape.
  • Crumple zones increase the impacts time, decreasing the force produced by the change in momentum.

SIDE IMPACT BARS: (metal tubes fitted into car door panels)

  • They help direct the kinetic energy of the crash away from the passengers to other areas of the car, such as the crumple zones.
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CAR DESIGN AND SAFETY 3

SEAT BELTS:

  • They stretch slightly, increasing the tim taken for the wearer to stop
  • This reduces the forces acting in the chest.
  • Some of the kinetic energy of the wearer is absorbed by the seat belt stretching

AIR BAGS:

  • Slows you down more gradually and prevent you from hitting hard surfaces inside the car

CARS HAVE DIFFERENT POWER RATINGS:

  • The size and design of car engines determine how powerful they are.
  • The more powerful an engine is, the more energy it transfers from its fuel every second, and so the faster its top speed can be.
  • Cars are also designed to be AERODYNAMIC. This means that they are shaped in such way that air flows very easily and smoothly past them, so minimising air resistance.
  • Cars reach their top speed when the resistive force equals the driving force provided by the engine. So, with less air resistance to overcome, the car can reach a higher speed before this happens.
  • Aerodynamic cars therefore have higher top speeds.
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