Physics P3.


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Mass, weight and gravity.

  • Gravity is the force of attraction between all masses.
  • Gravitaional field strength is acceleration due to gravity or g.
  • The value of g is about 10m/s2 on Earth.
  • Mass is the amount of matter in an object and is the same wherever you go.
  • Weight is caused by the pull of gravity.
  • Weight is a force measured in newtons.
  • Mass is not a force it is measured in kilograms.
  • Weight = mass x gravitational field strength.
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  • Gravity or weight act straight downwards.
  • Reaction force from a surface, usually acts straight upwards.
  • Thrust or push or pull due to an engine or rocket speeding something up.
  • Drag or air resistance or friction is slowing the thing down.
  • Lift is a force acting upwards that keeps something up.
  • Tension how tight something is.
  • Stationary object = all forces in balance...
  • The force of gravity is acting downwards this causes a reaction force pushing up on the object. This means it's in balance as these forces are equal.
  • Steady horizontal speed = all forces in balance.
  • Steady vertical speed = all forces balanced. 
  • Horizontal acceleration = unbalanced forces and vertical acceleration = unbalanced forces...
  • You only get acceleration with an overall resultant (unbalanced) force.
  • The bigger the unbalanced force the greater the acceleration.
  • On a force diagram the arrows will be unequal.
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Friction forces and terminal speed.

  • When an object is moving friction acts in the direction that opposes movement.
  • The frictional force will match the size of the force trying to move.
  • To travel at a steady speed you need a driving force to overcome the friction.
  • Friction occurs in three main ways...
  • Friction between solid surfaces which are gripping.
  • Friction between soild surfaces which are sliding past each other.
  • Resistance or drag from fluids - the larger the area of the object the larger the drag.
  • When objects first set off they have more froce accelerating them than resistance slowing them down.
  • As the speed increases so does the resistance which gradually reduces the acceleration untill the forces are equal.
  • It will have reached its maximum or terminal speed.
  • The terminal speed of any moving object depends on its drag compared to its driving force.
  • The greater the drag, the lower the 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, the object will acceleratein the direction of the force. The size of the acceleration is decided by F = ma which is newtons second law of physics.
  • The overall unbalanced force is often called the resultant force.
  • Any resultant force will produce acceleration and the formula is...
  • Force = mass x acceleration.
  • If the forces are parallel, the resultant force is found by just adding or subtracting them.
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Stopping distance.

  • The stopping distance of a car is the distance covered between the driver first spotting the hazard and the car coming to a complete stop.
  • Stopping distance = thinking distance = braking distance.
  • Thinking distance - the distance the car travels in the time between the driver noticing the hazard and applying the brakes. It's affected by two factors...
  • How fast your going - the faster you're going the further you'll go.
  • Your reaction time - this is affected by the tiredness, drugs, alcohol, distractions and a lack of concentration.
  • Braking distance - the distance taken to stop once the brakes have been applied. It's affected by four main factors...
  • How fast your going - the faster your going the further it takes to stop.
  • How heavily loaded the car is - with the same brakes, the heavier the vehicle the longer it takes to stop.
  • How good the brakes are - braking depends on how much force you apply. Also if brakes are worn or faulty you won't be able to brake as quickly. 
  • How good the grip is which depends on three things - road surface, weather conditions and tyres.
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More on stopping distances.

  • As a car speeds up the thinking distance increases at the same rate as speed .
  • The graph is linear - a straight line.
  • This is because the thinking time stays constant - but the higher the speed, the more distance you cover in the same time.
  • Braking distance, however increases faster the more you speed up.
  • The relationship between the speed and braking distance is a squared relationship.
  • This means as speed doubles, braking distance increases 4-fold.
  • If the speed trebles, braking distance is 9-fold - see card 10 for explanation. 
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  • The greater the mass of an object the greater its veleocity, the more momentum the object has.
  • Momentum (kg m/s) = mass (kg) x velocity (m/s).
  • Momentum has size and direction.
  • 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)
  • Its the amount of time taken  for a change in momentum that determines how big or small an force is.
  • If the change in momentum stays the same and t is small f will be big ans vice versa.
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Car safety

  • Car safety feactures reduce the forces acting in accidents.
  • In a collision, the force on an object can be lowered by slowing down the object over a long period of time.
  • This is because the longer it takes for a change in momentum, the smaller the forces acting.
  • These features increase the collision time to reduce the forces of deceleration to try and reduce injury...
  • Crumple zones crumple and change shape on impact, (absorb kinetic energy).
  • Seat belts stretch slightly.
  • Air bags also slow you down more gradually.
  • Roads can be made safer by having crash barriers and escape lanes.
  • ABS brakes help drivers keep control of a cars steering when braking. They automatically pump on and off to prevent skidding.
  • Safety features have to be tested using dummies and crash test cars.
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Work done and gravitational potential energy.

  • When a force makes an object move, energy is transferred and work is done.
  • When energy is supplied work is done and energy is transferred.
  • Work done = force x distance.
  • Gravitational potential energy (G.P.E.) is the energy that something has because of its height above the ground. 
  • The energy used to raise it is stored and can be changed into kinetic energy if it falls.
  • G.P.E. = mass x g x height
  • g is gravitational field strength.
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Kinetic energy.

  • The kinetic energy (K.E.) of something is the energy it haswhen moving.
  • It depends on both its mass and speed.
  • Kinetic energy = 1/2 x mass x (speed x speed).
  • To sptop a car the kinetic energy has to be converted to heat energy at the brakes and tyres.
  • Kinetic energy transferred = work done by brakes.
  • 1/2 x m x (v x v) = F x d
  • v = speed, F = maximum braking force, d = braking distance.
  • The braking distance increases as speed squared increases.
  • This means if you double the speed you double the value of v but the v x v means the K.E. is then increased by a factor of four.
  • F is the maximum possible braking force d must increase by a factor of four to make the equation balance.
  • In other words if you go twice as fast the braking distance must increase by a factor of four.
  • Doubling the mass of an object doubles the K.E. it has which will double the braking distance.  
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Falling objects and roller costers.

  • When something falls its G.P.E is converted into K.E.
  • Kinetic energy gained = gravitational potential energy lost.
  • 1/2m(v x v) = mgh.
  • h = height, g = 10
  • When a falling object reaches terminal speed its speed can't increase anymore so neither can its K.E. Instead the G.P.E. is transferred to internal energy of the object.
  • The formula can be rearanged to give h = (v x v) / 2g.
  • Roller coasters transfer energy...
  • At the top of a roller coaster the carriage has lots of G.P.E.
  • As the carriage descends G.P.E. is transferred to K.E. and the carriage speeds up.
  • As the carriage continues to descend it keeps accelerating.
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  • Power is the rate of doing work.
  • A powerful machine is one which transfers a lot of energy in a short space of time.
  • Power = work done / time taken.
  • Power is measured in watts.
  • Power means how much energy per second so watts are the same as joules per second.
  • Sometimes you want to find the power of something based on force and speed.
  • Power = force x speed.
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Fuel consumption and emissions.

  • The size and design of car engines determine how powerful they are and the more powerful and engine the more energy it transfers. 
  • The fuel consumption of a car is usually stated as the distance travelled using a certain amount of fuel.
  • A car with a low value for l/100km has a low fuel consumption but a car with a low value for mpg has a high fuel consumption.
  • Heavy cars have a high fuel consumption.
  • Driving style can affect fuel consumption for example frequently braking and accelerating.
  • The higher the fuel consumption the greater the emissions.
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