Physics P5

  • Created by: abbiedye
  • Created on: 29-07-18 11:53

What are vector quantities?

  • Have a magnitude and a direction
  • Include - force, velocity, displacement, acceleration, momentum
  • Represented by an arrow
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What are scalar quantities?

  • Only have a magnitude and no direction
  • Include - speed, distance, mass, temperature, time
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What are contact forces?

  • When two objects have to be touching for a force to act
  • Contact forces = friction, air, resistance, tension in ropes, normal contact force, etc
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What are non-contact forces?

  • If the objects do not need to be touching for the force to act
  • Non-contact forces = magnetic forces, gravitational force, electrostatic force, etc
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What is an interaction pair?

  • A pair of forces that are equal and opposite and act on two interacting objects (basically Newton's Third Law)
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What is gravity?

  • Attracts all masses
  • Important effects - makes all things fall towards the ground and gives everything a weight
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Differences between weight and mass

  • Mass = amount of 'stuff' in an object, same mass whether on earth or the moon and it is not a force (measured in kg with a mass balance)
  • Weight = force acting on an object due to gravity depends on the gravitational field at the location and it is a force measured in Newtons by a calibrated spring (newton metre)
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How are mass and weight linked?

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)

The gravitational field strength of the earth = 9.8N/kg

  • Mass and weight are directly proportional
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What is a resultant force?

  • When a force moves an object through a distance, energy is transferred and work is done on the object

Work done (J) = force (N) x distance (moved along the line of the force) (m)

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How do you calculate forces?

  • Use scale drawings to find resultant forces
  • An object is in equilibrium if the forces on it are balanced
  • You can split a force into components
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Stretching, compressing and bending in energy

  • Applying a force may cause it to stretch, compress or bend
  • Work done = force stretches or compresses an object - if it is elastically deformed - all energy transferred to the elastic potential energy store
  • elastically deformed = the object can go back to its original shape
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How does extension link to force?

  • The extension of a stretched spring (or another elastic object) is directly proportional to the load or force applied

Force (N) = spring constant (N/m) x extension (m)

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Link between extension and force

  • There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionality
  • When the force is great enough, the extension and force are not directly proportional
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Energy stored for linear relationships

  • As long as a spring is not stretched past its limit of proportionality, the work done in stretching (or compressing) a spring can be found using

elastic potential energy (J) = 1/2 x spring constant (m) x extension squared (N/m)

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What is distance and displacement?

  • Distance = how far an object has moved and it is a scalar quantity that doesn't involve direction
  • Displacement = vector quantity and measures the distance and direction in a straight line (starting point)
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What are speed and velocity?

  • Speed and velocity both measure how fast you're going, but speed is a scalar quantity and velocity is a vector quantity
  • Speed is just how fast you're going with no regard to the direction. Velocity is speed in a given direction.
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How do you calculate speed?

distance travelled (m) = speed (m/s) x time (s)

s = v x t

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What are typical everyday speeds?

  • A person walking = 1.5 m/s
  • A person running = 3 m/s
  • A person cycling = 6 m/s
  • A car = 25 m/s
  • A train = 30 m/s
  • A plane = 250 m/s
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What is acceleration?

  • Not the same as velocity or speed
  • Change in velocity in a certain amount of time

acceleration = change in velocity / time

deceleration = negative acceleration

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How do you calculate uniform acceleration?

final velocity - initial velocity = 2 x acceleration x distance

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What is terminal velocity?

  • Friction is always there to slow things down
  • Drag increases as speed increases
  • Objects falling through fluids reach a terminal velocity
  • Terminal velocity depends on shape and area (streamline)
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What is Newton's First Law?

  • If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it'll just carry on moving at the same velocity (same speed and direction)
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What is acceleration linked to?

  • The larger the resultant force acting on an object, the more the object accelerates. The force and the acceleration are directly proportional
  • Acceleration is also inversely proportional to the mass of the object 
  • Newton's second law = resultant force = mass x acceleration
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What is Inertia?

  • Inertia is the tendency for the motion to remain unchanged
  • An object's inertial mass measures how difficult it is to change the velocity of an object
  • Inertial mass can be found using Newton's Second Law rearranged into m = F / a
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What is Newton's Third Law?

  • Newton's Third Law says - When two objects interact, the forces they exert on each other are equal and opposite
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How do you calculate stopping distance?

stopping distance = thinking distance + braking distance

typical car braking distances - 14m at 30mph, 55m at 60mph and 75m at 70 mph

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What factors effect thinking distance?

  • Your speed
  • Your reaction time
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What factors effect braking distance?

  • Your speed
  • The weather or road surface
  • The conditions of your tyres
  • How good your brakes are
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How do reaction times differ?

  • Everyone's reaction time is different, but a typical reaction time is between 0.2 and 0.9 seconds. This can be affected by tiredness, drugs or alcohol
  • Distractions can also affect your ability to react
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What is momentum?

  • Momentum is mainly about how much 'oomph' an object has (it is a vector quantity)
  • greater the mass/velocity = more momentum
  • momentum = mass x velocity
  • momentum before = momentum after
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