Physics 3

An review of the Physics topic!!

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  • The moment of a force F about a pivot is F x D, where D is the perpendicular distance from the pivot to the line of action of the force.
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Centre of Mass

  • The centre of mass of an object is the point where its mass may be thought to be concentrated.
  • When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.
  • The centre of mass of a symmetrical object is along the axis of symmetry.
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Moments in Balance

  • For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.
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  • The stability of an object is increased by making its base as wide as possible and its centre of mass as low as possible.
  • An object will tend to topple over if the line of action of its weight is outside the base.
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Circular Motion

  • For an object moving in a circle at constant speed;
    • the object accelerates continuously towards the centre of the circle,
    • the centripetal force needed increases
      • as the mass or the speed of the object increases,
      • as the radius of the circle decreases.
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Gravitational Attraction

  • The force of gravity between two objects;
    • is an attractive force,
    • is bigger the greater the mass of each object is,
    • is smaller the greater the distance between the two objects is.
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Planetary Orbits

  • To stay in orbit at a particular distance, a small body must move at a particular speed around a larger body.
  • The larger an orbit is, the longer the orbiting body takes to go round the orbit.
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  • A satellite in a geostationary orbit has a period of 24 hours and stays at the same position directly above the Earth's equator.
  • Geostationary orbits are usually used for communication satellites.
  • Monitoring satellites are usually in low polar orbits.
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  • The normal at a point on a mirror is perpendicular to the mirror.
  • For a light ray reflected by a mirror,
    • angle of incidence = angle of reflection
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Curved Mirrors

  • The principal focus of a concave mirror is the point where parallel rays are focused to by the mirror.
  • A concave mirror forms;
    • a real image if the object is beyond the principal focus of the mirror,
    • a virtual image if the object is between the mirror and the principal focus.
  • A convex mirror always forms a virtual image of an object.
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  • A camera contains a converging that is used to form a real image of an object.
  • A magnifying glass is a converging lens that is used to form a virtual image of an object.
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  • Sound waves;
    • can travel through liquids and gases and in solids,
    • cannot travel in a vacuum,
    • are longitudinal waves,
    • can be reflected and refracted.
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Musical Sounds

  • The loudness of a note depends on the amplitude of the sound waves.
  • The pitch of a note depends on the frequency of the sound waves.
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  • Ultrasonic waves are;
    • sound waves that have a frequency above 18000 kHz,
    • partly reflected at a boundary between two substances,
    • non-ionisng.
  • Uses of ultrasonic waves include cleaning devices, flaw detectors and medical scanners.
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The Motor Effect

  • In the motor effect, the force;
    • is increased if the current or the strength of the magnetic field is increased,
    • is at right angles to the direction of the magnetic field and to the wire,
    • is reversed if the direction of the current or the magnetic field is reversed.
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Electromagnetic Induction

  • When a wire cuts the lines of a magnetic field, a potential difference is induced in a wire.
  • If the wire is part of a complete circuit, the induced p.d. causes a current in the circuit.
  • The current is increased if the wire moves faster or a stronger magnet is used.
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  • A transformer consists of a primary coil and a secondary coil wrapped on the same iron core.
  • Transformers only work using alternating current.
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Transformers and the National Grid

  • Transformers are used to step voltages up or down
  • The transformer equation is;

Primary p.d., Vp                      Np

Secondary p.d, Vs                  Ns

  • where Np = number of primary turns and Ns = number of secondary turns.
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  • As the universe expanded, it cooled and uncharged atoms formed.
  • The force of gravity pulled matter into galaxies and stars.
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The Life History of a Star

  • Low Mass Star;

Protostar --> Main Stage --> Red Giant --> White Dwarf --> Black Dwarf

  • High Mass Star (after the White Dwarf stage);

White Dwarf --> Supernova --> Neutron Star --> Black Hole (if sufficient mass)

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How the Chemical Elements Formed

  • Elements as heavy as iron are formed inside stars as a result of nuclear fusion.
  • Elements heavier than iron are formed in supernovas.
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