Physics 3

• 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.

• 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.

• For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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

• 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.

• 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.

• Sound waves;
  • can travel through liquids and gases and in solids,
  • cannot travel in a vacuum,
  • are longitudinal waves,
  • can be reflected and refracted.

• 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.

• 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.

• 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.

• 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.

• A transformer consists of a primary coil and a secondary coil wrapped on the same iron core.

• Transformers only work using alternating current.

• 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.

• As the universe expanded, it cooled and uncharged atoms formed.

• The force of gravity pulled matter into galaxies and stars.

• 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)

• Elements as heavy as iron are formed inside stars as a result of nuclear fusion.

• Elements heavier than iron are formed in supernovas.