# Physics 3

An review of the Physics topic!!

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## Moments

• 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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## Stability

• 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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## Satellites

• 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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## Reflection

• 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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## Refraction

• 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

• 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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## Ultrasound

• 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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## Transformers

• 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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## Galaxies

• 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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