- 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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- 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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- 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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- 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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- 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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- 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,
- 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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- 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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