GCSE PHYSICS UNIT 3 KEY POINTS ON EVERY TOPIC

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 TURNING EFFECT OF THE FORCE CALLED THE MOMENT OF THE FORCE CAN BE INCREASED BY:

• INCREASING THE SIZE OF THE FORCE
• USING A SPANNER WITH A LONGER HANDLE AS YOU WILL NEED TO EXERT LESS FORCE TO UNTIGHTEN THE NUT
• PUSH AT RIGHT ANGLES TO THE SPANNER

YOU CAN WORK OUT THE MOMENT FORCE USING THIS EQUATION

MOMENT (Nm) = FORCE (N) X PERPENDICULAR DISTANCE FROM THE PIVOT TO THE LINE OF ACTION OF THE FORCE (m)

THE CENTRE OF MASS OF AN OBJECT IS THE POINT WHERE ITS MASS MAY BE THOUGHT TO BE CONCENTRATED

WHEN A SUSPENDED OBJECT IS AT EQUILIBRIUM, ITS CENTRE OF MASS IS DIRECTLY BELOW THE POINT OF SUSPENSION

THE CENTRE OF MASS OF A SYMMETRICAL BODY IS ALONG THE AXIS OF SYMMETRY

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

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 ITS BASE.

BODIES WITH A LOW CENTRE OF MASS AND A BROAD BASE ARE MORE STABLE THAN BODIES WITH A HIGH CENTRE OF MASS AND A NARROW BASE.

FOR AN OBJECT MOVING IN A CIRCLE AT A 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
• 

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 2 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.

TO STAY IN ORBIT AT A PARTICULAR DISTANCE, A PLANET MOVE AT A PARTICULAR SPEED AROUND THE SUN

• IF ITS SPEED IS TOO LOW, IT WILL SPIRAL INTO THE SUN.
• IF ITS SPEED IS TOO HIGH, IT WILL FLY OFF ITS ORBIT AND MOVE AWAY FROM THE SUN.

THE FURTHER THE PLANET IS FROM THE SUN, THE LESS ITS SPEED IS AS IT MOVES ROUND THE SUN.

THATS BECAUSE THE FORCE OF GRAVITY IS WEAKER AS IT IS FURTHER FROM THE SUN.

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 SUALLY 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 OBY THE MIRROR

CURVED MIRRORS

THE 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
• THE MAGNIFICATION OF THE IMAGE IS: THE IMAGE HEIGHT/THE OBJECT HEIGHT

REFRACTION OF LIGHT IS THE CHANGE OF DIRECTION OF A LIGHT RAY WHEN IT CROSSES A BOUNDARY BETWEEN TWO TRANSPARENT SUBSTANCES

IF THE SPEED IS REDUCED, REFRACTION IS TOWARDS THE NORMAL (E.G AIR TO GLASS)

IF THE SPEED IS INCREASED, REFRACTION IS AWAY FROM THE NORMAL (E.G GLASS TO AIR)

A REAL IMAGE IS FORMED BY CONVERGING LENS IF THE OBJECT IS FURTHER AWAY THAN ITS PRINCIPAL FOCUS

A VIRTUAL IMAGE IS FORMED BY DIVERGING LENS AND BY CONVERGING LENS IF THE OBJECT IS NEARER THAN THE PRINCIPAL FOCUS

USING LENSES

A CAMERA CONTAINS A CONVERGING LENS 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, GASES AND SOLIDS,
• CANNOT TRAVEL IN A VACUUM
• ARE LONGITUDINAL WAVES
• CAN BE REFLECTED AND REFRACTED

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

ULTRA SONIC WAVES ARE:

• SOUND WAVES THAT HAVE A FREQUENCY ABOVE 18,000 kHz
• PARTLY REFLECTED AT A BOUNDARY BETWEEN 2 SUBSTANCES,
• NON-IONISING

USE OF ULTRASONIC WAVES INCLUDE:

• CLEANING DEVICES
• FLAW DETECCTORS
• 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 TO THE WIRE
• IS REVERSED IF THE DIRECTION OF THE CURRENT OR THE MAGNET FIELD IS REVERSED

GRAPHITE IS A FORM OF CARBON WHICH CONDUCTS ELECTRICITY AND IS VERY SLIPPY. IT THEREFORE CAUSES VERY LITTLE FRICTION WHEN IT IS IN CONTACT WITH THE ROTATING COMMUTATOR

WHEN A WIRE CUTS THE LINES OF A MAGNETIC FIELD, A POTENTIALDIFFERENCE 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

TRANSFORMERS

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 TRANSFORMERS EQUATION IS:

p.d ACROSS PRIMARY, Vp / p.d ACROSS SECONDARY, Vs =

NUMBER OF TURNS ON PRIMARY, Np /NUMBER OF TURNS ON SECONDARY,Ns

• AS THE UNIVERSE EXPANDED, IT COOLED AND UNCHARGED ATOMS FORMED
• THE FORCE OF GRAVITY PULLED MATTER INTO GALAXIES AND STARS
•  

THE LIFE HISTORY OF A STAR

LOW MASS STAR:

• PROTOSTAR TO MAINSTAGE TO RED GIANT TO WHITE DWARF TO BLACK DWARF

HIGH MASS STAR, AFTER THE WHITE DWARF STAGE:

• WHITE DWARF TO SUPERNOVA TO NEUTRON STAR TO BLACK HOLE IF SUFFICIENT MASS
• ELEMENTS AS HEAVY AS IRON ARE FORMED INSIDE STARS AS A RESULT OF NUCLEAR FUSION