Centre of Mass
An object's centre of mass is the point at which all of the object's mass seems concentrated.
The time period of a pendulum is the time taken to complete a full swing.
Moment is the turning effect of a force.
Clockwise and Anticlockwise moments
When an object is balaced, the total clockwise moment about the pivot is equal to the total anitclockwise moment.
This is the principle of moments.
Levels multiply forces using moments.
An object will topple if the line of action of its weight falls outside the base of the object.
Stability is increased by making the base wider and the centre of mass lower.
Pressure in liquids
Liquids are almost incompressible.
Pressure in liquids is transmitted equally in all directions.
Hydraulic systems multiply force.
An object moving in a circle is constantly changing direction, so is constantly accelerating towards the centre of the circle.
The force between an object in circular motion and the centre of the circle is a centripetal force.
Flow of electric current through a wire produces a magnetic field around the wire.
If a current-carrying wire is placed within another magnetic field, the magnetic fields interact, creating the motor effect.
When a wire cuts across a magnetic field, a potential difference is induced across the ends of the wire.
A transformer has an iron core with two coils of wire around it.
If the primary coil is connected to an a.c. supply an alternating potential difference is induced in the secondary coil.
In a step-up transformer, potential difference is higher across the secondary coil than the primary coil.
In a step-down transformer, potential difference is lower across the secondary coil than the primary coil.
Potential difference and turns of the coils
Potential differences across the primary and secondary coils and the number of turns in the coils are related.
100% efficient transformer
In a transformer with 100% efficiency, the power input to the primary coil is equal to the output from the secondary coil.
Most electronic devices use a lower potential difference than the 230V mains supply.
Step-down transformers are used.