Moments and Centre of Mass
Moment (Nm) = force (N) x distance from pivot (m)
If the anitclockwise moment = clockwise moment, the object is stable.
To increase the moment, increase the force used, or the distance from the pivot (eg using a spanner to tun a bolt).
Centre of Mass
Centre of Mass: the point at which the total mass in an object appears to act.
To find an objects centre of mass:
- Tie the object from a corner to a piece of string and put a plumline (weighted piece of string) on the other end.
- Hang this over something (eg a nail in a wall/clamp stand).
- Because there is no moment, the plubline hangs straight through the centre of mass. Draw this line along the object (tricky)
- Repeat but rotate object. Draw another line - where these cross is the centre of mass.
This picture shows the objects hanging and the lines drawn on.
(I do not own any pictures)
Stable: Balanced. centre of mass below the object => no turning force
Unstable: Will not balance as centre of mass is not beneath object => a turning force
Neutral: can easily be knocked over
For an object moving in a circle at a constant speed:
- the object accelerates towards the centre of the circle constantly
- the centripetal force increases as:
the mass or the speed increases
the radius of the circle decreases
Mass: the amount of matter in an object
Weight: the force of gravity acting upon an object with a mass. Measured in Newtons
Circular orbits have one focus point
Elliptical orbits have 2 focus points. The sum of the distance between them is equal
Geostationary: orbits at a certain height above the equator with the same speed as the earth. Looks stationary.
Low polar orbit: orbits over the poles at a low height. Used for monitoring.
tall, thin image = convex [bends out () ]
short, broad image = concave [bends in )( ]
Mirror image: refelction of light
The normal at a point on a mirror is perpendicular to the mirror
For a light ray reflected by a mirror angle of incidence ray = angle of reflection
Real images are used in films
Virtual images cannot be projected
principal focus is at the point the parallel rays are focused by the mirror
forms a real image if the object is beyond the principal focus
or a virtual image if the object is between the mirror and the principal focus
ALWAYS forms a virtual image ()
Refraction is when an object changes the speed light is travelling at as it goes through (it slows down)
Parallel light rays converge (meet) at the principal focus. Any image is upside down.
Parallel light rays diverge (spread out). Tthe principal focus is where the ray leaves the lens
Focal length: distance from principal focus to the centre of the lens.
contain converging lens to get a real image
contain film/pixels that pick up light and form image
can travel through liquids, gases and solids Human ear range: 20-20,000 Hz
can't travel through a vacuum
are longitudinal waves (across => )
can be reflected (echoed) and refracted
- Pitch increases as frequency increase
- Loudness increases as amplitude (height) increases
- Quality of sound depends on the waveform
Shape of sound wave: triangle; weak and mellow. Sqaure; hollow. Sawtooth ( /|/|/| ); buzzy, brassy. Pulse: thin ready sound.
20,000 Hz + (out of human range)
Electronic systems can produce them
Partially refelcted when they meet a boundary between 2 different mediums
Time taken for reflections to reach detector is how far away the boundary is
Can be used for industrial cleaning and quality control
Frequency = 1/Time (secs)
Speed = freq (Hz) x wavelength (m)
If a wire cuts across a magnetic field, potential difference/a current is induced.
Direction of flux, current and motion are at right angles to each other.
Increased 'flux cutting' can be done by:
a stronger magnet
varying the magnetic field (faster)
moving the coil of wire (faster)
DYNAMO EFFECT: produces electricity from the motion (induction)
LEFT HAND RULE:
- ThuMb = Motion
- First = Field
- SeCond = Current
[thumb up, first finger out (like a gun) second figner at right angle to first finger. If confusing, Google 'Newton's left hand rule' for a picture]
Motor effect contd. and Electricity
When the coil of wire cuts perpendicular through the magnetic field, it induces a current. Because it cuts the magnetic fierld and the wire is attatched to a circuit, the current that was induce can travel through the complete circuit, powering a motor.
Step up-step downTransformers:
Step up: from power station to pylons. More efficient, less lost as heat and over the distance travelled.
Stepdown: from pylons into homes. Appliences can't handle such voltage, average plug voltage in EU 240v.
- A star to be is called a protostar. Particles of dust and gas gather together under their gravity. They merge and become more concentrated.
- As it becomes denser, the temperature increases. The nuclei of hydrogen and other light elements fuse together. The core gets hotter and it shines.
- Our sun shines because of the hydrogen fusion at the core. This is the main stage of a stars life. Can continue for billions of years.
- Radiation flows out of the star steadily. This prevents it from collapsing in on itself. This stays balanced until all the hydrogen fuses in the core.
- When the star runs out of hydrogen, it expands. It cools down and becomes a red giant.
- Helium and other elements fuse to form heavier elements.
- When there is no more elemetns to fuse, fusion stops and the star collapses in on itself, becoming a white dwarf. This is a small, hot and dense star. They eventually go cold. Red->Yellow->White.
Supernovs and black holes
Bigger stars collapse and reach the white dwarf stage. Then in a dramatic explosion called a supernova.
This compresses the core into a neutron star, made from neutrons. If it has a large enough mass it will become a black hole. The gravity of a black hole means nothing can escape it, it is so strong.
Low mass star: protostar > main stage > red giant > white dwarf > black dwarf
High mass star: protostar > main stage > red giant > white dwarf > supernove > neutron star > black hole if mass sufficient