Physics - Moments
- The turning effect of a force is the 'moment'.
- The moment of a force depends how big the force is in the first place.
- The unit for moments is Newton meters (Nm).
- The size of a turning force can be calculated by this equation --> Force x perpendicular distance from centre.
- Shorter distances need more force. For example if you use a short spanner to undo a bolt it will need more force than if you use a longer one.
- The moment depends on the perpendicular distance from the line of action of the force to the axis of rotation.
Physics - Centre of mass
Centre of mass -
- For symetrical objects, the centre of mass is along the lines of symmetry.
- When an object is hanged, it will come to rest with the centre of mass directly under the point of suspension.For example a child swinging on monkey bars, their hands would be the suspension point and they would come to rest directly beneah them.
- An objects centre of mass is the point it comes to when all the other parts of the object have equal mass. For example the centre of a circle is the centre of mass as everywhere else on the shape has the same amount of mass on it. A scalene triangle would be harder to find the centre of mass because of all the different weight around the different sides.
Physics - circular motion
Circular motion -
- The force in a circle is called centripetal force.
- The amount of centripetal force neede to keep an object moving depends on:
- The mass of the object. (The heavier the object, the more force needed)
- The speed it is travelling. (The higher the speed, the more force needed)
- The radius of its 'flight'. (The smaller the circle, the more force is needed)
- Examples of centripetal force is a tennis ball on a string being swung around your head, motorbikes going around a corner on a racetrack, jets performing tight turn manouvers in the air and also fairground rides.
Physics - Gravity and the solar system
Gravity and the solar system -
- All the planets in our solary system are in 'elliptical orbit' around the sun. (Ellipse is like a squashed circle/oval).
- In the ellipse there are two points each called a focus.
- Gravity is the centripetal force that keeps the planets in orbit.
- The size of the force depends on the masses of the 'bodies' and the distance between them. For example the sun and earth.
- The further the planet is from the sun, the longer the orbit takes. This is because they orbit slower because the gravitational pull on them is less, the further away they are.
- The speed of a planet in orbit does not depend on its mass. The force needed to keep a 'body' in a circular motion does increase the mass as the mass of the 'body' increases, but the force of gravity between two 'bodies' also increases if the mass increases, so the two effects cancel each other out.
Physics - Satellites
- A 'satellite' is any 'body' that orbits around another 'body'. Such as the Earth is a satellite to the sun.
- The time a satellite takes to orbit the earth depends on it's distance from Earth.
- The higher the orbit, the longer the orbit.
- Satellites must have an exact speed to be travelling when it hits orbit, so it can stay in orbit. To much speed and the satellite will shoot off into space, to little and it'll come crashing down to Earth. Fast.
- Communication satellites are in 'Geostationary orbit' above the equator. Weather satellites are also sometimes put into this orbit.
- Many other satellites such as 'Monitoring satellites' are put into polar orbit. In this case, the satellites orbit around the wolrd 'vertically'. From pole to pole.
Physics - Stars and planets
Stars and planets -
- The sun shines because of the 'nuclear fusion reactions' within it.
- When the sun was formed, it consisted mainly of hydrogen nuclear reactions, such as converting hydrogen nuceli to helium nuclei. At present the sun is about 70% hydrogen.
- Stars form when clouds of space dust and gas are pulled together by gravity. The cloud gets denser and heats up, which causes it to glow. As more and more mass is attracted, the temperatures and preassures in the centre become high enough to force hydrogen atoms to 'fuse' together. The radiation pressure tries to make the star expand but this is then balanced by gravitational forces. The star stabilizes, and remains stable for millions of years with the constant reactions happening within.
- The main idea for how the planets were formed is the 'Nebular hypothesis theory'. This theory suggests that when the sun was formed, it started spinning, making it flatten. The gas towards the edge of the 'disc' cooled and began to clump together into 'grains'. These 'grains' then moved around the 'disc' hitting each other and 'sticking together'. This meant that the 'bodies' gradually built up. They then generated their own gravitational pull. Pulling in more dust and 'grains' and eventually, they settled and became planets.
- The asteroids are 'grains' that did not grow enough to become planets.
Physics - Life cycle of stars
Life cycle of stars -
- The life cycle of a normal sized star goes like this -->
- Stellar nebular (500 million years)
- Star (10 million years)
- Red giant (1000 million years)
- Planetary nebula (gradually cools)
- White dwarf
- The life cycle of a 'massive star' goes like this -->
- Stellar nebula ( 0.1 million years )
- Massive star ( 15 million years )
- Red supergiant ( 1 million years )
- Supernova ( A few minutes )
- Black hole OR Neutron star
Physics - Plane mirrors
Plane mirrors -
- Plane / flat mirrors are the simplest kind of mirrors.
- Angles of reflected rays are always measured from the 'normal', this is a line which is at a right angle to the mirror.
- The 'angle of incidence' is always equal to the angle of reflection. This line is the angle at which the 'incident ray' hits the mirror. The point of incidence is where it hits the mirror. And the angle of reflection is the angle at which the rays reflect off the mirror.
- A mirror reflection is called a 'virtual image'. The rays of light cannot pass through the image but only 'appear' to.
- A projected image is known as a 'real image'. The rays of light 'go to' the image and you can touch it.
- The image in a plane mirror is -->
- The same size as the object it is reflecting. For example a person.
- The same distance from the mirror as the object.
Physics - Curved mirrors
Curved mirrors -
- There are two types of curved mirrors. These are 'convex' and 'concave' mirrors.
- Convex mirrors make light 'diverge / spread out'. If you shine a set of parallel rays at a convex mirror, they appear to come from a point behind the mirror itself. These images are always upright and 'virtual'. They are also always 'diminished / smaller than the object'. Convex mirrors are used at awkwrad junctions on roads for example.
- Concave mirrors make light 'converge / come together'. They produce different kinds of images, depending on how far away from the mirror the object is. To show how these mirrors work, you can draw a 'ray diagram'. On these diagrams, you only need to draw three rays of light.
- To work out the magnification in a mirror, use this equation -->
- Magnification = 'Image height' divided by 'object height'
- The nature of the image changes with the distance of the object. This is how it changes -->
- Further than 2F - Real / Inverted / Diminished / Closer to mirror.
- 2F - Real / Inverted / Same size / Same distance from mirror.
- Between 2F and F - Real / Inverted / Magnified / Further from mirror.
- F - No image (reflected rays are parallel to axis).
- Closer than F - Virtual / Upright / Magnified / Behind mirror.
Physics - Refraction
- Refraction is when a ray of light changes speed, and direction. The light 'bends'.
- The way the light bends depends on how fast the light travels in two materials, and also the angle at which it hits the 'interface'. The greater the difference in speed in the two materials, the more the light is bent.
- Different situations effect how the light bends. These are the effects -->
- Light slows down when it enters glass from air. If it is travelling along the 'normal', it does not change direction.
- If the light slows down and is not on the normal, it refracts towards the 'normal'.
- If the light speeds up and is not on the normal, it refracts away from the 'normal'.
- White light consists of a 'spectrum' of light of different colours.
- Red light is refracted by the smallest angle, and violet light the greatest angle.
- These different spectrum colours 'dispearse / spread out' when refracted.
- The best way to 'dispearse' the colours is to use a triangular prism.
Physics - Lenses
- Lenses are pieces of glass or transparent material that are shaped to bend light in particular ways. The two main lenses are 'converging' and 'diverging' lenses.
- There are three main ways to work how images will be formed using curved lenses -->
- A ray parallel to the axis bends so that it passes through the focus of the lense.
- A ray from the focus emerges parallel to the axis.
- A ray through the centre of the lense passes straight through without bending.
- Ray diagrams can be used to help work out the above.
- Different lenses and different distances make the images change. This is how -->
- Diverging - Object can be any distance - Virtual / Upright / Diminished / Closer to lense than object.
- Converging - Further than 2F - Real / Inverted / Diminished / Between F and 2F.
- 2F - Real / Inverted / Same size / at 2F.
- Between 2F and F - Real / Inverted / Magnified / Further than 2F.
- F - No image (emerging rays are parallel to axis).
- Closer than F - Virtual / Upright / Magnified / Same side of lense as object.
- The formula for working out magnification is -->
- Magnification = 'Image height' divided by 'Object height'.
Physics - Sound
- Sound is made when things 'vibrate'. A vibrating object makes the air around it vibrate, passing it on to other air particles and so on until the vibration energy runs out.
- Sound always needs a 'medium' in which to travel (Solid / Liquid / Gas). This also means that sound cannot travel through a vacuum.
- Sound waves can be refracted when they change speed like light can. For example sound travels faster in water than in air, meaning the direction of sound changes when it enters water.
- Sound varys in 'pitch', which is how high or low they sound.
- The number of sound waves in one second is the 'frequency' and this is measured in 'Hertz (Hz)'. The higher the frequence, the higher the pitch of the note.
- The loudness of the sound depends on the amplitude. The larger the amplitude, the louder the note.
- On an 'Oscilloscope', the height of the wave is the amplitude and the length of the wave is the time (or the pitch of the note).
- The 'Oscillioscope's' reading is called the 'waveform' and this is called the 'quality' of the note.
Physics - Ultrasound
- Sounds higher than what the human ear can hear (20 - 20,000 Hz) are called ultrasounds.
- Ultrasound is used to 'see' things we cannot see with 'visible light'.
- An 'Ultrasound scanner' works like this -->
- The probe emits and receives Ultrasound waves.
- A gel is used to stop the Ultrasound just reflecting from the skin.
- Some of the sound is reflected when the Ultrasound waves pass into a different medium, such as fat or bone.
- On an Ultrasound screen, the further down the screen, the longer the echo took to get back to the machine.
- Ultrasound waves can be used for cleaning objects.
- Distance = Speed x Time.
Physics - Electric motors
Electric motors -
- Electrical currents create 'Magnetic fields'.
- When a current flows through a wire it creates a magnetic field around it. If the wire is then placed in a magnetic field of a magnet, the two magnetic fields affect each other and the wire experiences a force. This is known as a 'motor effect'.
- The size of the force produced can be increased by -->
- An increasing current.
- Increasing the length of the magnetic field.
- The direction of movement reverses if -->
- The direction of the current reverses.
- The direction of the magnetic field reverses.
Physics - Making electricity
Making electricity -
- Electricity generated in power stations is made by rotating coils of wire in a magnetic field. This process is called 'Electromagnetic induction'.
- When a piece of wire is moved through a magnetic field, a potential difference is 'induced' across the ends of the wire. If the wire is part of a circuit, the potential difference causes a current to flow in the wire. The wire must 'cut' the magnetic field. No potential difference is 'induced' if the wire moves along the magnetic field lines.
- If the direction of movement is changed, the direction of the 'induced' current is changed. The currents direction also depends on which way round the magnet is held.
- To create a continuous current, you must keep the magnet moving 'relative' to the coil of wire. It does not matter whether it is the coil, or the magnet that is moving. To induce the current, the coil must be moved over the magnet or the magnet through the coil.
- The induction also occurs if you spin a coil in a magnetic field, because the wires in the coil 'cut' the magnetic field as the coil spins. An example of this is a bike 'dynamo' which powers the lights on a pushbike.
Physics - Transformers
- The size of an alternating potential difference can be changed using a 'transformer'.
- A transformer is made of two coils of wire, wrapped around an iron core. The primary coil acts as an electromagnet, and magnetises the iron core. Because the direction of the current in an alternating supply keeps changing, the magnetic field produced by the coil keeps changing.
- If there are more coils on the secondary wire, there will be a higher output, the voltage will be higher than the primary wire. This is known as a 'step-up transformer'. If there are less on the secondary, the opposite will happen and less output will be produced with less voltage than the primary wire. This is a 'step-down transformer'.