# Physics 3

Revision cards for physics 3 exam

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• Created by: Katie
• Created on: 05-10-11 19:26

## Moments

MOMENT = FORCE X PERPENDICULAR DISTANCE FROM PIVOT

• A turning effect
• Cause objects to turn or rotate around a fixed point- the pivot or fulcrum
• Are caused by forces but are not forces themselves
• Act in two ways- clockwise or anticlockwise
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## Centre of mass

• The point at which the mass of an object is thought to be concentrated﻿
• Where lines of symmetry cross is the centre of mass of a regular shape
• When suspended, the centre of mass of an object is below the suspended point
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## Toppling and stability

• The extent to which an object resists toppling
• Sliding is not toppling
• When the centre of mass acts within an object, it will not topple
• The taller an object, the higher the centre of mass, causing the topple angle to become smaller meaning that the object will topple easier
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## Newtons laws

1) An object will stay at rest or stay in constant motion unless acted on by an external force.

• Basically, unless something else happens to it, an objects motion will not change

2) FORCE = MASS X ACCELERATION

• A bigger mass will have a smaller acceleration when pushed by the same force
• Increasing the force will increase the acceleration of an object

3) For every action, there is an equal and opposite reaction

• If an object exerts a force on another object of the same mass, the force will be 'reflected'
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## Resultant forces

• The result of all forces acting on an object
• Forces in the same direction are added together
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## Centripetal force

FORCE =(MASS X VELOCITY²) / RADIUS

• Holds an object in circular motion

• Measured in newtons
• Effected by:
• Mass of the object
• Radius of the circle
• Velocity of the object
• Increased by:
• Increased mass
• Increased speed
• Decreased by:
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## Gravity

• Acts between all objects that have mass
• Infinite range
• Big things have a large gravitational pull
• Small things have a small gravitional pull
• Inversely proportional to distance.. the greater the distance, the smaller the force
• Provides the centripetal force to keep planets and moons in orbit
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## Satellites

• Natural and unnatural
• Always falling towards the earth
• Two types of orbit:
• Geostationary- have an orbital period that is the same as the earths rotational period, making them seem stationary from the earth's surface. Mostly used for communication and navigation
• Polar- orbits almost perpendicularly to the equator so that it passes over both poles in each orbit. Mainly used for monitoring things like weather
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## Mirrors

• Three types:
• Plane
• Concave
• Convex
• The normal line is perpendicular to the mirror, all angles are measured against this line
• Dashes on the back of a line shows that the surface is reflective
• Incidence ray- the ray from the source to the mirror (on the way)
• Reflected ray- the ray from the mirror to the environment (after hitting the mirror)
• In a plane mirror:
• The image is as far behind the mirror as the object is in front
• The line joining the object to the image is the normal line
• The size of the object is the same size as the image
• The image formed behind the mirror is virtual
• The image is laterally inverted (mirrored)
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## Curved mirrors

• Centre of curvature- The centre of the circle the mirror is part of
• Principle axis- The line connecting centre of curvature to the mirror, usually drawn horizontally
• Focal point- The point at which all reflected rays will pass
• Focal length- The distance between the mirror and the focal point
• Focal point is always on the principle axis
• To find the focal point, parallel rays of incidence must be shon onto a mirror, the point at which they all cross is the focal point
• To say an object is inside the focal point, means that it is between the focal point and the mirror
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## Convex mirrors

• When parallel rays are shon onto a convex mirror, the rays of reflection are diverged and never cross
• The focal point is found by tracing the rays of reflection back behind the mirror, this is called carrying on the line of reflection
• Focal points of convex mirrors are virtual- the light doesn't actually go there
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## Waves through a differentiating media

• The speed and direction of waves change as they enter or exit different materials
• As light enters a more dense material, it slows down and bends towards the nomal line
• As light enters a less dense material, it slows down and bends away from the normal line
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## Converging lenses

• As light passes through a converging lens, the rays are refracted
• The image on the opposite side is real because the light actually goes there but we cannot see it because the light is not reflected back to the eye
• When an object is far away from the lens, the image is real, inverted and smaller
• As the object moves closer to the lens, the image becomes real, inverted and bigger
• When the object is really close, the rays are diverged and will not meet again. However, the observer still sees an image, believing that is comes from behind the object. This image is virtual, upright and bigger
• Uses:
• The eye
• Magnifying lens
• Cameras

Drawn like this:

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## Diverging lenses

• Causes all rays of incidence to diverge
• Meaning that all images and focal points are virtual because the light doesn't actually go to where we are seeing the image
• To find the focal point of a diverging lens, shine parallel rays through the lens and trace back the refracted rays

Drawn like this:

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## Magnification

MAGNIFICATION = IMAGE HEIGHT / OBJECT HEIGHT

• Image is larger than object when magnification is >1
• Image is the same size as object when magnification =1
• Image is smaller than object (diminished) when magnification is <1
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## Prisms

• Refract light into different colours
• Violet=shortest wave length, bent the most
• Red=longest wave lenth, bent the least
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## Sound waves

• Caused by vibrations
• Drawn by oscilloscopes
• Loud sounds have a high amplitude
• High pitch sounds have a high frequency
• Cannot be heard in a vacuum
• Ultrasound- Sound with a frequency too loud for humans to hear
• Uses:
• Sonography- baby scans
• Sanitation- bacteria can be disintegrated by ultrasonic waves
• Welding- Ultrasonic waves can create heat energy between two objects and cause them to melt together
• Humans can hear sounds with frequency between 20-20,000 Hz

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## Magnetism

• Two poles on a magnet:
• North
• South
• Magnetic field- The area around a magnetic object in which other materials experience the force of the magnet
• Only three magnetic elements:
• Iron
• Nickel
• Cobalt
• When drawing a magnetic field, arrows point from north to south to show direction
• Closer lines=stronger field
• Magnetic field lines crossing causes a force and makes things move
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## Electricity and magnetism

• An electric current enduces a magnetic field
• The motor effect:
• A wire carrying a current has magnetic field lines
• This wire is put in the magnetic field of a magnet
• The two sets of magnetic field lines cross, causing the wire to get flung away from the magnet
• However, if the wire is parallel to the magnetic field, it will not experience the motor effect
• Direction of force is reversed if:
• Direction of current is reversed
• Direction of magnetic field is reversed (magnet is turned around)
• Force can be increased if:
• The magnet is stronger
• The current in the wire is bigger
• The amount of wire in the field is greater (coil up the wire)
• MOTOR EFFECT IS MAGNET+ELECTRIC CURRENT = MOVEMENT
• GENERATOR EFFECT IS MANET+MOVEMENT = ELECTRIC CURRENT
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## Transformers

• Electromagnetic Induction- The creation of electricity when magnetic field lines cross
• As electricity is sent through the coiled wire on the primary transformer, magnetic field lines grow, alternating current causes the field lines to constantly grow and shrink
• The magnetic field lines cutting through the wires of the secondary transformer induces a current
• The voltage is induced when there is relative motion between a magnetic field and a conductor caused by:
• A conductor cutting magnetic field lines
• Magnetic field lines cutting a conductor
• A change in the strength of the magnetic field linking a conductor
• The produced voltage can be increased by:
• Increasing the speed of movement
• Increasing the strength of magnetic field
• Increasing the number of turns on the coil
• Increasing the area of the coil
• In order to keep the current being made, we must use an alternating current to keep the motion to generate electricity
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