# FORCES AND WAVES

Physics

HideShow resource information
• Created by: katie
• Created on: 28-05-12 10:15

## turning forces and centre of mass

A moment is the turning effect of a force:

MOMENT (Nm) = FORCE (N) x perpendicular DISTANCE (m) between line of actions and pivot

EXAMPLE - the force of the spanner causes a turning effect or moment on the nut, a larger force would mean a larger moment. Using a longer spanner, the same force can exert a larger moment becayse the distance from the pivot is greater. To get the maximum moment (or turning effect) you need to push at right angles (perpendicular) to the spanner. Pushing at any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot smaller.

The centre mass hangs directly below the point of suspension:

You can think of the centre of mass of an object as the point at which the whole mass is concentrated. A free suspended object will swing until its centre of mass is vertically below the point of suspension. This means you find the centre of mass of any flat shape.

1 of 14

## balanced moments and stability

If the anticlockwise moments are equal to the clockwise moments, the objects WONT turn.

Total anticlockwise moments = total clockwise moments

If they are not equal...

If the anticlockwise moments do not equal the total clockwise moments, there will be a resultant moment (so the object will turn)

Example - low and wide objects are most stable because they have a wide base and a low centre of mass.

2 of 14

## circular motion

Circular motion - velocity is constantly changing

• velocity is both the speed and direction of an object
• if an object is travelling in a circle it is constantly changing direction, which mens it's accelerating
• this emans there must be a force acting on it
• this force that keeps something moving in a circle is called a centripetal force
• in the exam you might be asked to say WHICH FORCE is providing the centripetal force in a given situation. It can be TENSION or FRICTION or even GRAVITY.

Centripetal force dependson mass, speed and radius

• the faster an objects, the bigger the centripetal force has to be to keep it moving in a circle
• likewise, the heavier the object, the bigger the centripetal force has to be to keep it moving in a circle
• and you need larger force to keep something moving in a smaller circle - it has 'more turning' to do
3 of 14

## gravity and planetary orbits

Gravity is the centripetal force that keeps planets in orbit:

• gravity is the force of attraction between masses (the larger the masses the greater the force)
• this gravitational force can act as the centripetal force that keeps one object moving in orbit (an orbit is possible when theres a balance between forward motion of the object and the gravitiational force pulling it inwards)
• planets always orbit around stars
• these orbits are all slightly ellipitcal (elongated circles) with the sun at one focus of the ellipse
• the further the planet is from the sun, the longer is takes to orbit

Gravity decreases quickly as you get further away:

• with very large masses like stars and planets, gravity is very big and is felt a long way out
• the closer you get to a star or a planet, the stronger the force of attraction
• to counteract the stronger gravity, planets nearer the sun move faster covering their orbit quicker
• comets are also held in orbit by gravity as are moons and satellites and space stations
• the size of the force of gravity decreases very quickly with increasing distance
4 of 14

## gravity and planetary orbits

1) GEOSTATIONARY SATELLITES ARE USED FOR COMMUNICATIONS

• are in high orbits over the equator
• take exactly 24 hours to complete
• stay above the same point on the earths surface because the earth rotates with them
• makes them ideal for telephone and TV becuse they are always in same place
• can transfer signals from one side of earth to another in a fraction of a second
• ﻿

2) LOW POLAR ORBIT SATELLITES ARE FOR WEATHER AND SPYING

• in a low polar orbit
• satellit sweeps over both poles whilst earth rotates beneath it
• takes just a few hours for a full orbit
• each time satellite comes round it can scan the next bit of the globe
• allows whole surface of the planet to be monitored each day
• geostationary satellites are too high to take good weather or spying photos but satellites in polar orbits are nice and low
5 of 14

## images

A real image is actually there - a virtual image is not -> to describe an image properly, you need to say 3 things: 1) HOW BIG IT IS compared to the object 2) whether is UPRIGHT OR INVERTED (upside down) 3) whether its REAL OR VIRTUAL

• a real image is where the light from an object comes together to form an image on a 'sceen'  (like the image formed on an eyes retina - the screen at the back of an eye)
• a virtual image is when the rays are diverging, so thr light from the object appears to be coming from a completely different place
• Reflection of light lets us see things:
• relection of light is what allows us to see objects (light bounces off them into our eyes). When light reflects from an uneven surface such as a peice of paper the light reflects off at all angles and you get a diffuse reflection. When light reflects from an even surface (smooth and shiny like a mirror) then it's all reflected at the same angle and you get a clear reflection
• Refraction - light bends as it changes speed:
• refraction of light is wqhen the waves change direction as they enter a different medium. This is caused entirely by the change in speed of the waves. Thats what makes ponds look shallower than they are - light reflects off the bottom and speeds up when it leaves the water making the bottom look like its nearer than it is.
• ﻿
6 of 14

## mirrors

Draw a ray diagram for an imagine in a plane mirror

Curved mirrors are more complicated

7 of 14

## mirrors

Draw a ray diagram for an image in a concave mirrow

1) an incident ray parallel to the axis will pass through the focal point when its reflected 2) an incident ray passing through the focal point will be parallel to the axis when its reflected

Draw a ray diagram for an imagine in a convex mirror

1) an incident ray parallel to the axis will reflect so that the reflected ray seems to come from the focal point 2) an incident ray that can be extended to pass through the focal point will be parallell to the axis when its reflected

8 of 14

## lenses

Draw a ray diagram for an image through a converging lens

Distance from the lens affects the image

Draw a ray diagram for an image through a diverging lens

9 of 14

## uses - magnification and cameras

Magnifying glasses use convex lenses

Magnification formula -> Magnification = image height/object height

Taking a photo forms an image on the film

• the image of the film is a real image because light rays actually meet there
• the image is smaller than the object because the objects alot further away than the focal length of the lens
• the image is inverted - upside down
10 of 14

## sound waves

Sound travels as a wave -

caused by vibrating objects (longitudinal waves)

Sound waves can reflect and refract -

reflected by hard flat surfaces  (carpets and curtains act as absorbing surfaces which will absorb sounds rather than reflect them). Sound waves will also refract (change direction) as they enter different media. As they enter denser material, they speed up.

We hear sounds in the range 20 -20,000 Hz -

frequency of a wave (in Hz) is the number of waves in one second

Sound does not travel in a vacuum -

they are transmitted by vibrating particles so they cant travel through a vacuum (BELL JAR EXPERIMENT)

11 of 14

## sound waves

Loudness increases with amplitude: the greater the amplitude of a wave, the more energy it carries, this means in sound it will be louder (bigger amplitude means a louder sound)

The higher the frequency, the higher the pitch: high frequency waves sound high pitched and low frequency waves sound low pitches. Frequency is the number of complete vibrations each second

Quality of a note depends on the waveform: CRO trace - clear pure sound produces a smooth rounded waveform called a SINE wave.

12 of 14

## ultrasound

• Ultrasound is sound with a higher frequency than we can hear:
• Electical devices can be made which produce electrical oscillations of any frequency, can easily be converted into mechanical vibrations to produce sound waves beyond the range of human hearing. This is called ultrasound and it has loads of uses:
• You can use CRO Traces to compare amplitudes and frequencies:
• Ultra sound waves get partially reflected at a boundary between media:
• When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media and some is transmitted (and refracted). This is partial reflection.
• You can point a pulse of ultrasound at an object and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back.
• The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is.
• You can use Oscilloscope Traces to find boundaries:
• The CRO trace can show an ultrasound pulse reflecting off two seperate boundaries. In exam, they might give you the frequency and wavelength of the ultrasound and leave you to work out the speed using = v = f (upside down y)
13 of 14

## ultrasound

Ultrasound vibrations are used in industrial cleaning: - can be used to clean delicate mechanisms without them being dismantled, the ultrasound waves can be directed onto very precise areas and are effective at removing dirt and other deposits with form on the delicate equipment, the high frequency vibrations of ultrasound make the components of a piece of equipment vibrate at a high frequency, the dirt on the equipment vibrates too and breaks up the dirt and crud into small particles which fall off equipment (same technique used by dentists to clean teeth)

Ultrasound is used in industrial quality control: - ultrasound waves can pass through something like a metal casting and whenver they reach a boundary between two different media (like metal and air) some of the wae is reflected back and detached, the exact timing and distribution of these echoes gives detailed info about the internal structure, echoes are usually processed by computer to produce a visual display of what object must be like inside, cracks that shouldnt be there will show up

Ultrasound imaging is used for pre-natal scanning of a fetus: - follows same principle as industrial quality control because as the ultrasound hits boundaries between different media some of the wave is reflected back, in the uterus there are boundaires between the amniotic fluid that the fetus floats in and the bosy tissues of the fetus itself, the reflected waves are processed by computer to produce a video image of the fetus and can show what sex the fetus is

Noone knows for sure whether ultrasound is safe in all cases but xrays would definately be dangerous to the fetus.

14 of 14