# Physics module 3 101

GCSE Notes for physics module 3!

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• Created by: robert
• Created on: 24-04-10 11:16

## Physics 3 - forces and waves & magnetism and stars

Including:

turning forces and centre of mass, balanced moments and stability, circular motion, gravity and planetary orbits, images, mirrors, lenses, uses magnifactation and cameras, sound waves, ultrasound.

magnetic fields, the motor effect, simple electric motor, electromagnetic induction, generators, transformers, stars and galaxies, the life cycle of stars.

hope it helps

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## turning forces and centre of mass

Moment - turning effect of a force: MOMENT (Nm) = FORCE (N) x perpendicular DISTANCE (m)

1) the force on the spanner causes a turning effect or moment on the nut. a larger force would mean a larger moment. 2) using a longer spanner the same force can exert a larger moment because the distance from the pivot is greater. 3) to get the maximum moment (or turning effect) you need to push at righ angles (perpendicular) to the spanner. 4)pushing at any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.

The centre of mass hangs directly below the point of suspension

1) you think of the centre of mass of an object as the point at which the whole mass is concentrated. 2) a freely suspended object will swing until its centre of mass is vertically below the point of suspension. 3) this means you can find the centre of any flat shap like this: 1. suspend the shape and a plumb line from the same point and wait till they stop moving. 2. draw a line along plumb line. 3. do same again from a different pivot point. 4. centre of mass is were both lines cross.

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## Balanced moments and stability

Balance = moments being equal

if the anti clockwise and the clockwise moemnts of an object are equal the object wont turn.

example: your younger brother ways 300 N and sits 2m from the pivot of a seesaw. if you weigh 700 N where would you need to sit to balance the seesaw?

300 x 2 = 700 x Y so Y= 0.86m

1) most stable objects have a wide base and low centre of mass. 2) an object will begin to tip if its centre of mass moves beyond the edge of the base. 3) again because of moments if the weight doesnt act in line with the pivot it'll cause a resultant moment. this will either right the object, or tip it over.

Stable - low centre of mass, Unstable - may tip over - high centre of mass

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## Circular motion

Circular motion - velocity is constantly changing

1) the velocity is both the speed an direction of an object. 2) if an object is travelling in a circle it is contantly changing direction, which means its accelerating. 3) this means there must be a force acting on it. 4) this force acts towards the centre of the circle. 5) this force that keeps something moving in a circle is called centripetal force (sen-tree-pee-tal)

in the exam, you can be asked to say which force is actually providing the centripetal force in a given situation. it can be tension or friction or even gravity (next card for gravity) FRICTION - a car going round a bend: imagine the bend is part of a circle - the centripetal force is towards the centre of this circle- the force is from friction between the tyres and the road TENSION - a bucket whirling on a rope: the centripetal force comes from tension in the rope. break the rope and the bucket flies of at a tangent.

the faster an object - the bigger the centropetal force. the heavier the bigger the centropetal force. larger force needed to keep something moving in a smaller circle

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## Gracity and planetary forces (1)

Gravity is the centripetal force that keeps planets in orbit

1) gravity is the force of attraction between masses - the larger the masses the greater the force of the gravity between them (you're strongly attracted to a big mass like the earth, but not to a small mass like a toaster). 2) the gravitational force can act as the centripetal force that keeps one object moving in a circular path (orbit) round another. an orbit is possible when theres a balance between the forward motion of the object and the gravitational force pulling it inwards. 3) planets orbit around stars E.g. the earth orbits the sun, and the centripetal force needed is provided by the gravitational attraction between the earth and the sun. 4) these orbits are all slightly eliptical (elongated circles) with the sun at one focus of the ellipse. 5) the further the planet from the sun the longer it takes to orbit.

Gravity decreases quickly as you get further away and vice versa.

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## Gracity and planetary forces (2)

Artificial satellites are used for: monotoring, communications, space research, and spying.

Geostationary satellies are used for communications

1) geostationary satelites are in high orbits over the equator which takes exactly 24 hours to complete. 2) this means that they stay above the same point on the earths surface because the earth rotates with them. 3) this makes them ideal for telephone and TV because they're always in the same place and they can transfer signals from one side of the earth to another in a fraction of a second. geostationary is high to get all signals and cover more area.

Low polar orbit satellites are for weather and spying

1) in a low polar orbit, the satellite sweeps over both poles whilst the earth roates beneath it. 2) few hours for full orbit. 3) each time the satelite goes round it can scan next bit of globe. 4. this allows the whole surface of the planet to be monitored each day. polar orbits are nice and low.

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## Images

1) A real image is where the light from an object comes together to form an image on a 'screen' - like the image formed on an eyes retina. 2) a virtual image is when the rays are diverging, so the kight from the object apears to be coming from a different place. 3) when you look in a mirror you see a virtual image of your face - because the object (your face) appears to be behind the mirror. 4) you can get a cirtual image looking at an object through a magnifying lense- the virtual images looks bigger and further away that the object actually is.

1) reflection of light is what allows us to see objects. light bounces of them into out eyes. 2) when the light reflects from and uneven surface such as a piece of paper the light reflects of at all different angles and you get a diffuse relfection. 3)when light reflects from an even surface (smooth and shiny like a mirror) then its all reflected at the same angle and you get a clear relfection. 4) but dont forget the law of refelection applies to every reflected ray: Angle of incidence = Angle of reflection

1) Refraction of light is when the waves change direction as they enter a different medium. 2) this is caused entirely by the change in speed of the waves. 3) thats what makes ponds look shallower than they are.

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## Mirrors

You need to be able to reproduce an entire diagram of how an image is formed in a plane mirror!

1) first of draw the virtual image. dont try to draw the rays first: the images is the same size as the object. it is as far behind the mirror as the object is in front. it is formed from diverging rays which means its a virtual image. 2) next draw a reflected ray going from the top of the virtual image to the top of the eye. draw a bold line for the part of the ray between the mirror and the eye and a dotted line for the part of the ray between the mirror and virtual image. 3) now draw the incident ray going from the top of the object to the mirror. the incident and reflected rays follow the law of relfection - but you do not have to measure any angles. just draw from the object to the point where the relfected ray meets the mirror. 4) now you have an incident ray and refected ray from the top of the image. do steps 2 and 3 again for the bottom of the eye - reflected ray going from the image to the bottom of the eye, then an incident ray from the object to the mirror.

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