# Physics GCSE (1)

Revision for physics

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• Created by: daniel
• Created on: 10-05-11 20:14

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## Turning forces and centre mass

A moment is a turning effect of a force.                                                                                 Moment (Nm) = force (N) x Perpendicular distance (M)

The force of the spanner causes a turning effect or moment on the nut. Larger force would mean a larger moment. Using a longer spanner, same force can exert a larger moment because distance from the pivot is greater. to get maximum moment you need to push at right angles. Pushing at any other angles means smaller moment.

The centre of mass hangs directly below the point of suspension.

you can think of the centre of mass of an object as the point where the whole mass is concentrated. A freely suspended object will swing until centre of mass is vertically below the point of suspension. Simple shapes you can guess quickly where centre of mass is by looking at the line of symmetry.

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## Balances moments and stability

If the anticlockwise moment are equal to the clockwise moment, the object won't turn.

E.g younger brother weighs 300N and sits 2M from the pivot of a seesaw, if you weigh 700N, where should you sit to balance the seesaw?                                           Anticlockwise moment = clockwise moment                                                                                        300 x 2 = 700 x y                                                                                                                  y = 0.86m

Low and wide objects are most stable.

Unstable objects tip over easily - stable ones don't. position of centre of mass is all important. An object will tip if it's centre of mass moves beyond the edge of its base.

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## Circular motion

Circular motion - Velocity is constantly changing

Velocity is both the speed & direction of an object. If object is travelling in a circle    its constantly changing direction (Accelerating). Must be a force acting on it.            This force is acting towards the centre of the circle. Force that keeps something moving in a circle is called a centripetal force

Centripetal Force depends on Mass, Speed & Radius

Faster object is moving, bigger the centripetal force to keep it moving in a circle. Heavier the object, bigger the centripetal force to keep it moving in a circle. Larger force to keep something moving in a smaller circle.

Three things that mean you need a bigger centripetal force are:

More Speed, More Mass & Smaller Radius.

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## Gravity and planetary orbits

Gravity is the centripetal force that keeps planet in orbit.

The larger the masses greater gravity is between them, Gravitational force keeps the orbit in a circular path. Planet orbit around starts. further planet is from the sun longer it takes to orbit.

Gravity decreases quickly as you get further away

large masses like planets, gravity is bigger. closer to star or planet stronger the force. comets, moons and satellites are held in by gravity.

Geostationary satellites are used for communications they are over the equator and take 24 hours to orbit.

Low polar orbit satellites are for weather and spying, satellite sweep over both poles while the earth rotates beneath it. allows whole surface of the earth to be monitored.

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## Images

A real image is where light from an object comes together to form a image on a 'screen'

A virtual image is when a ray are diverging, so the light from the object appears to be coming from somewhere else.

Reflection of light lets us see things

Reflection of light allows us to see things, light bounces of them into our eyes. When light reflects from uneven surface e.g paper light reflects off at different angles and you get a different reflection. When light reflects from uneven surface then its all reflected at same angle and get a clear reflection.

Refraction - Light bends as it changes speed

Refraction is when the waves change direction as they enter a different medium e.g water.

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## Mirrors

Three important points to learn: Th is same size as the object, it's as far behind the mirror as the object in front & it's formed from diverging rays, which means it's a virtual image.

Curved Mirrors are a little more complicated

concave mirrors are shiny on inside of curve and concave mirrors are shinyon the outside. Halfway between centre of curvature(object) and the vertex is the focus point. the centre of curvature, vertex & focal point all lie on a line down the middle of the mirror called the axis.

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