# AQA Physics 3

These will explain about centre of mass circular motion, satillites, life of stars and moments

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• Created by: James
• Created on: 12-04-10 15:34

## Centre of Mass

What is the Centre of Mass?

The centre of mass of an object is the point at which all the mass of the object is centred.

How to find the Centre of Mass

To find the Centre of Mass of an object you must place the object on a pivot with a weight on string and drawing a line on it after reapeating this at another two different locations to the object the lines should meet, which is where the centre of mass is.

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## Circular Motion(Centripetal Force)

What is Circular Motion?

This is when an object moves in a circle at a steady speed, the direction of its motion is constantly changing. There is always a force towards the centre of the circle which is required to change the velocity constantly.The centripetal force needed is greater when:

• the mass of the object is greater
• the speed of the object is greater
• the smaller the radius of the circle

Circular motion is always caused by a force this could be tention, friction or gravity

Example

A bucket filled with water was swung around on a rope. The water stayede inside the bucket due to the centripetal force. However when holes where put into the bottom of the bucket the water was able to escape.

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## Satillites

What is a Satellite?

An object which orbits around the earth in space.

Geostationary Orbit has a high orbit and stays over on particualar spot over the earth

Polar orbit has a low orbit, moves fast,clearer view of the earth and does many orbits per day

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## The Life of Stars

The solar system began as a nebula of dust, ice, gas. They were pulled together by gravity. The sun is a star: energy from nuclear fusion. Gravitational foces balance raditaion pressure. There are millions of stars in the Milky Way Galaxy.

The life cycle of our star is

Sun---.>red giant--->white dwarf--->black dwarf

The life cycle of a larger star is

Larger star(blue giant) ---> red supergiant--->supernova--->neutron star or balck hole

New elements are made in stars by nuclear fusion and spread through the universe by supernova explosions.

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## Moments

What is a Moment?

A moment is a "turning force" for example opening a door. The size of a moment is given by

Moment (Nm) = Force (N) X Perpendicular distance from pivot (m)

Most exam questions will have a "balance" and ask you to find either the distance from the pivot or the force.

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