Physics 3B AQA GCSE

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  • Created by: Han2812
  • Created on: 21-05-13 10:48

Centre Of Mass

The centre of mass is the point of an object which the whole mass in concentrated

You can find the centre of mass by an experiment:


  • Object
  • String
  • Bung (or weight)
  • Pivot (eg a pin)


  • Put the pin through a point in the object
  • Attach the weight to the string, then hang the string off the pin
  • Wait until the object stops swinging and draw a line where the string is
  • Repeat the first few steps, but at a different point in the object
  • Where alll the lines cross, thats where the centre of mass is
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A moment is the turning affect of a force

Moment = Force x Distance       M = F x D

  • Moment: Measure in Nm
  • Force: Measure in N
  • Distance: Measure in m

The force on an object causes a turning effect or moment on the object acting like a pivot             (eg a spanner turning a nut)

By using a longer object, the same force can be excert a larger moment because the distance from the pivot is greater

To get maxium moment (or turning effect) you need to push at 90 degrees (right angle) so it is perpendicular from the pivot

To have balanced moments, the anticlockwise and clockwise moments must be the same

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Leavers use balanced moments to make it easier for us to do work (eg. lifting a load)

Leavers increase the distance from the pivot at which the force is applied - so less force is needed to get the same moment

They are known as force multipliers

Eg. crow bar, wheel barrow

If the effort is closer to the pivot, then it is a distance magnifer

Eg. Fishing rod, muscles

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Moments, Stability and Pendulums


If the Total Anticlockwise Moments do not equal the Total Clockwise Moments, there will be a Resultant Moment, so the object will turn

To make an object stable, they must have a wide base and low centre of mass

An object will begin to tip over if its centre of mass moves beyond the edge of its base


A simple pendulum is made by swinging a weight from a piece of string

The time for one pendulum swing depends on its length. This is called a time period

Time period = 1/Frequency       T = 1/f       (T measured in s, F measured in Hz)

The longer the pendulum, the greater the time period. So the shorted the pendulum, the shorter the time period

Uses of pendulums: Swings, clocks, fairground rides

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Liquids and solids cannot be squashed because the molecules are closer together.

Pressure put on liquids acts equally in all directions at each point in a liquid

Any force applied to a liquid is transmitted through it

This means any change in pressure is the same at all point

Hydrolics uses liquids as they can pass on pressure, like a force magnifier

Pressure = Force (N) / Cross Sectional Area (m2)

If pressure remains constant then:    

         ForceA / CSAa  =  ForceB / CSAB     rearranged: Forcex CSAB / CSAB = Force

so having a larger CSA on the output magnifies the force

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Circular Motion and Centripetal Force

Newtons First Law:

Objects will remain at rest or moving at a constant velocity unless acted on by a resultant force

Objects moving around, accelerate towards the centre of the circle due to the change in velocity. The force that causes this is called centripetal force

The centripetal force needed to keep an object in circular motion increases if the mass of an object increases or the speed - important to cars and cornering

Centripetal force depends on mass, speed and radius

1) The faster an object is moving (speed), the bigger the centripetal force

2) The larger the mass of an object, the bigger the centripetal force

3) The smaller the circe, the greater the centripetal force

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Magnetic Field

A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force action on them

1) Electromagnets: current flowing though a wire produces a magnetic field - this makes magnets go in circles

2) If you place 2 magnets near each other they will attract or repel

3) Magnetic fields flow from North to South

4) This can be used to generate motion

The magnetic field round a coil of wire is strong and uniform. You can increase this by adding magnetically 'soft' iron core' - turns this into an ELECTROMAGNET

An electromagnet must be constantly supplied with current - as thats whate produces the magnetic field. So if the current stops , then it stops being magnetic

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Fleming's Left Hand Motor Effect Rule

Placing a current carrying conductors in a magnetic field produced a force. The direction of the force can be worked out using FLEMING'S LEFT HAND RULE

Thumb = THRUST (Motion)

Forefinger = FIELD

SeCond Finger = CURRENT

The force experienced by a current carrying wure in a magnetic field is known as the motor effect

Simple Electric Motor:

2 Factors which speed it up: 1) more CURRENT     2) STRONGER MAGNETIC FIELD


- CD players, food mixers, fan heaters, fans, printers, drills, hair dryers, cement mixers

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ELECTROMAGNETIC INDUCTION: The creation of a POTENTIAL DIFFERENCE across a conductor which is which is experiencing a CHANGE IN MAGNETIC FIELD

Wrapping coil of wire around a magnet creates a voltage - so magnetism is used to generate electricity

   The MORE TURNS or STRONGER MAGNET or                                       MORE MOTION (CURRENT) = MORE ELECTRICITY


  • These use electromagnetic induction to change the potential difference (p.d)
  • Use only a.c (alternating current) electricity
  • There are two types of transformers: STEP UP and STEP DOWN

STEP UP: Step up voltage, more turns of secondary coil than the primary coil 

STEP DOWN: Step down voltage, less turns on secondary coil, more on primary coil

SOFT IRON: stops being magnetic when current is switched off

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