Physics 3B AQA GCSE
- 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:
Equipment:
- Object
- String
- Bung (or weight)
- Pivot (eg a pin)
Method:
- 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
Moments
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
Levers
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
Moments, Stability and Pendulums
Stability
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
Pendulums
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
Hydrolics
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: ForceA x CSAB / CSAB = Force
so having a larger CSA on the output magnifies the force
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
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
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
Uses:
- CD players, food mixers, fan heaters, fans, printers, drills, hair dryers, cement mixers
Transformers
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
Transformers:
- 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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