Physics unit 3 - GCSE notes

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Physics Unit 3
Moments
The turning effect of a force is known as the moment of a force
Moment = force x perpendicular distance from pivot
We can increase the moment by:
o increasing the applied force
o increasing the distance from the pivot
For an object to be balanced (in equilibrium), the anti-clockwise moment must equal the clockwise
moment
Centre of mass and stability
The centre of mass of an object is the point where its mass may be thought to be concentrated
Finding a centre of mass:
o Dangle an object from a point on a clamp
o Attach a piece of string to the clamp
o Draw a straight line vertically down along the string
o Repeat again with a different point
o The place where all the lines intersect is the centre of mass
An object will remain stable as long as the line of action of its weight lies inside the base
If the line of action falls out of the base, the object will topple
Make an object more stable by:
o Lowering the centre of mass
o Having a wider base
Circular Motion
An object moving at a constant speed around a circle has a constantly changing velocity (it's
changing direction) and therefore accelerate
The centripetal force needed increases as:- (gets quicker)
o the mass increases
o the radius decreases
Force = Mass(kg) x Velocity2 (m/s2)
Radius
Gravitational Attraction and Orbits
Gravity causes small masses to orbit larger ones in space
Force increases with:
o size
o smaller distance
The stronger the force, the shorter the orbit
The orbits of the planets are elliptical(ellipses) with the sun centred at one of the foci.
Types of satellite orbit
There are two types of satellites you must know for your exam!
1) Geostationary
24 hour orbit
Over the equator
It has a further distance to travel than the earth so it must travel quicker

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At the same point over the earth at all times
Communications!
2) Polar Orbit
Over the poles
The lower it is, the better the resolution
Monitoring!
Reflection
Images in flat mirrors are always virtual and upright
Angle of incidence = angle of reflection
Convex mirrors
Always virtual
Concave mirrors
Real image (not upright) if the object is behind the focus
Virtual image if the object is in front of the focus
Refraction
Moving light waves from one medium to another, slowing down , and sometimes,…read more

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Diverging - always virtual
Sounds
Can be refracted like light
Have longitudinal waves and therefore, cannot travel through a vacuum
Frequency = pitch (Hz = number of waves/ time taken)
Amplitude = Loudness (measured in m)
Humans can hear up to 20KHz, or 20,000Hz
Ultrasound
Anything above 20KHz (20,000 Hz)
Used for industrial cleaning
Detecting flaws ( distance of flaw = speed of wave x time taken for reflection)
prenatal scans
The Motor Effect
A current causes a magnetic field in the wire
The two…read more

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Transformers
Uses an iron core as it easily gains and loses charge
Voltage of primary/Voltage of secondary = N.o coils primary/ N.…read more

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