# revision form 4

```Upthrust force:
Weight of fluid displaced is equal to the upthrust force.
W&gt; U ( sinks)
W&lt; U ( floats)
W = U ( just floats)
PE and KE
When an object is falling energy is conserved. PE is converted into KE ( assuming no air resistance)
PE = mgh
KE = &frac12; mv2.
The velocity can be found using the velocity v = √
2πΎπΈ
π
Stopping distance = thinking distance + braking distance
Thinking distance. Moving at constant speed. This depends on speed and mental state of driver
Braking distance. Deceleration. This depends on speed, state of road and car.
Friction:
Streamlined shape
Separating surfaces with air.
Smoothen surface
Lubricate the surface
Earth and beyond:
Planets in order : Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Definition of planet. Dwarf planet. Galaxy
Force of gravity depends on mass of planets and distance between planets.
Difference between a planet and a star
Satellites. Geostationary, polar orbit.
Solar power in space
Heat
Difference between temp and heat.
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Conduction—good and bad conductors . trapped air is a good insulator
Needs a material to pass through
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Convection – the density of the fluid changes as heat is absorbed.
Needs a material to pass through
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Does not need a material to pass through
Specific heat capacity
π
Is a property of the material c = πβπ
Waves
Transverse and longitudinal waves.
units J/kg&deg;C
Transverse waves such as water waves and light waves
They experience reflection, refraction and diffraction
Properties of light waves
Characteristics of Light
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Light is a form of energy produced by luminous objects.
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Light can travel through vacuum.
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Light can penetrate through transparent materials but cannot pass through opaque objects.
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Light travels in a straight line in an optically homogeneous medium.
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Light bounces back when made to fall on polished surfaces such as mirrors or metal surfaces.
This bouncing back of light is described as reflection.
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The change in the velocity of light when it travels from one transparent medium to another is
described as refraction.
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Light appears to have a dual nature. During propagation, light exhibits wave characteristics but
when it interacts with matter, it behaves like particles.
Properties of reflection.
Laws of reflection
Refraction.
Total internal reflection
Conditions for total internal reflection
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Angle of incidence is larger than the critical angle
Ray of light must be passing from a dense to a less dense material
Examples of total internal reflection:
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Optical fibres
Rear reflectors
Renewable and non renewable energy sources:
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Non-renewable : limited supply. Causes pollution.
Coal, oil and gas are fossil fuels. Nuclear energy.
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Renewable : unlimited supply. Does not cause pollution.
Solar, wind are siutable for Malta. hydroelectric, tidal, geothermal
Conservation of energy
Energy cannot be created or destrioyed, it can only be converted from one form to another.
Efficiency = useful energy output x 100%
Energy input
Efficiency = useful power output x 100%
Power input
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