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Hot water freezes faster than cold?

The Mpemba effectThe Mpemba effect - why hot water freezes more quickly than cold water - is named after a Tanzanian studentA study led by Xi Zhang at the Nanyang Technological University in Singapore, said the reason for the Mpemba effect is down to the unique properties of the molecular bonds that hold water together

A single water molecule is made from a large oxygen atom joined to two hydrogen atoms with covalent bonds (a chemical bond that involves the sharing of two electron pairs between atoms).When the liquid warms up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart.The stretching in the hydrogen bonds allows the covalent bonds to relax and shrink somewhat, which causes them to give up their energy. The process of covalent bonds giving up their energy is essentially the same as cooling, and so warm water should in theory cool faster than cold.

Measure the distance from earth to sun

The easiest way to calculate the distance from the earth to the sun is by using the known distance to one of the other planets.Take Venus for example; we can use radar signals to find the distance to Venus. Signals sent from earth will bounce off the surface of Venus and then travel back to earth where we can detect them. Because radar travels at the speed of light you can easily calculate the distance between Venus and Earth at any time.

distance = 0.5*(time for radar to travel both ways)*(speed of light)Now that we know the distance to Venus we can calculate the distance from the Earth to the Sun using trigonometry.where the Venus-Sun line and the Earth-Sun are perpendicular (90 degrees apart)(distance of Sun-Earth) = (distance of Earth-Venus)*(cosine(angle)) This is how we get the distance of Sun-Earth to be 92,955,830 miles! This is also known as one astronomical unit (AU)