1
Estimate the lifetime of the sun.
From a box of cereal we know that 1 oz of cereal gives 120 Calories of energy.
Note that 1 calorie is the amount of energy needed to raise 1 gm of water 1◦ C, and that
1 Calorie = 1 kcal = 1000 calories
1 oz = 28.35 gm ≈ 30 gm
1calorie = 4.2joules
Assume: The chemical process in the sun is about as efficient as that of the body.
The mass of the sun is 2 × 1030 kg.
The distance to the sun is 1.5 × 108 km.
The rate at which energy from the sun lands on earth can be measured by putting a glass
of water outside at noon on a sunny day and is about 300 cal/ m2 sec
The challenge is to use the above facts to estimate the lifetime of the sun if it were
actually a large lump of burning coal.
In a problem like this it is best and sensible to give a rough estimate of the lifetime because
of the rough approximations being made. If you include more than one significant figure,
then you will appear to be inexperienced, i.e. a rookie :-( If you do all of the arithmetic on
the back of an envelope and don’t even look at your calculator then the overall experience
is much more enlightening!