Physics 1.1 - The Realm Of Physics

Explanation
The mass of Earth is about  kg =  g.
Suppose all this mass is made up by the lightest molecule, hydrogen, which is 2 g/mol. Then there would be  mol. Each mole will contain kg of molecules. So the number of hydrogen molecules in  kg would be equal to  mol/ mol . Each hydrogen molecule contains two hydrogen atoms, so the number of atoms would be of order . This gives us an upper limit.
 
Suppose all this mass is made up by some heavy element, say iron, which is 56 g/mol. Then there would be about mol. Each mole again contains  iron atoms. So the number of iron atoms equals to  mol / mol . This gives us a lower limit. So C should be the right answer.
 
If you forget to convert kilograms to grams, you might choose the wrong answer B.
 

Explanation
Earth's radius is about  m. Each atom is about m.

It takes  =  atoms to be as long as Earth's radius. Since Earth is three-dimensional, it should contain = atoms.
If you forgot to convert the unit from kilometers to meters, you will end up with the wrong answer B.

Explanation
The number of heartbeats in a human's lifetime is  heartbeats/min  min/hr  hr/day days/year  years heartbeats.

Explanation
The thickness of a piece of paper is  .
Pay attention to the units. It is 0.0001 in meters. But answer E is wrong.

Explanation
14 billion years years  days/year  hr/day  sec/min  seconds. 
The ratio is