The Physics
Opus in profectus


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practice problem 1

Which of the following fruits has a weight that is closest to one newton?


Use the weight formula.

W = mg

Solve for mass. Substitute one newton for weight and one standard earth gravity for gravity.

m =  W  =  1 N  
g 9.8 m/s2  
m =  0.102 kg = 102 g  

The 96.7 gram tangerine comes closest to this value. Not all tangerines weigh 98.7 grams, however, so this is only a rule of thumb. There are certainly apples, bananas, oranges, tomatoes, and other fruits out there with a mass of approximately 102 grams and a weight of approximately one newton.

Those of you familiar with multiple choice tests should have eliminated the chicken egg as a possible answer. A chicken egg is only metaphorically the "fruit of the chicken".

practice problem 2

A problem for Americans only. Verify the rule of thumb that one newton is approximately equal to a quarter pound.


Here's the way I usually do it — using values I've memorized from years of use.

W =  mg
2.2 lb =  (1 kg)(9.8 m/s2)
1 lb =  4.45… N
1 N =  0.224… lb

Here's a more accurate way to do it — using values that are exact by definition.

W =  mg
1 lb =  (0.45359237 kg)(9.80665 m/s2)
1 lb =  4.44822162… N
1 N =  0.224808943… lb

Not quite a quarter pound, but you get the idea.

0.20 lb  <  0.224808943… lb  <  0.25 lb
 ⅕ lb   <  1 N  <  ¼ lb

The fraction 9/40 gives a decimal expansion of 0.225, which is accurate to three significant figures. Not my favorite fraction, but it gets the job done. With sixteen avoirdupois ounces in a pound, one newton is also about 3½ ounces.

1 N ≈  9 lb  ×  16 oz  =  18 oz  = 3½ oz
40 1 lb 5

practice problem 3

Write something different.


Answer it.

practice problem 4

Write something completely different.


Answer it.