The Physics
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Opus in profectus

Weight

Practice

practice problem 1

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

solution

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.

solution

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.