dynamics i: force

difference mass and weight summary

discussionsummarypracticeproblems

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
g9.8 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
401 lb5

practice problem 3

Write something different.

solution

Answer it.

practice problem 4

Write something completely different.

solution

Answer it.

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