practice problem 1
Which of the following fruits has a weight that is closest to one newton?
|McIntosh apple (151.3 g)||Red delicious apple (216.4 g)||Cavendish banana (160.0 g)|
|Valencia orange (178.0 g)||Tangerine (96.7 g)||Tomato (152.8 g)|
|Large chicken egg (65.9 g)|
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|
|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
Here’s the way I usually do it — using values I’ve memorized from years of use.
|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.
|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|
practice problem 3
practice problem 4