Weight

Practice

practice problem 1

Which of the following fruits has a weight that is closest to one newton?
macintosh apple red delicious apple cavensish banana
McIntosh Apple (151.3 g) Red Delicious Apple (216.4 g) Cavendish Banana (160.0 g)
valencia orange tangerine tomato
Valencia Orange (178.0 g) Tangerine (96.7 g) Tomato (152.8 g)
chicken egg    
Chicken Egg (65.9 g)    

solution

Use the weight formula.

W = m g

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

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

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 metaphoricalluy 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.

solution

Answer it.

practice problem 4

Write something completely different.

solution

Answer it.

The Physics Hypertextbook

  • No condition is permanent.