Addition & Subtraction and Significant Digits.  Add the three following masses: 12.7 cm + 8.8 cm + 7.642 cm  12.7 cm + 8.8 cm + 7.642 cm = 29.142 cm.

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Presentation transcript:

Addition & Subtraction and Significant Digits

 Add the three following masses: 12.7 cm cm cm  12.7 cm cm cm = cm (according to my calculator)  Does this answer make sense?  The answer has 5 significant digits! Should the answer be this accurate?  We know that the number of sig dig represents accuracy. Can the answer be more accurate than the question?

 A sculptor carves a 1200 kg slab of marble. When finished the sculpture weighs 645 kg. How much marble was removed?  1200 kg – 645 kg = 555 kg  Does this answer make sense?  The answer has more significant digits than the question!

 We know an answer can’t be more precise than the question! So here is the rule:  The sum or difference should have the same number of decimal places as the least precise value in the numbers being added or subtracted.  OR the sum or difference should be accurate to the same place value as the accurate numbers being added or subtracted.  NOTE that the words significant digits were NOT used!

2.35 m m m ⇒this is the least precise number ( mdecimal place. Therefore the answer (6.9 m)can only have 1 decimal place)  Remember to add the numbers then round!  This is the precision rule.  Your answer can not be any more precise than the least precise measuring instrument used.

 12.7 cm cm cm = cm (according to my calculator)  The least accurate value in the question has 1 decimal place. It could also be said that the value is only accurate to the tenths column.  Therefore the answers should be rounded down to 1 decimal place.  cm ⇒ 29.1 cm  1200 kg – 645 kg = 555 kg  The least accurate value is only accurate to the hundreds place value.  Therefore the answers should be rounded to the closest hundred.  555 kg ⇒ 560 kg

 If the digit after the one you want is greater than 5, then round up For example: To obtain 2 significant digits: 3.47 rounds to 3.5 and rounds to 3.5  If the digit after the one you want is less than five then the preceding number stays the same For example: To obtain 2 significant digits: 3.44 rounds to 3.4 and rounds to 3.4  If the single digit after the one you want is 5, round to the closest even number For example: To obtain 2 significant digits: 2.55 is rounded to 2.6 and 2.25 is rounded to 2.2

Numbers obtained from counting are not measured. They do not affect the number of significant digits in the answer! Ex.Each section of a bridge weighs 2430 tonnes. The bridge has 24 sections, what is the weight of the bridge? Since the 24 is a counted number, we still use the 3 significant digits in the first number to obtain the number of sig dig in the answer. 24 x 2430 tonnes = tonnes ⇒ tonnes

All answers must have the correct number of significant digits and the correct units. 1) 46.8 m m m 2) 12.0 L L 3) kg kg 4) mm mm 5) mm mm mm 6) km – km 7) (202 m x 170 m) – m 2 8) (12.75 g – 11.1 g)/ 2.04 cm 3

 6)Numbers obtained from counting are not measured. They do not affect the number of significant digits in the answer! Ex.Each section of a bridge weighs 2430 tonnes. The bridge has 24 sections, what is the weight of the bridge? Since the 24 is a counted number, we still use the 3 significant digits in the first number to obtain the number of sig dig in the answer. 24 x 2430 tonnes = tonnes ⇒ tonnes