Mathematics Grade 9 15 min

Which metric unit of weight is appropriate?

Which metric unit of weight is appropriate?

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1

Introduction & Learning Objectives

Learning Objectives Calculate the final mass of a substance undergoing exponential growth or decay. Convert fluently between metric units of weight, including kilograms, grams, milligrams, and micrograms. Analyze the result of an exponential function to determine its order of magnitude. Select the most appropriate metric unit of weight to express a very large or very small calculated value. Justify their choice of a metric unit based on the context of an exponential growth or decay problem. Interpret real-world scenarios involving exponential change in mass, such as bacterial growth or radioactive decay. Ever wonder how scientists measure the weight of a single bacterium that doubles every hour? 🦠 How would you write that number down after a full day of growth? In this les...
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Key Concepts & Vocabulary

TermDefinitionExample Exponential GrowthA pattern where a quantity increases by a fixed percentage over equal time intervals. The growth rate becomes more rapid as the quantity increases.A bacterial culture starts at 1 milligram and its mass doubles every hour. After 1 hour it's 2 mg, after 2 hours it's 4 mg, after 3 hours it's 8 mg, and so on. Exponential DecayA pattern where a quantity decreases by a fixed percentage over equal time intervals. The rate of decrease slows down as the quantity gets smaller.A 100-gram sample of a radioactive substance has a half-life of one year. After 1 year, 50 g remain; after 2 years, 25 g remain; after 3 years, 12.5 g remain. Metric Prefixes for WeightSymbols used before the base unit 'gram' (g) to represent powers of 10, making...
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Core Formulas

Exponential Growth Formula W(t) = W_0 (1 + r)^t Calculates the final weight, W(t), after 't' time periods. W_0 is the initial weight, and 'r' is the growth rate per period (as a decimal). Exponential Decay Formula W(t) = W_0 (1 - r)^t Calculates the final weight, W(t), after 't' time periods. W_0 is the initial weight, and 'r' is the decay rate per period (as a decimal). For half-life problems, the base (1-r) is simply 0.5. Metric Conversion Hierarchy 1 kg = 1000 g; 1 g = 1000 mg; 1 mg = 1000 µg To convert from a larger unit to a smaller unit (e.g., g to mg), multiply by 1000. To convert from a smaller unit to a larger unit (e.g., mg to g), divide by 1000.

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Sample Practice Questions

Challenging
After 48 hours of exponential growth where its mass doubled every 8 hours, a bacterial culture has a final mass of 3.2 mg. What was the initial mass of the culture? Express the answer in the most appropriate unit.
A.5 g
B.500 µg
C.0.05 g
D.50 µg
Challenging
Isotope A starts at 1 kg with a half-life of 10 years. Isotope B starts at 800 g with a half-life of 20 years. After 40 years, what is the most appropriate unit to express the *difference* in their masses?
A.Kilograms (kg)
B.Grams (g)
C.Milligrams (mg)
D.Micrograms (µg)
Challenging
A self-replicating nanomaterial has an initial mass of 12 µg. Its mass increases by a factor of 2.5 every 30 minutes. What is its approximate mass after 2.5 hours, and which unit is most appropriate?
A.1.17 g
B.117.19 mg
C.117,187.5 µg
D.0.117 kg

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