Accumulation Functions, Definite Intervals, Applied Contexts

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AP Calculus AB › Accumulation Functions, Definite Intervals, Applied Contexts

Questions 1 - 10
1

A website gains users at rate $u(t)$ users/day, $t$ days after launch. What does $\int_{7}^{14} u(t),dt$ represent?

The change in user-gain rate from day 7 to day 14, in users per day

The average number of users gained per day from day 7 to day 14, in users

The rate of gaining users on day 14, in users per day

The net number of users gained from day 7 to day 14, in users

The number of users on day 14, in users

Explanation

This problem involves interpreting a definite integral as accumulation when dealing with user growth rates. The function u(t) represents the rate at which the website gains users in users per day, and integrating this rate from t=7 to t=14 days gives the total number of users gained during this one-week period. The integral ∫₇¹⁴ u(t)dt accumulates all new users added between day 7 and day 14 after launch. Choice C is a distractor because it mentions "average number of users gained per day," but that would be (1/7)∫₇¹⁴ u(t)dt. The units verify our interpretation: (users/day) × days = users, confirming the integral measures net users gained.

2

A medication enters the bloodstream at rate $m(t)$ mg/hour, $t$ hours after dosing. What does $\int_{0}^{4} m(t),dt$ represent?

The change in entry rate from $t=0$ to $t=4$, in milligrams per hour

The time per milligram entering in the first 4 hours, in hours per milligram

The medication-entry rate at $t=4$, in milligrams per hour

The total amount of medication that enters in the first 4 hours, in milligrams

The average amount entering per hour in the first 4 hours, in milligrams

Explanation

This question involves interpreting a definite integral as accumulation in a pharmacokinetic context. The integrand m(t) represents the rate at which medication enters the bloodstream in milligrams per hour, and integrating this rate from t=0 to t=4 hours gives the total amount of medication absorbed during this time period. The integral ∫₀⁴ m(t)dt accumulates all medication that enters the bloodstream over the first 4 hours after dosing. Choice C is incorrect because it suggests the integral gives an average amount per hour, but that would be (1/4)∫₀⁴ m(t)dt. The dimensional check confirms our answer: (milligrams/hour) × hours = milligrams, showing the integral measures total medication absorbed.

3

A company’s profit changes at rate $p'(t)$ dollars/day, $t$ days after a campaign starts. What does $\int_{0}^{14} p'(t),dt$ represent?

The net change in profit over the first 14 days, in dollars

The company’s profit on day 14, in dollars

The average profit over the first 14 days, in dollars

The change in profit-change rate from day 0 to day 14, in dollars per day

The profit-change rate on day 14, in dollars per day

Explanation

This question requires understanding definite integrals when the integrand is a rate of change function. Since p'(t) represents the company's profit change rate in dollars per day, integrating this rate from t=0 to t=14 days gives the total change in profit during the first 14 days after the campaign starts. By the Fundamental Theorem of Calculus, ∫₀¹⁴ p'(t)dt = p(14) - p(0), representing the net change in the company's profit over this two-week period. Choice C is tempting because it mentions "average profit," but that would be (1/14)∫₀¹⁴ p(t)dt, not the integral of p'(t). The units confirm our interpretation: (dollars/day) × days = dollars, showing the integral measures net profit change.

4

A river’s flow rate is $f(t)$ cubic meters/second, $t$ hours after sunrise. What does $\int_{0}^{2} f(t),dt$ represent?

The time required for one cubic meter to pass, in seconds per cubic meter

The change in flow rate over the first 2 hours, in cubic meters per second

The average flow rate in the first 2 hours, in cubic meters

The total volume of water passing a point in the first 2 hours, in cubic meters

The flow rate at $t=2$, in cubic meters per second

Explanation

This problem requires understanding definite integrals as accumulation in fluid flow contexts. The function f(t) represents the river's flow rate in cubic meters per second, and integrating this rate over time gives the total volume of water that passes a given point. The integral ∫₀² f(t)dt calculates the total volume of water flowing past a fixed location during the first 2 hours after sunrise. Choice C is a common distractor because it mentions "average flow rate," but that would be (1/2)∫₀² f(t)dt, not the integral itself. The units support our interpretation: (cubic meters/second) × seconds = cubic meters, confirming the integral measures total water volume.

5

A robot moves with velocity $v(t)$ cm/s, $t$ seconds after activation. What does $\int_{10}^{15} v(t),dt$ represent?

The robot’s displacement from $t=10$ to $t=15$, in centimeters

The change in velocity from $t=10$ to $t=15$, in centimeters per second

The robot’s velocity at $t=15$, in centimeters per second

The robot’s average velocity from $t=10$ to $t=15$, in centimeters

The time per centimeter traveled from $t=10$ to $t=15$, in seconds per centimeter

Explanation

This question requires understanding definite integrals as accumulation in robotics motion contexts. The function v(t) represents the robot's velocity in centimeters per second, and integrating velocity over time gives displacement (change in position). The integral $\int_{10}^{15} v(t), dt$ calculates the robot's displacement during the time interval from t=10 to t=15 seconds after activation. Choice C is tempting because it mentions "average velocity," but that would be $(1/5) \int_{10}^{15} v(t), dt$, not the integral itself. The dimensional analysis supports our answer: $(\text{centimeters/second}) \times \text{seconds} = \text{centimeters}$, confirming the integral measures displacement.

6

A cyclist’s power output is $p(t)$ watts, $t$ minutes after starting. What does $\int_{0}^{10} p(t),dt$ represent?

The time per watt over the first 10 minutes, in minutes per watt

The total energy expended in the first 10 minutes, in watt-minutes

The change in power from $t=0$ to $t=10$, in watts

The cyclist’s power at $t=10$, in watts

The cyclist’s average power over the first 10 minutes, in watts

Explanation

This question requires understanding definite integrals as accumulation in energy contexts. The integrand p(t) represents the cyclist's power output in watts, and integrating power over time gives total energy expended. The integral $\int_0^{10} p(t) , dt$ calculates the total energy the cyclist uses during the first 10 minutes of cycling, measured in watt-minutes (which can be converted to other energy units like joules). Choice A is tempting because it mentions average power, but that would be $\frac{1}{10} \int_0^{10} p(t) , dt$, not the integral itself. The dimensional analysis supports our answer: $ \text{watts} \times \text{minutes} = \text{watt-minutes} $ (energy units), confirming the integral measures total energy expenditure.

7

A tank fills at rate $r(t)$ liters/min, $t$ minutes after noon. What does $\int_{0}^{30} r(t),dt$ represent?

The change in filling rate from $t=0$ to $t=30$, in liters per minute

The average filling rate of the tank over $0\le t\le 30$, in liters per minute

The filling rate at exactly $t=30$, in liters per minute

The time required to add one liter during $0\le t\le 30$, in minutes per liter

The total amount of water added to the tank from noon to 12:30, in liters

Explanation

This question requires interpreting a definite integral as accumulation in an applied context. The integrand r(t) represents the tank's filling rate in liters per minute, and when we integrate this rate function over the time interval from t=0 to t=30 minutes, we accumulate the total amount of water added. Since rate × time = quantity, the integral ∫₀³⁰ r(t)dt gives us the total volume of water that enters the tank during the first 30 minutes after noon. Choice A is tempting because it mentions the average filling rate, but that would be (1/30)∫₀³⁰ r(t)dt, not the integral itself. To verify the correct interpretation, check that the units work out: (liters/minute) × minutes = liters, confirming the integral measures total volume.

8

A delivery app receives orders at rate $o(t)$ orders/min, $t$ minutes after 6 PM. What does $\int_{0}^{45} o(t),dt$ represent?

The order rate at 6:45 PM, in orders per minute

The average order rate from 6:00 to 6:45 PM, in orders

The total number of orders received from 6:00 to 6:45 PM, in orders

The change in order rate from 6:00 to 6:45 PM, in orders per minute

The time per order from 6:00 to 6:45 PM, in minutes per order

Explanation

This problem tests the interpretation of definite integrals as accumulation in service contexts. The function o(t) represents the delivery app's order-receiving rate in orders per minute, and integrating this rate over the time interval from t=0 to t=45 minutes gives the total number of orders received during the first 45 minutes after 6 PM. The integral ∫₀⁴⁵ o(t)dt accumulates all orders placed throughout this period. Choice C is a common distractor because it mentions "average order rate," but that would be (1/45)∫₀⁴⁵ o(t)dt, not the integral itself. The dimensional analysis supports our answer: (orders/minute) × minutes = orders, confirming the integral measures total orders received.

9

A bakery’s bread cools at rate $C(t)$ °C/min, $t$ minutes after removal from oven. What does $\int_{0}^{30} C(t),dt$ represent?

The bread’s temperature at $t=30$, in degrees Celsius

The net change in bread temperature over the first 30 minutes, in degrees Celsius

The change in cooling rate from $t=0$ to $t=30$, in degrees Celsius per minute

The cooling rate at $t=30$, in degrees Celsius per minute

The average bread temperature over the first 30 minutes, in degrees Celsius

Explanation

This problem tests the interpretation of definite integrals when the integrand is a rate of change function. Since C(t) represents the bread's cooling rate in degrees Celsius per minute, integrating this rate from t=0 to t=30 minutes gives the total change in bread temperature during this time period. By the Fundamental Theorem of Calculus, ∫₀³⁰ C(t)dt represents the net change in bread temperature over the first 30 minutes after removal from the oven. Choice D is a distractor because it mentions "average bread temperature," but that would be (1/30)∫₀³⁰ T(t)dt, not the integral of the cooling rate. The units confirm our interpretation: (°C/minute) × minutes = °C, showing the integral measures net temperature change.

10

A candle’s wax melts at rate $m(t)$ grams/min, $t$ minutes after lighting. What does $\int_{0}^{25} m(t),dt$ represent?

The average melting rate over the first 25 minutes, in grams

The melting rate at $t=25$, in grams per minute

The total mass of wax melted in the first 25 minutes, in grams

The mass of wax remaining at $t=25$, in grams

The change in melting rate from $t=0$ to $t=25$, in grams per minute

Explanation

This problem involves interpreting a definite integral as accumulation in combustion contexts. The integrand m(t) represents the candle's wax melting rate in grams per minute, and integrating this rate over the time interval from t=0 to t=25 minutes gives the total mass of wax that melts during the first 25 minutes after lighting. The integral ∫₀²⁵ m(t)dt accumulates all wax consumed throughout this period. Choice D is a distractor because it mentions "average melting rate," but that would be (1/25)∫₀²⁵ m(t)dt, not the integral itself. The dimensional check confirms our answer: (grams/minute) × minutes = grams, verifying the integral measures total wax melted.

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