Rates of Change in Applied Concepts

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AP Calculus BC › Rates of Change in Applied Concepts

Questions 1 - 10
1

A vineyard’s sugar content is $S(t)=12+2\ln(2t+1)$ Brix, $t$ in weeks. What is $S'(3)$?

$\dfrac{4}{7}$ Brix

$\dfrac{7}{4}$ weeks per Brix

$\dfrac{2}{7}$ Brix per week

$\dfrac{4}{7}$ Brix per week

$2\ln 7$ Brix per week

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change of a vineyard's sugar content at t=3 weeks. The derivative S'(t) represents the rate at which Brix is changing per week. Differentiate S(t)=12+2 ln(2t+1) to get S'(t)=4/(2t+1). At t=3, S'(3)=4/7 Brix per week, indicating increase. A tempting distractor is B, 2/7 Brix per week, but that's if you miss the chain rule factor of 2. When interpreting derivatives in applied contexts, always express the rate with units of output over input for clarity.

2

A population is modeled by $N(t)=800e^{0.03t}$ organisms, with $t$ in weeks. What is $N'(0)$?

$24$ organisms

$26.7$ weeks per organism

$0.03$ organisms per week

$24$ organisms per week

$800$ organisms per week

Explanation

This problem tests applied rate reasoning by requiring the instantaneous rate of change in a population at t=0. The derivative N'(t) = 24 $e^{0.03t}$ represents the growth rate in organisms per week. At t=0, N'(0) = 24, indicating the population is increasing by 24 organisms per week initially. The exponential model shows continuous growth proportional to the current population. A tempting distractor is 24 organisms (choice A), which ignores the 'per week' aspect and treats it as a total rather than a rate. To interpret rates in applied contexts, evaluate the derivative at the given point and ensure units express the rate of change over time.

3

The concentration of CO$_2$ is $C(t)=410+0.8t$ ppm, $t$ in months. What is the rate of change of $C$?

$0.8$ ppm per month

$410.8$ ppm per month

$1.25$ months per ppm

$410$ ppm per month

$0.8$ ppm

Explanation

This problem tests applied rate reasoning by identifying the constant rate of change in CO2 concentration. The derivative C'(t) = 0.8 represents a steady increase of 0.8 ppm per month. This linear model implies a constant rate regardless of t. The simplicity highlights straightforward rate interpretation. A tempting distractor is 410 ppm per month (choice A), which confuses the initial value with the rate. To interpret rates in applied contexts, differentiate linear functions to find constant slopes and include time units.

4

A reservoir’s volume satisfies $V'(t)=40-2t$ cubic meters per hour. What is the rate of change at $t=12$ hours?

$16$ cubic meters

$16$ cubic meters per hour

$\dfrac{1}{16}$ hours per cubic meter

$40$ cubic meters per hour

$-16$ cubic meters per hour

Explanation

This problem tests applied rate reasoning by evaluating the given derivative at $t=12$. The function $V'(t) = 40 - 2t$ directly gives the rate in cubic meters per hour. At $t=12$, $V'(12) = 40 - 24 = 16$, indicating volume increases at 16 cubic meters per hour. The linear decline suggests slowing inflow. A tempting distractor is -16 cubic meters per hour (choice C), from sign error. To interpret rates in applied contexts, use provided derivatives and note directional changes.

5

A freezer’s ice mass is $M(t)=50-3t+0.1t^2$ kg, $t$ in days. What is the average rate of change from $t=0$ to $t=10$?

$-20$ kg

$0.5$ days per kg

$-2$ kg

$2$ kg per day

$-2$ kg per day

Explanation

This problem tests applied rate reasoning by asking for the average rate of change of a freezer's ice mass from t=0 to t=10 days. The average rate is the change in kg divided by change in days, representing overall rate over the interval. Compute [M(10)-M(0)]/10, where $M(t)=50-3t+0.1t^2$, giving (30-50)/10=-2 kg per day. This shows net decrease over the period. A tempting distractor is B, -20 kg, but that's the total change, not the rate. When interpreting rates in applied contexts, distinguish average rates as total change over interval for overall trends.

6

A city’s electricity use is $E(t)=900+120\sin(\pi t/12)$ MWh, $t$ in hours. What is $E'(0)$?

$\dfrac{1}{10\pi}$ hours per MWh

$120$ MWh per hour

$10\pi$ MWh

$10\pi$ MWh per hour

$0$ MWh per hour

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change of a city's electricity use at t=0 hours. The derivative E'(t) represents the rate at which MWh are changing per hour. Differentiate E(t)=900+120 sin(π t/12) to get E'(t)=10π cos(π t/12). At t=0, E'(0)=10π MWh per hour, showing initial increase. A tempting distractor is A, 0 MWh per hour, but that's at a different time like t=6. When interpreting derivatives in applied contexts, always express the rate with units of output over input for clarity.

7

A device’s reliability score is $R(t)=\sqrt{100-t}$, $t$ in months. What is $R'(36)$?

$16$ months per score unit

$-\dfrac{1}{16}$ score units

$-\dfrac{1}{16}$ score units per month

$-\dfrac{1}{8}$ score units per month

$\dfrac{1}{16}$ score units per month

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change of a device's reliability score at t=36 months. The derivative R'(t) represents the rate at which the score units are changing per month. Differentiate R(t)=sqrt(100-t) to get R'(t)=-1/(2 sqrt(100-t)). At t=36, R'(36)=-1/16 score units per month, indicating a decrease. A tempting distractor is D, -1/16 score units, but it omits the 'per month' for the rate. When interpreting derivatives in applied contexts, always express the rate with units of output over input for clarity.

8

The mass of salt in a tank is $S(t)=10+4\ln(t+1)$ kg, $t$ in hours. What is the instantaneous rate at $t=3$?

$4$ kg per hour

$1$ kg

$\ln 4$ kg per hour

$4$ hours per kg

$1$ kg per hour

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change in salt mass at t=3. The derivative $S'(t) = \frac{4}{t+1}$ represents the rate at which salt is accumulating in kg per hour. At t=3, $S'(3) = \frac{4}{4} = 1$, meaning the mass is increasing at 1 kg per hour. This logarithmic growth slows over time as the denominator increases. A tempting distractor is 1 kg (choice A), which forgets the 'per hour' and confuses the rate with the total mass. To interpret rates in applied contexts, differentiate the function and include units that denote the change per unit time.

9

A factory’s emission level is $E(t)=90e^{0.05t}$ grams per hour, $t$ in hours. What is $E'(0)$?

$0.05 , \text{grams per hour}$

$4.5 , \text{grams}$

$4.5 , \text{grams per hour}^2$

$4.5 , \text{grams per hour}$

$90 , \text{grams per hour}^2$

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change in emission level at t=0. Since E(t) is in grams per hour, $E'(t) = 4.5 e^{0.05t}$ represents change in that rate, in grams per hour squared. At t=0, $E'(0) = 4.5$, indicating the emission rate increases at 4.5 grams per hour per hour. This models accelerating emissions. A tempting distractor is 4.5 grams per hour (choice A), which misses the second 'per hour' for the derivative of a rate. To interpret rates in applied contexts, consider units of derivatives when the original function is already a rate.

10

A pond’s pollutant amount is $P(t)=60-10\ln(t+1)$ grams, $t$ in days. What is $P'(5)$?

$-\dfrac{10}{6}$ grams per day

$-10\ln 6$ grams per day

$\dfrac{6}{10}$ days per gram

$-\dfrac{10}{6}$ grams

$\dfrac{10}{6}$ grams per day

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change of a pond's pollutant amount at t=5 days. The derivative $P'(t)$ represents the rate at which the pollutant grams are changing per day. To find it, differentiate $P(t)=60-10 \ln(t+1)$ to get $P'(t)= -\frac{10}{t+1}$. At t=5, $P'(5)= -\frac{10}{6}$ grams per day, showing the pollutant is decreasing. A tempting distractor is D, $-\frac{10}{6}$ grams, but it forgets the 'per day' unit essential for rates. When interpreting derivatives in applied contexts, always express the rate with units of output over input for clarity.

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