Rates of Change in Applied Concepts

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AP Calculus BC › Rates of Change in Applied Concepts

Questions 1 - 10
1

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

$800$ organisms per week

$24$ organisms

$26.7$ weeks per organism

$0.03$ organisms per week

$24$ 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.

2

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

$1.25$ months per ppm

$410$ ppm per month

$0.8$ ppm per month

$0.8$ ppm

$410.8$ ppm per month

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.

3

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 $t=10$?

$-2$ kg per day

$2$ kg per day

$-2$ kg

$-20$ kg

$0.5$ days per kg

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.

4

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

$0$ MWh per hour

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

$10\pi$ MWh

$120$ MWh per hour

$10\pi$ 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.

5

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

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

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

$16$ months per score unit

$\dfrac{1}{16}$ 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.

6

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 $t=3$?

$4$ hours per kg

$4$ kg per hour

$1$ kg per hour

$1$ kg

$\ln 4$ 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.

7

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

$4.5 , \text{grams per hour}$

$4.5 , \text{grams}$

$0.05 , \text{grams per hour}$

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

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

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.

8

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

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

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

$-10\ln 6$ grams per day

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

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.

9

A pool’s chlorine level is $C(t)=3+0.4t-0.05t^2$ ppm, $t$ in hours. What is $C'(2)$?

$0.2$ ppm

$-0.2$ ppm per hour

$0.2$ ppm per hour

$0.4$ ppm per hour

$5$ hours per ppm

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change of a pool's chlorine level at t=2 hours. The derivative C'(t) represents the rate at which ppm is changing per hour. Differentiate $C(t)=3+0.4t-0.05t^2$ to get C'(t)=0.4-0.1t. At t=2, C'(2)=0.2 ppm per hour, indicating increase. A tempting distractor is B, 0.2 ppm, but it omits the 'per hour' rate. When interpreting derivatives in applied contexts, always express the rate with units of output over input for clarity.

10

The amount of medicine in blood is $M(t)=80e^{-0.2t}$ mg, $t$ in hours. What is $M'(5)$?

$-0.2e^{-1}$ hours per mg

$-16e^{-1}$ mg

$-80e^{-1}$ mg per hour

$-16e^{-1}$ mg per hour

$16e^{-1}$ mg per hour

Explanation

This problem tests applied rate reasoning by asking for the instantaneous rate of change in medicine amount at t=5. The derivative M'(t) = -16 $e^{-0.2t}$ represents the decay rate in mg per hour. At t=5, M'(5) = -16 $e^{-1}$, indicating the amount is decreasing at that rate. The negative exponential reflects rapid initial decay slowing over time. A tempting distractor is -16 $e^{-1}$ mg (choice D), which omits 'per hour' and mistakes the rate for a total amount. To interpret rates in applied contexts, evaluate the derivative with units that capture the per-unit-time change.

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