Exploring Accumulations of Change
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AP Calculus BC › Exploring Accumulations of Change
A runner’s speed is $v(t)=2+\cos t$ m/s for $0\le t\le2\pi$; what distance is traveled?
$4\pi$ meters
$0$ meters
$2\pi$ meters
$6\pi$ meters
$2$ meters
Explanation
This problem assesses the skill of exploring accumulations of change by calculating the total distance traveled from a speed function. The speed is $v(t) = 2 + \cos t$ meters per second over $0$ to $2\pi$ seconds. Since speed is always positive, the distance is the integral of $v(t)$ from $0$ to $2\pi$, accumulating the rate of distance over time. This gives $[2t + \sin t]$ from $0$ to $2\pi$ = $4\pi + 0 = 4\pi$ meters. A tempting distractor is $2\pi$ meters, from integrating only the constant 2 and ignoring $\cos t$, but the full function must be included. In general, to find the net accumulation of a quantity from its rate of change, compute the definite integral of the rate function over the specified interval.
For $0\le t\le6$, a machine’s production rate is $p(t)=\begin{cases}2t,&0\le t\le3\\12-2t,&3\le t\le6\end{cases}$; how many items are produced?
$36$ items
$6$ items
$12$ items
$18$ items
$9$ items
Explanation
This problem evaluates the skill of exploring accumulations of change by determining total production from a piecewise rate function. The production rate is 2t for 0 to 3 and 12 - 2t for 3 to 6. Total items produced is the integral over each piece, accumulating the rate over time. This gives [t²] from 0 to 3 = 9, plus [12t - t²] from 3 to 6 = 36 - 27 = 9, totaling 18 items. A tempting distractor is 36 items, from doubling the first integral without computing the second properly, but both pieces must be integrated separately. In general, to find the net accumulation of a quantity from its rate of change, compute the definite integral of the rate function over the specified interval.
A battery’s charge changes at $Q'(t)=-\frac{3}{t+1}$ coulombs/min for $0\le t\le3$; what is $Q(3)-Q(0)$?
$-\dfrac{9}{4}$ coulombs
$-3\ln 4$ coulombs
$\ln 4$ coulombs
$3\ln 4$ coulombs
$-9$ coulombs
Explanation
This problem assesses the skill of exploring accumulations of change by finding the net change in charge from a rate function. The rate is Q'(t) = -3/(t+1) coulombs per minute over 0 to 3 minutes. The net change Q(3) - Q(0) is the integral of Q'(t) from 0 to 3, accumulating the rate over time. This gives -3 [ln(t+1)] from 0 to 3 = -3(ln4 - ln1) = -3 ln4 coulombs. A tempting distractor is 3 ln4, from omitting the negative sign in the rate, but the sign indicates discharge. In general, to find the net accumulation of a quantity from its rate of change, compute the definite integral of the rate function over the specified interval.
A function satisfies $f'(x)=\frac{x}{1+x^2}$ for $0\le x\le2$; what is $f(2)-f(0)$?
$\dfrac{2}{5}$
$\ln 5$
$\ln 2$
$\dfrac{1}{2}\ln 5$
$2$
Explanation
This problem assesses the skill of exploring accumulations of change by finding the net change in a function from its derivative. The derivative is $f'(x) = \frac{x}{1 + x^2}$ over 0 to 2. The change $f(2) - f(0)$ is the integral of $f'(x)$ from 0 to 2, accumulating the rate over the interval. Using substitution $u = 1 + x^2$, this yields $\frac{1}{2} [\ln u]$ from 1 to 5 = $\frac{1}{2} \ln 5$. A tempting distractor is $\ln 5$, from forgetting the 1/2 factor in the substitution, but the $du = 2x , dx$ requires it. In general, to find the net accumulation of a quantity from its rate of change, compute the definite integral of the rate function over the specified interval.
A cyclist’s velocity is $v(t)=\sin t$ m/s for $0\le t\le\pi$; what is the cyclist’s displacement?
$\pi$
$2\pi$
$1$
$0$
$2$
Explanation
This problem tests the skill of exploring accumulations of change by using integration to find the displacement from a velocity function. The cyclist's displacement is the definite integral of $v(t) = \sin t$ from $t=0$ to $t=\pi$, which accumulates the position changes over the interval. The antiderivative $-\cos t$ evaluated at the limits gives $-\cos(\pi) - (-\cos(0)) = -(-1) - (-1) = 1 + 1 = 2$ meters. This positive accumulation reflects the velocity being non-negative throughout the interval, leading to forward movement. A tempting distractor like 0 might come from confusing displacement with total distance or misapplying the fundamental theorem, but it fails as the integral clearly yields 2. In general, to find the net accumulation over an interval, compute the definite integral of the rate function from the lower to upper limit.
A leak changes oil amount at rate $A'(t)=-5\sqrt{t}$ liters/hour for $0\le t\le 4$; what is net change in oil?
$-20$ liters/hour
$\frac{40}{3}$ liters
$-40$ liters
$\frac{80}{3}$ liters
$-\frac{80}{3}$ liters
Explanation
This problem uses accumulation reasoning to find net change in oil from the leak rate. The net change equals ∫A'(t)dt = ∫(-5√t)dt from t = 0 to t = 4. Computing: ∫(-5t^(1/2))dt = -5(2t^(3/2)/3) = -10t^(3/2)/3 evaluated from 0 to 4 gives -10(8)/3 - 0 = -80/3 liters. Choice C incorrectly gives a rate (liters/hour) rather than the accumulated volume change. When integrating power functions with fractional exponents, apply the power rule carefully and track negative signs.
For $0\le t\le 3$, a population changes at rate $P'(t)=2e^{-t}$ thousand/day; what is total population increase?
$2(1-e^{-3})$ thousand
$2e^{-3}$ thousand
$6e^{-3}$ thousand/day
$2(1+e^{-3})$ thousand
$6$ thousand
Explanation
This problem requires accumulation reasoning to find total population increase from the rate of change. The total increase equals ∫P'(t)dt = ∫2e^(-t)dt from t = 0 to t = 3. Computing the integral: ∫2e^(-t)dt = -2e^(-t) evaluated from 0 to 3 gives -2e^(-3) - $(-2e^0$) = -2e^(-3) + 2 = 2(1 - e^(-3)) thousand. Choice C incorrectly gives a rate (thousand/day) rather than the accumulated population change. To find total change from an exponential rate, integrate carefully and evaluate at the bounds.
A battery’s charge changes at rate $Q'(t)=\frac{6}{t+1}$ coulombs/hour for $0\le t\le 2$; find net charge change.
$6\ln 3$ coulombs/hour
$3\ln 3$ coulombs
$6\ln 3$ coulombs
$12$ coulombs
$6\ln 2$ coulombs/hour
Explanation
This problem requires accumulation reasoning to find net charge change from the charging rate. The net change equals ∫Q'(t)dt = ∫6/(t+1)dt from t = 0 to t = 2. Computing: ∫6/(t+1)dt = 6ln|t+1| evaluated from 0 to 2 gives 6ln(3) - 6ln(1) = 6ln(3) - 0 = 6ln(3) coulombs. Choice E incorrectly includes units of rate (coulombs/hour) rather than accumulated charge (coulombs). To find accumulated change from a rational function rate, integrate using logarithms and evaluate at the bounds.
For $2 \le t \le 5$, temperature changes at rate $T'(t) = \frac{1}{t} - 2$ °C/min; what is net temperature change?
$-6$ °C/min
$\ln!\left(\frac{5}{2}\right)$ °C/min
$6 - \ln!\left(\frac{5}{2}\right)$ °C
$\ln!\left(\frac{5}{2}\right) + 6$ °C
$\ln!\left(\frac{5}{2}\right) - 6$ °C
Explanation
This problem requires accumulation reasoning to find net temperature change from the rate. The net change equals $\int T'(t) , dt = \int \left( \frac{1}{t} - 2 \right) dt$ from t = 2 to t = 5. Computing: $\int \left( \frac{1}{t} - 2 \right) dt = \ln |t| - 2t$ evaluated from 2 to 5 gives $(\ln 5 - 10) - (\ln 2 - 4) = \ln 5 - \ln 2 - 6 = \ln\left( \frac{5}{2} \right) - 6$ °C. Choice C incorrectly gives units of rate (°C/min) rather than accumulated temperature change. To find net change, integrate each term separately and combine the results at the bounds.
Water flows with rate $r(t)=\frac{6}{t+1}$ L/min for $0\le t\le3$; how many liters enter total?
$18\ln 4$
$\frac{18}{4}$
$\ln 4$
$6\ln 4$
$\frac{6}{4}$
Explanation
This problem tests the skill of exploring accumulations of change by using integration to find the total water volume from a flow rate. The total liters entering is the definite integral of r(t) = 6/(t+1) from t=0 to t=3, accumulating the flow over the interval. The antiderivative 6 ln|t+1| evaluated gives 6(ln4 - ln1) = 6 ln4 liters. This reflects the decreasing flow rate still yielding a positive accumulation. A tempting distractor like ln4 might arise from omitting the coefficient 6, but it fails as the integral requires multiplying by that factor. In general, to find the net accumulation over an interval, compute the definite integral of the rate function from the lower to upper limit.