Word Problem Involving Velocities, Acceleration, Limits
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This is the problem:

I took the integral of a(t) to get v(t) and got $v(t)$ = $ln(t+2)$ $-$ $ln(t+3)$ + C . I plugged in the initial conditions and got C = 0. Then when I took the limit, I know it's a telescoping series so everything cancels out except the $ln(2)$ but that's not an answer choice so I'm confused what I did wrong.
calculus integration
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add a comment |
$begingroup$
This is the problem:

I took the integral of a(t) to get v(t) and got $v(t)$ = $ln(t+2)$ $-$ $ln(t+3)$ + C . I plugged in the initial conditions and got C = 0. Then when I took the limit, I know it's a telescoping series so everything cancels out except the $ln(2)$ but that's not an answer choice so I'm confused what I did wrong.
calculus integration
$endgroup$
add a comment |
$begingroup$
This is the problem:

I took the integral of a(t) to get v(t) and got $v(t)$ = $ln(t+2)$ $-$ $ln(t+3)$ + C . I plugged in the initial conditions and got C = 0. Then when I took the limit, I know it's a telescoping series so everything cancels out except the $ln(2)$ but that's not an answer choice so I'm confused what I did wrong.
calculus integration
$endgroup$
This is the problem:

I took the integral of a(t) to get v(t) and got $v(t)$ = $ln(t+2)$ $-$ $ln(t+3)$ + C . I plugged in the initial conditions and got C = 0. Then when I took the limit, I know it's a telescoping series so everything cancels out except the $ln(2)$ but that's not an answer choice so I'm confused what I did wrong.
calculus integration
calculus integration
edited Dec 19 '18 at 4:44
Andrei
12.4k21128
12.4k21128
asked Dec 19 '18 at 4:40
krauser126krauser126
474
474
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Here I assume that the initial velocity refers to $v(0) = ln frac{2}{3}$:
$$v(t) = ln frac{t+2}{t+3} = ln frac{1+frac{2}{t}}{1+frac{3}{t}}stackrel{t to infty}{longrightarrow}ln 1 = 0$$
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$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
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– krauser126
Dec 19 '18 at 4:53
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@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
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– trancelocation
Dec 19 '18 at 5:07
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[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
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– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
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@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
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– trancelocation
Dec 19 '18 at 5:34
|
show 2 more comments
$begingroup$
You have
$$
v(t)=lntfrac23+int_0^tfrac1{u^2+5u+6},du=lntfrac23+lnfrac{t+2}{t+3}+lntfrac32,
$$
where the $ln tfrac32$ comes from evaluating the antiderivative at $0$. Then
$$
lim_{ttoinfty}v(t)=ln tfrac23+ln tfrac32=0.
$$
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$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
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Now I see that I misread something. Give me a second.
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– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Here I assume that the initial velocity refers to $v(0) = ln frac{2}{3}$:
$$v(t) = ln frac{t+2}{t+3} = ln frac{1+frac{2}{t}}{1+frac{3}{t}}stackrel{t to infty}{longrightarrow}ln 1 = 0$$
$endgroup$
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
|
show 2 more comments
$begingroup$
Here I assume that the initial velocity refers to $v(0) = ln frac{2}{3}$:
$$v(t) = ln frac{t+2}{t+3} = ln frac{1+frac{2}{t}}{1+frac{3}{t}}stackrel{t to infty}{longrightarrow}ln 1 = 0$$
$endgroup$
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
|
show 2 more comments
$begingroup$
Here I assume that the initial velocity refers to $v(0) = ln frac{2}{3}$:
$$v(t) = ln frac{t+2}{t+3} = ln frac{1+frac{2}{t}}{1+frac{3}{t}}stackrel{t to infty}{longrightarrow}ln 1 = 0$$
$endgroup$
Here I assume that the initial velocity refers to $v(0) = ln frac{2}{3}$:
$$v(t) = ln frac{t+2}{t+3} = ln frac{1+frac{2}{t}}{1+frac{3}{t}}stackrel{t to infty}{longrightarrow}ln 1 = 0$$
edited Dec 19 '18 at 4:58
answered Dec 19 '18 at 4:47
trancelocationtrancelocation
12.3k1826
12.3k1826
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
|
show 2 more comments
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
That makes sense. However, why doesn't using the fact that they're telescopic work in this case?
$endgroup$
– krauser126
Dec 19 '18 at 4:53
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
@krauser126 Could you please write down the telescoping sum you are referring to? This might clear up what went wrong.
$endgroup$
– trancelocation
Dec 19 '18 at 5:07
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
[ln (2) + ln(3) + ln(4) + ...] - [ln (3) + ln (4) + ln (5) + ...] Since its from t = 0 to t = infinity. So wouldnt all the terms in the negative set cancel with those equivalent in the positive set, leaving + ln(2) as the first term left over.
$endgroup$
– krauser126
Dec 19 '18 at 5:15
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 This expression would be adding the velocities at specific points: $v(0) + v(1) + cdots $ and does not represent the limit for $t to infty$
$endgroup$
– trancelocation
Dec 19 '18 at 5:26
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
$begingroup$
@krauser126 Besides this: what you wrote is an undefined expression: $infty - infty$. If you set $v(k) = ln (k+2) - ln (k+3)$ you would get $$Rightarrow sum_{k=0}^n v(k) = sum_{k=0}^n (ln (k+2) - ln (k+3)) = ln 2 - ln (n+3)$$ Now ask yourself whether this reflects the question to be answered.
$endgroup$
– trancelocation
Dec 19 '18 at 5:34
|
show 2 more comments
$begingroup$
You have
$$
v(t)=lntfrac23+int_0^tfrac1{u^2+5u+6},du=lntfrac23+lnfrac{t+2}{t+3}+lntfrac32,
$$
where the $ln tfrac32$ comes from evaluating the antiderivative at $0$. Then
$$
lim_{ttoinfty}v(t)=ln tfrac23+ln tfrac32=0.
$$
$endgroup$
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
add a comment |
$begingroup$
You have
$$
v(t)=lntfrac23+int_0^tfrac1{u^2+5u+6},du=lntfrac23+lnfrac{t+2}{t+3}+lntfrac32,
$$
where the $ln tfrac32$ comes from evaluating the antiderivative at $0$. Then
$$
lim_{ttoinfty}v(t)=ln tfrac23+ln tfrac32=0.
$$
$endgroup$
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
add a comment |
$begingroup$
You have
$$
v(t)=lntfrac23+int_0^tfrac1{u^2+5u+6},du=lntfrac23+lnfrac{t+2}{t+3}+lntfrac32,
$$
where the $ln tfrac32$ comes from evaluating the antiderivative at $0$. Then
$$
lim_{ttoinfty}v(t)=ln tfrac23+ln tfrac32=0.
$$
$endgroup$
You have
$$
v(t)=lntfrac23+int_0^tfrac1{u^2+5u+6},du=lntfrac23+lnfrac{t+2}{t+3}+lntfrac32,
$$
where the $ln tfrac32$ comes from evaluating the antiderivative at $0$. Then
$$
lim_{ttoinfty}v(t)=ln tfrac23+ln tfrac32=0.
$$
edited Dec 19 '18 at 4:58
answered Dec 19 '18 at 4:49
Martin ArgeramiMartin Argerami
127k1182183
127k1182183
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
add a comment |
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
I didn't understand a thing you did. Doesn't it tell us that the velocity is initially ln(2/3) so that we can use that as an initial condition to plug t = 0 and solve for C, which I did already and got C = 0. So why did you do that additional step.
$endgroup$
– krauser126
Dec 19 '18 at 4:52
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
Now I see that I misread something. Give me a second.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:56
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
$begingroup$
You can do it as I did, or you find that $C=0$. In any case, the limit is zero.
$endgroup$
– Martin Argerami
Dec 19 '18 at 4:59
add a comment |
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