Why below question is considered as Linear and not exponential growth?












0












$begingroup$



A ball falls from a height of $2$ meters onto a firm surface and jumps
after each impact each back to $80%$ of the height from which it fell. ¨
Set up the function, which indicates the height of the ball after the
$n$th impact reached. How high does the ball jump after the $5$th impact?




Below question is considered as a linear growth and not exponential growth .
I dont understand why its linear growth or decay.
$$y=2*0.8^5$$




For the percentage neither it add to $1$ nor minus from $1$.




I wish to ask why it's not $y=2*.2^5$ (I took $80%$ as decay and did $1-.8=.2$)










share|cite|improve this question











$endgroup$

















    0












    $begingroup$



    A ball falls from a height of $2$ meters onto a firm surface and jumps
    after each impact each back to $80%$ of the height from which it fell. ¨
    Set up the function, which indicates the height of the ball after the
    $n$th impact reached. How high does the ball jump after the $5$th impact?




    Below question is considered as a linear growth and not exponential growth .
    I dont understand why its linear growth or decay.
    $$y=2*0.8^5$$




    For the percentage neither it add to $1$ nor minus from $1$.




    I wish to ask why it's not $y=2*.2^5$ (I took $80%$ as decay and did $1-.8=.2$)










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$



      A ball falls from a height of $2$ meters onto a firm surface and jumps
      after each impact each back to $80%$ of the height from which it fell. ¨
      Set up the function, which indicates the height of the ball after the
      $n$th impact reached. How high does the ball jump after the $5$th impact?




      Below question is considered as a linear growth and not exponential growth .
      I dont understand why its linear growth or decay.
      $$y=2*0.8^5$$




      For the percentage neither it add to $1$ nor minus from $1$.




      I wish to ask why it's not $y=2*.2^5$ (I took $80%$ as decay and did $1-.8=.2$)










      share|cite|improve this question











      $endgroup$





      A ball falls from a height of $2$ meters onto a firm surface and jumps
      after each impact each back to $80%$ of the height from which it fell. ¨
      Set up the function, which indicates the height of the ball after the
      $n$th impact reached. How high does the ball jump after the $5$th impact?




      Below question is considered as a linear growth and not exponential growth .
      I dont understand why its linear growth or decay.
      $$y=2*0.8^5$$




      For the percentage neither it add to $1$ nor minus from $1$.




      I wish to ask why it's not $y=2*.2^5$ (I took $80%$ as decay and did $1-.8=.2$)







      calculus linear-algebra exponential-function






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 16 '18 at 8:55







      tomtom

















      asked Dec 15 '18 at 12:02









      tomtomtomtom

      63




      63






















          1 Answer
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          1












          $begingroup$

          Here's what you've done.



          With each impact, the height of the bounce decreases to $80%$ of what it was. But you've made it decrease by $80%$ (to $20%$) by subtracting the $80%$ from $1$.



          I think that's where your confusion is.



          You need to multiply the height by $80%$ with each bounce, and the decay comes from the fact that $80%$ is smaller than $1$.



          Now, looking at it step by step:



          After no bounces (the starting position), you've multiplied by $0.8$ no times, so the height in metres is $$y=2=2*0.8^0$$



          ($0.8^0$ simply means "Don't multiply by $0.8$ at all": anything to the power of $0$ is $1$. But putting it in helps to show the pattern.)



          After $1$ bounce, you've multiplied by $0.8$ once, so $$y=2*0.8=2*0.8^1$$



          After $2$ bounces, you've multiplied by $0.8$ twice, so now $$y=2*0.8*0.8=2*0.8^2$$



          After $3$ bounces, you've multiplied by $0.8$ $3$ times, making $$y=2*0.8*0.8*0.8=2*0.8^3$$



          After $n$ bounces, you've multiplied by $0.8$ $n$ times, so $$y=2*0.8^n$$ which is the equation the question wants you to use.



          Then just put $n=5$ to get the height after $5$ bounces.



          Edit: When a problem like this is described in words, "of" is very often the same as "multiplied by": e.g. "two thirds of $x$" means $frac23*x$ and "$80%$ of $y$" means $80%*y$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            My question is why its linear growth and not an exponential growth
            $endgroup$
            – tomtom
            Dec 16 '18 at 8:56










          • $begingroup$
            It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
            $endgroup$
            – timtfj
            Dec 16 '18 at 11:40










          • $begingroup$
            i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
            $endgroup$
            – tomtom
            Dec 16 '18 at 20:03












          • $begingroup$
            If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:13










          • $begingroup$
            It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:17













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          $begingroup$

          Here's what you've done.



          With each impact, the height of the bounce decreases to $80%$ of what it was. But you've made it decrease by $80%$ (to $20%$) by subtracting the $80%$ from $1$.



          I think that's where your confusion is.



          You need to multiply the height by $80%$ with each bounce, and the decay comes from the fact that $80%$ is smaller than $1$.



          Now, looking at it step by step:



          After no bounces (the starting position), you've multiplied by $0.8$ no times, so the height in metres is $$y=2=2*0.8^0$$



          ($0.8^0$ simply means "Don't multiply by $0.8$ at all": anything to the power of $0$ is $1$. But putting it in helps to show the pattern.)



          After $1$ bounce, you've multiplied by $0.8$ once, so $$y=2*0.8=2*0.8^1$$



          After $2$ bounces, you've multiplied by $0.8$ twice, so now $$y=2*0.8*0.8=2*0.8^2$$



          After $3$ bounces, you've multiplied by $0.8$ $3$ times, making $$y=2*0.8*0.8*0.8=2*0.8^3$$



          After $n$ bounces, you've multiplied by $0.8$ $n$ times, so $$y=2*0.8^n$$ which is the equation the question wants you to use.



          Then just put $n=5$ to get the height after $5$ bounces.



          Edit: When a problem like this is described in words, "of" is very often the same as "multiplied by": e.g. "two thirds of $x$" means $frac23*x$ and "$80%$ of $y$" means $80%*y$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            My question is why its linear growth and not an exponential growth
            $endgroup$
            – tomtom
            Dec 16 '18 at 8:56










          • $begingroup$
            It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
            $endgroup$
            – timtfj
            Dec 16 '18 at 11:40










          • $begingroup$
            i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
            $endgroup$
            – tomtom
            Dec 16 '18 at 20:03












          • $begingroup$
            If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:13










          • $begingroup$
            It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:17


















          1












          $begingroup$

          Here's what you've done.



          With each impact, the height of the bounce decreases to $80%$ of what it was. But you've made it decrease by $80%$ (to $20%$) by subtracting the $80%$ from $1$.



          I think that's where your confusion is.



          You need to multiply the height by $80%$ with each bounce, and the decay comes from the fact that $80%$ is smaller than $1$.



          Now, looking at it step by step:



          After no bounces (the starting position), you've multiplied by $0.8$ no times, so the height in metres is $$y=2=2*0.8^0$$



          ($0.8^0$ simply means "Don't multiply by $0.8$ at all": anything to the power of $0$ is $1$. But putting it in helps to show the pattern.)



          After $1$ bounce, you've multiplied by $0.8$ once, so $$y=2*0.8=2*0.8^1$$



          After $2$ bounces, you've multiplied by $0.8$ twice, so now $$y=2*0.8*0.8=2*0.8^2$$



          After $3$ bounces, you've multiplied by $0.8$ $3$ times, making $$y=2*0.8*0.8*0.8=2*0.8^3$$



          After $n$ bounces, you've multiplied by $0.8$ $n$ times, so $$y=2*0.8^n$$ which is the equation the question wants you to use.



          Then just put $n=5$ to get the height after $5$ bounces.



          Edit: When a problem like this is described in words, "of" is very often the same as "multiplied by": e.g. "two thirds of $x$" means $frac23*x$ and "$80%$ of $y$" means $80%*y$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            My question is why its linear growth and not an exponential growth
            $endgroup$
            – tomtom
            Dec 16 '18 at 8:56










          • $begingroup$
            It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
            $endgroup$
            – timtfj
            Dec 16 '18 at 11:40










          • $begingroup$
            i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
            $endgroup$
            – tomtom
            Dec 16 '18 at 20:03












          • $begingroup$
            If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:13










          • $begingroup$
            It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:17
















          1












          1








          1





          $begingroup$

          Here's what you've done.



          With each impact, the height of the bounce decreases to $80%$ of what it was. But you've made it decrease by $80%$ (to $20%$) by subtracting the $80%$ from $1$.



          I think that's where your confusion is.



          You need to multiply the height by $80%$ with each bounce, and the decay comes from the fact that $80%$ is smaller than $1$.



          Now, looking at it step by step:



          After no bounces (the starting position), you've multiplied by $0.8$ no times, so the height in metres is $$y=2=2*0.8^0$$



          ($0.8^0$ simply means "Don't multiply by $0.8$ at all": anything to the power of $0$ is $1$. But putting it in helps to show the pattern.)



          After $1$ bounce, you've multiplied by $0.8$ once, so $$y=2*0.8=2*0.8^1$$



          After $2$ bounces, you've multiplied by $0.8$ twice, so now $$y=2*0.8*0.8=2*0.8^2$$



          After $3$ bounces, you've multiplied by $0.8$ $3$ times, making $$y=2*0.8*0.8*0.8=2*0.8^3$$



          After $n$ bounces, you've multiplied by $0.8$ $n$ times, so $$y=2*0.8^n$$ which is the equation the question wants you to use.



          Then just put $n=5$ to get the height after $5$ bounces.



          Edit: When a problem like this is described in words, "of" is very often the same as "multiplied by": e.g. "two thirds of $x$" means $frac23*x$ and "$80%$ of $y$" means $80%*y$.






          share|cite|improve this answer











          $endgroup$



          Here's what you've done.



          With each impact, the height of the bounce decreases to $80%$ of what it was. But you've made it decrease by $80%$ (to $20%$) by subtracting the $80%$ from $1$.



          I think that's where your confusion is.



          You need to multiply the height by $80%$ with each bounce, and the decay comes from the fact that $80%$ is smaller than $1$.



          Now, looking at it step by step:



          After no bounces (the starting position), you've multiplied by $0.8$ no times, so the height in metres is $$y=2=2*0.8^0$$



          ($0.8^0$ simply means "Don't multiply by $0.8$ at all": anything to the power of $0$ is $1$. But putting it in helps to show the pattern.)



          After $1$ bounce, you've multiplied by $0.8$ once, so $$y=2*0.8=2*0.8^1$$



          After $2$ bounces, you've multiplied by $0.8$ twice, so now $$y=2*0.8*0.8=2*0.8^2$$



          After $3$ bounces, you've multiplied by $0.8$ $3$ times, making $$y=2*0.8*0.8*0.8=2*0.8^3$$



          After $n$ bounces, you've multiplied by $0.8$ $n$ times, so $$y=2*0.8^n$$ which is the equation the question wants you to use.



          Then just put $n=5$ to get the height after $5$ bounces.



          Edit: When a problem like this is described in words, "of" is very often the same as "multiplied by": e.g. "two thirds of $x$" means $frac23*x$ and "$80%$ of $y$" means $80%*y$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Dec 15 '18 at 16:54

























          answered Dec 15 '18 at 12:48









          timtfjtimtfj

          2,238420




          2,238420












          • $begingroup$
            My question is why its linear growth and not an exponential growth
            $endgroup$
            – tomtom
            Dec 16 '18 at 8:56










          • $begingroup$
            It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
            $endgroup$
            – timtfj
            Dec 16 '18 at 11:40










          • $begingroup$
            i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
            $endgroup$
            – tomtom
            Dec 16 '18 at 20:03












          • $begingroup$
            If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:13










          • $begingroup$
            It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:17




















          • $begingroup$
            My question is why its linear growth and not an exponential growth
            $endgroup$
            – tomtom
            Dec 16 '18 at 8:56










          • $begingroup$
            It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
            $endgroup$
            – timtfj
            Dec 16 '18 at 11:40










          • $begingroup$
            i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
            $endgroup$
            – tomtom
            Dec 16 '18 at 20:03












          • $begingroup$
            If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:13










          • $begingroup$
            It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
            $endgroup$
            – timtfj
            Dec 16 '18 at 20:17


















          $begingroup$
          My question is why its linear growth and not an exponential growth
          $endgroup$
          – tomtom
          Dec 16 '18 at 8:56




          $begingroup$
          My question is why its linear growth and not an exponential growth
          $endgroup$
          – tomtom
          Dec 16 '18 at 8:56












          $begingroup$
          It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
          $endgroup$
          – timtfj
          Dec 16 '18 at 11:40




          $begingroup$
          It's not linear—that would mean adding or subtracting a fixed amount with each bounce. It's exponential because we're multiplying by a fixed amount ($0.8$) each time, and it's decay because repeatedly multiplying something by $0.8$ makes it get smaller and smaller. If we were multiplying by something bigger than $1$ (e. g. $1.1$ in $y=a*1.1^n$) then it would be growth.
          $endgroup$
          – timtfj
          Dec 16 '18 at 11:40












          $begingroup$
          i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
          $endgroup$
          – tomtom
          Dec 16 '18 at 20:03






          $begingroup$
          i am quite lost . eg the value of a new car in 2015 is 4000. it depreciates 7% every year. how much will the car be worth in 2024. here i notice that solution is first find (1-0.07)= 0.93 and then 4000(0.93)^9 is the answer. may i ask why some time for decay we minus decaly percentage from 1 and in above we just dont . may i ask why
          $endgroup$
          – tomtom
          Dec 16 '18 at 20:03














          $begingroup$
          If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
          $endgroup$
          – timtfj
          Dec 16 '18 at 20:13




          $begingroup$
          If it depreciates by $7%$, it goes down to $93%$ of what it was. So, multiply by $0.93$, $9$ times (for the $9$ years): $4000*0.93^9=2081.64$
          $endgroup$
          – timtfj
          Dec 16 '18 at 20:13












          $begingroup$
          It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
          $endgroup$
          – timtfj
          Dec 16 '18 at 20:17






          $begingroup$
          It's the difference between decreasing by a certain amount (which needs subtracting from $100%$) and decreasing to a certain amount. The key thing is to work out what multiple the new value is of the old one.
          $endgroup$
          – timtfj
          Dec 16 '18 at 20:17




















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