9.4.5 Identify the equilibrium values

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In summary, there are three equilibrium values for the differential equation $y'=(y-3)(y-4)(y-5)$, which are 3, 4, and 5. The equilibrium value of 3 is unstable, while the values of 4 and 5 are stable. This can be seen by graphing the derivative function and observing the behavior of solutions near each equilibrium value. The Desmos calculator was used to generate the graph for a better understanding of the concept.
  • #1
karush
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5
$\tiny{9.4.5}$
$$y'=\sqrt{5},y>0$$
Identify the equilibrium values
which are stable and unstable?

I assume 0 is the only one
 
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  • #2
Why would you "assume" that? Are you clear on what an equilibrium solution is?

An "equilibrium" position is one that does not change, such as a ball sitting at the top of a hill or at the bottom of valley as opposed to a ball on the side of the hill or the valley.
An equilibrium position is "stable" if a slight change from that equilibrium position does not result is a large change and "unstable" if it does. The ball sitting on the top of a hill is in an "unstable equilibrium" because as soon as it moves off the top it will roll all the way down the hill. A ball sitting at the bottom of a valley is in a "stable equilibrium" because if it moves slightly from that position, it will quickly roll back to the bottom.

An "equilibrium solution" to a differential equation is a constant solution- if y(t) is an equilibrium solution then y'= 0. Here we are told that [tex]y'= \sqrt{5}[/tex], a non-zero constant. Since y' is never 0 there is no equilibrium solution.
 
  • #3
that was helpful
l have more to do so hope it sinks in

the next one is$\tiny{9.4. 7}$
$$y'=(y-3)(y-4)(y-5)$$
identify the equilibrium values
3,4,5
which are stable and which are unstable?
 
  • #4
Let's look at the behavior of the solutions near the equilibrium solution $y=3$. Suppose we have:

\(\displaystyle y(0)=2.9\)

Then we see:

\(\displaystyle y'<0\)

And for:

\(\displaystyle y(0)=3.1\)

We find:

\(\displaystyle 0<y'\)

So, we find solutions are moving away from $y=3$, so that's an unstable equilibrium solution. What do you find near the other equilibrium solutions?
 
  • #5
karush said:
that was helpful
l have more to do so hope it sinks in

the next one is$\tiny{9.4. 7}$
$$y'=(y-3)(y-4)(y-5)$$
identify the equilibrium values
3,4,5
which are stable and which are unstable?
Okay, 3, 4, and 5 are equilibrium values because:
when y= 3, y'= (3- 3)(3- 4)(3- 5)= 0
when y= 4, y'= (4- 3)(4- 4)(4- 5)= 0
when y= 5, y'= (5- 3)(5- 4)(5- 5)= 0.

Now, if y< 3, all of y- 3, y- 4, and y- 5 are negative and the product of three negative numbers is negative. For y< 3, y'< 0.

If 3< y< 4, y- 3 is positive while y- 4 and y- 5 are still negative. The product of two negative and one positive number is positive. For 3< y< 4, y'> 0.

Think about what that means. If y is close to 3 but a little less than 3, y' is negative so y is decreasing, moving away from 3. If y is close to 3 but a little more than 3, y' is positive so y is increasing, moving away from 3. In either case, y is moving away from 3. y= 3 is an unstable equilibrium.

If 4< y< 5, both y- 3 and y- 4 are positive while y- 5 is still negative. The product of two positive and one negative number is negative. For 4< y< 5, y'< 0.

Now, if y is close to 4 but less than 4, y' is positive so y is increasing, moving toward 4. If y is close to 4 but a little more than 4, y' is negative so y is decreasing, moving toward 4. 4 is a stable equilibrium.

Finally, for y> 5, all three of y- 3, y- 4, and y- 5 are positive. The product of three positive numbers is positive. For y> 5, y'> 0.

So if y is close to 5 but less than 5, y'< 0 so y is decreasing, moving away from 5. If y' is close to 5 but more than 5, y'> 0 so y is increasing, moving away from 5. y= 5 is an unstable equilibrium.
 
  • #6
much mahalo
great help much more ready for test

which is tomorrow...yikes
 
  • #7
karush said:
much mahalo
great help much more ready for test

which is tomorrow...yikes

Here's the method I was taught as a student:

1.) Graph \(\displaystyle y'=f(y)\).

2.) Near the roots, where \(\displaystyle f(y)<0\) draw an arrow along the curve, in the direction of decreasing $y$ and where \(\displaystyle 0<f(y)\) draw an arrow along the curve in the direction of increasing $y$.

For this problem, we would have:

View attachment 6443

Now it's easy to see that:
  • \(\displaystyle y=3\) is unstable.
  • \(\displaystyle y=4\) is stable.
  • \(\displaystyle y=5\) is unstable.
 

Attachments

  • equilibrium.png
    equilibrium.png
    4.3 KB · Views: 58
  • #8
well that sure made it less confusing

looks like Desmos
how did you put in the arrows?

the colors looked the same to me??
 
  • #9
karush said:
well that sure made it less confusing

looks like Desmos
how did you put in the arrows?

the colors looked the same to me??

Yes, I used the Desmos calculator to generate the curve, then grabbed a screenshot and edited the image to add the arrows. If I had had more time, I would have tried to do it using TikZ. :D
 

FAQ: 9.4.5 Identify the equilibrium values

What does it mean to identify equilibrium values?

Identifying equilibrium values refers to determining the points at which a system reaches a balance between opposing forces or influences. This can be seen in various scientific phenomena, such as chemical reactions, physical systems, and economic markets.

How is equilibrium measured?

Equilibrium can be measured in different ways depending on the system in question. For chemical reactions, equilibrium is often determined by measuring the concentrations of reactants and products. In physical systems, equilibrium can be measured by analyzing forces and motion. For economic markets, equilibrium is often identified through supply and demand analysis.

What factors affect equilibrium values?

There are several factors that can affect equilibrium values, such as temperature, pressure, concentration, and the presence of catalysts. These factors can shift the equilibrium point in a system, leading to changes in the overall balance.

How can equilibrium values be calculated?

Equilibrium values can be calculated using mathematical equations and principles, such as the law of mass action and the equilibrium constant. These calculations involve taking into account the initial conditions, concentrations, and reaction rates of the system.

Why is identifying equilibrium values important?

Identifying equilibrium values is crucial in understanding and predicting the behavior of various systems. It allows scientists to determine the stability and balance of a system and how it may change under different conditions. This information is essential in fields such as chemistry, physics, and economics, as it helps in designing and optimizing processes and systems.

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