In this quiz, we are going to develop some intuition behind the “limiting process.” To do this:

- We will first look at an example with a mass on a spring.
- We will then use that example to define the more general problem of limit calculation.
- From there, we will look at some weird function behavior and use those examples to develop some intuition behind what a limit is.

Let us look at an example similar to the $\frac{\text{distance}}{\text{time}}$ example we saw in the last quiz.

Let $x(t) = \sin(t)$ model the position of a mass on a spring. Suppose we want its “instantaneous velocity” at the time $t = 0.$

To do this, let us look at the average velocity on the interval $[0, t]$ and examine what happens as we let $t$ approach 0, as in the last quiz.

Which of the following expressions involving $t$ correctly models the average velocity $\frac{\text{change in position}}{\text{change in time}}$ of the mass on the interval $[0, t]?$

The average velocity of a mass on a spring up to a time $t$ is $\frac{\sin(t)}{t}.$

Similar to what we saw in the previous quiz, if we plug in 0 for $t$, the expression evaluates to the $\frac{0}{0}$ indeterminate form, so we can't immediately find the value of the instantaneous velocity.

We can, however, look at the value that $\frac{\sin(t)}{t}$ *approaches* as $t$ approaches $0.$

Using the code environment and graph below, investigate what happens to $\frac{\sin(t)}{t}$ as $t$ gets closer and closer to zero.

Try, for example, $t = 1$, $t = 0.1$, and $t = 0.01$. What seems to be happening to $\frac{\sin(t)}{t}?$

We saw that as $t$ gets closer and closer to $0,$ the expression $\frac{\sin(t)}{t}$ gets closer and closer to $1.$

We'll have a lot more to say about the details, but this is the basic idea behind saying “the limit as $t$ approaches $0$ of $\frac{\sin(t)}{t}$ is 1.” Symbolically, we write

$\lim_{t \to 0}\frac{\sin(t)}{t} = 1.$

So far, we have worked with limits in relation to a function that models velocity. However, there is no reason we can't generalize to all kinds of functions. This generalization allows us to define the limit problem.

Limit Problem:How can we determine whether or not ageneralfunction $f(x)$ approaches a specific value at $x = a?$ And if it does, what is this value?

For most functions, limit computation is fairly straightforward, but there are a lot of interesting exceptions with weird behavior. Let's analyze one of them so we can get a better understanding of what a limit is.

Suppose we want to determine how the function $\sin\left(\frac{1}{x}\right)$ behaves as $x$ approaches $0.$ In other words, we wish to determine

$\lim_{x \to 0} \sin\left(\frac{1}{x}\right).$

Does the function approach a specific value near $x = 0?$ If so, what value is it?

Use the code environment and graph below to help out.

The function $\sin\left(\frac{1}{x}\right)$ doesn't seem to approach any particular value as $x$ approaches $0.$ When this happens, we say that $\lim\limits_{x \to 0} \sin\left(\frac{1}{x}\right)$ **does not exist**.

Which of these arguments best explains this behavior?

**A.** $\hspace{1mm}$ As $x$ gets close to $0,$ $\sin(x)$ also gets close to 0. Therefore, when we take the reciprocal of $\sin(x)$, we get something that gets bigger and bigger, so it doesn't approach any particular number.

**B.** $\hspace{1mm}$ As $x$ gets close to $0,$ $\sin(x)$ oscillates between $0$ and $1,$ and so does the reciprocal, so it never gets close to any particular number.

**C.** $\hspace{1mm}$ When you actually plug in $x=0,$ you get $\sin\left(\frac{1}{0}\right),$ which is undefined, so the limit does not exist.

**D.** $\hspace{1mm}$ As $x$ gets close to $0,$ $\frac{1}{x}$ gets larger and larger. Since $\sin$ oscillates forever between $-1$ and $1$ as its input gets large, the final result doesn't approach any particular value.

Unusual oscillatory behavior can lead to a non-existent limit. Is this pattern true in general?

Let's investigate another oddball example of similar type: $\lim\limits_{x \to 0} \; x \sin\left(\frac{1}{x}\right).$

What happens to $x \sin\left(\frac{1}{x}\right)$ as $x$ approaches $0?$

Use the code environment and graph visualization below to help out.

**A.** $\hspace{1mm}$ It gets closer and closer to $0,$ because even though $\sin\left(\frac{1}{x}\right)$ is doing crazy things as $x$ gets small, we're also multiplying by $x$ and that's going to $0.$

**B.** $\hspace{1mm}$ It doesn't approach any particular number, because as we saw in the last question, the $\sin\left(\frac{1}{x}\right)$ term oscillates between $-1$ and $1$ as $x$ gets small.

**C.** $\hspace{1mm}$ It gets closer and closer to $1.$

To end this quiz, let's recap what we have done so far:

- We first examined the notion of a limiting process by analyzing the instantaneous velocity of a mass on a spring at a certain time.
- We then formulated the limit problem for more general functions.
- Finally, we examined two oddball examples $x\sin\left(\frac{1}{x}\right)$ and $\sin\left(\frac{1}{x}\right)$ in order to analyze more interesting edge cases and develop intuition on how to compute limits.

While the two examples seen were stranger than most, these functions give insight into the heart of what a limit actually is.

In the next quiz, we will look at some more examples to gain a better understanding of the limit process.