Main content

## 8th grade

### Course: 8th grade > Unit 3

Lesson 11: Constructing linear models for real-world relationships- Linear functions word problem: fuel
- Linear functions word problem: pool
- Modeling with linear equations: gym membership & lemonade
- Graphing linear relationships word problems
- Linear functions word problem: iceberg
- Linear functions word problem: paint
- Writing linear functions word problems

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Linear functions word problem: iceberg

Sal is given a verbal description of a real-world relationship involving a melting iceberg, and is asked to find the formula of the function that represents this relationship.

## Want to join the conversation?

- The notation confuses me.

I have always thought of parenthisis as multiplication. So S(0) should be zero.

Just like 2(x+1) = 2x+2

What do I have to put into my brain to make this make sense to me?(10 votes)- You REALLY need to get used to the meaning of parenthesis with respect to context that they are being used.

If I say, what is "f(2)" I want to know the output value of function f with the input value 2.

If I am told there is a function called S(t) and I am asked, what is the value of the function S when t=0, then I write S(0). In these cases, functional notation is the context, so f or S is the name of the function and 2 or 0 or t is the value at which to evaluate the function.

If I say, what is 2(x+1) or x(2y + 7x), then the context is obviously multiplication.

With more time, it will become second nature!

AND - there are some other meanings of ( ) as well!

Keep Studying!(27 votes)

- hi this is from 2022 im 13 :D(11 votes)
- its 2023 and im 15(4 votes)

- At0:19what exactly does denote mean in math terms?(4 votes)
- It basically just means "represent." Sal is saying that S(t) means "the thickness of the ice (represented by S) as a function of time (represented by t)."(12 votes)

- What if the rate of change was not constant? How much harder would this become? I bet this is the advent of calculus?(5 votes)
- Nice question!

Yes, you are correct that functions with non-constant rates of change are the advent of calculus! For example, suppose we are given the ice sheet's initial thickness and also given the rate of change of the thickness as a function of time. To find the formula for the thickness as a function of time, we would need to 1) find the anti-derivative of the rate of change function, then 2) use the initial thickness to solve for the constant term that appears in the anti-derivative. (Derivatives and anti-derivatives are the two main operations in calculus, and both operations are ultimately based on the concept of a limit.)

Have a blessed, wonderful day!(10 votes)

- can you do this in the form
?**Y=mx+b**(5 votes)- At about @4:20in the video, Sal shows you that he is using y=mx+b, but the the variables are swapped to different letters to match the ones used in the problem description.(7 votes)

- Why did Sal use the slope-intercept form to find the equation for this question?(6 votes)
*Good question. We need to use the slope-intercept form, because we are asked to create an equation. While creating an equation, we use this form so that we get the x and y-intercept, which serves as the constant rate to our answer.*

For example, let's say that you bought 7 apples each month, and you already had 4 apples at home. You're asked to create its equation.(I know this is a terrible question) How will you do that? Well, we can use the slope-intercept form to figure it out!

The constant rate will be 7, since you bought 7 apples each 4 months. How will we create an equation out of it? Simple.

y = mx+b

y =(the constant rate)x+(what we started out with, or b)

y= 7x+4

Didn't use a great vocabulary, but hope this helps!!(4 votes)

- So what's the difference between a graph and t-chart?(3 votes)
- A graph is a coordinate plane that expresses data using a line, points, etc. On the other hand, a t-chart uses only numbers, or words, to express data. The t-chart is not very visual, while the graph is.(8 votes)

- At5:06, Sal's equation is S(t) = -0.25t +2. Is S(t) the dependent variable or just S?(5 votes)
- S(t) is a function not a dependent variable.

A function is kind of a box which takes a number and throws a different number back.

The name of the box is S and the number you put in is t ( which could be any number ).

Let us say that t is 4 and we throw 4 in the S box, inside the box, we multiply 4 * -0.25 which equals -1 , then we add 2 to it.

If we see it in the equation form it will look like this:

S(4) = -0.25 * (4) + 2(5 votes)

- me big smart you big smart we all big smart(7 votes)
- Why do we write functions as, for example
*S(0)=2*?(4 votes)- This is to show that we are setting the independent variable to 0 to produce a value for the dependent variable, S.(2 votes)

## Video transcript

- [Voiceover] "A lake near
the Arctic Circle is covered "by a 2-meter-thick sheet of ice "during the cold winter months. "When spring arrives, the warm
air gradually melts the ice, "causing its thickness to
decrease at a constant rate." It's gonna decrease at a constant rate. "After 3 weeks, the sheet
is only 1.25 meters thick. "After 3 weeks, the sheet
is only 1.25 meters thick. "Let S(t) denote the
ice sheet's thickness S "(measured in meters) as a function "of time (measured in weeks). "Write the function's formula." Alright, so we have some
interesting things here. They've given us some
values for this function. We know when time is equal to zero. We know that S of zero,
when time equals zero, that's when the sheet is two meters thick. So S of zero is equal to two. And they also tell us
that after three weeks, the sheet is only 1.25 meters thick. And when we have the function S of t, S is measured in meters,
time is measured in weeks. So after zero weeks,
we're two meters thick, and then they tell us, after three weeks-- So S of three. After three weeks,
we're 1.25 meters thick. Or another way to think about it, I could write t here in
weeks and S in meters, and when time is zero,
we're two meters thick, and when time is 1.-- Sorry, when time is three weeks, we are 1.25 meters thick. So when our change in time
is equal to positive three, we increased our time by three, what's our change in thickness? Our change in thickness,
the triangle here, that's the Greek letter delta,
shorthand for "change in," well, this was negative 0.75. So what was the rate of
change over this time? And they tell us that the rate of change is at a constant rate. So whatever it is between
these two periods of time, between zero weeks and three weeks, it would be that same rate
between any two periods of time, between zero week and one week, or one week and two weeks,
or 1 1/2 weeks and 1.6 weeks. So what is the rate of change of thickness relative to time? Well, it's going to be change in thickness over change in time. How much does our
thickness change per time? Well, we saw right over here. Our thickness went down, set 0.75 meters in three weeks. Or we could say that this right over here is equal to, let's see. 75 divided by three is 25, so
0.75 divided by three is 0.25. We have the negative out there, negative 0.25 meters per week. So how can we take the information we have and express this as a function? It's going to be a linear function, because we see that we are
changing at a constant rate. Let's think about it a little bit. Linear functions, one way
we could write it is in-- So we could write it-- If we were dealing with x and y, you might recognize y
is equal to mx plus b, often written as slope-intercept form. This is when you're dealing with x as the, I guess you could say
the independent variable, y as the dependent variable, and b would be where you start. What happens when x equals zero and m is your rate of
change, it's your slope? So in this case, we don't have y and x, we're going to have S and t. We have S as a function of time, and it's going to be equal
to the rate of change... times time, plus where we started, plus b. Now, what is b going to be? Well, one way to think about it is, well, what's S of zero going to be? S of zero is going to
be m times zero, plus b. S of zero is going to be b. Well, we already know that this ice sheet, it starts off at two meters thick. So S of zero is equal
to b, is equal to two. So b is equal to two. And what is m? Well, we've already said,
that's our rate of change, that is our slope, that is how much our thickness changes with respect to time. And we already figured out
that that's negative 0.25. So m is negative 0.25. You could say that m is the
slope between this point, between the point zero comma two, and the point three comma 1.25, if we were plotting these points
on a t/S coordinate plane. So now we can write what
the function's going to be. Maybe I'll do this in a
new color just for fun. S of t, thickness as a function of time, is going to be equal to m, negative 0.25, times time, plus two. Or if you want, you
could write it like this, two minus 0.25t. I actually like this
form a little bit better. In my brain, it kind of describes what's happening a little bit more. When time is equal to
zero, you're starting at two meters thick, and then every week that goes by, as t increases by one, you're going to lose a quarter of a meter. You're going to lose, you
have a negative value right over here, you're gonna lose 0.25 meters. And if you really want
to kind of get this even in a deeper level, I
encourage you to graph it, and it'll become even
clearer what's going on here. That this right over here
is this right over here, is this right over here, this is the slope of the line that represents the solution
set to this equation, and this two, this would
be your vertical intercept. In this case, it would be your S-intercept as opposed to your y-intercept,
when y is the vertical axis.