This is going to be a quick video about the GUESS method, which is a specific problem-solving tool
we use in high school physics. The GUESS method is a way of solving word problems. It helps you
find which equations to use specifically, that's the main reason why we care about
it. Each letter in the word GUESS here stands for a different step in the problem-solving
method. G stands for the given information, so when you're solving a physics problem the first
thing that you should do is write down all the variables that you know, the values of,
so set those variables equal to specific numbers from the problem. U stands for the unknown,
that's the thing that you're trying to solve for in the problem. E means the equation, which is the
equation that you will use. The equation always has the unknown, some or all of your givens,
and nothing else. That's going to be the most important part of the GUESS method: identifying
which equation to use. The next step is solve, which means to isolate your unknown variables
using algebra, to rearrange the equation that you have so that the thing that you're looking
for the unknown variable is on one side of the equation and everything else is on the other side,
and the other S stands for substitute which means to plug in your givens into your equation and
calculate your answer for your unknown. I'm not going to go too in-depth in the examples here,
I'm just going to do one quick example because we're going to be working with the GUESS method
a lot in this unit. So I'm going through the steps of the GUESS method one at a time starting with
writing down my given information G, and I'm using this example problem for the GUESS method: a car
drives at 20 m/s for 600 meters, how much time did this take? So you might already know how to solve
this. This video isn't really about solving this problem, it's about how to use the GUESS method,
so just make sure you're understanding each step. So every physics problem begins by decoding a word
problem to understand what information it's giving you, so here I know that this is giving me 2
things: it's telling me that the velocity of a car is 20 m/s and the displacement that it covers s is
600 meters. So that's the information that I'm given by the problem, so whenever you're writing
that you need to make sure to write the variable is equal to a number. Don't just write the number,
the variable is going to be really important to helping you solve the problem. Also make sure
you're including units as well. The next step is unknown, which is what you need to know, so this
is where you write the variable that you're trying to find. So in this case I'm trying to find the
time t, so I'm going to write t = ?. That's the thing that I'm looking for in this problem. Okay,
this is the most important step: identifying the equation. The equation always has the unknown,
some or all of the givens, and nothing else. I find that when students walk into physics they
sometimes assume that physicists have some really in-depth understanding of every single physics
equation and can understand all the implications of an equation just by looking at it, and that's
not exactly true. A lot of times when we're doing problem solving we're just pulling together things
that we know, finding equations that connect them, and then moving on without thinking too much about
what the equation actually implies, and that's what's going to happen in the GUESS method too.
You don't really need to think so much about the problem to identify the equation, you just
need to say "Oh I need t and I also have v and displacement s so there must be some equation
that has t and either v or s or both and nothing else" because as a rule if you only have one
missing variable in your equation you can always solve for that missing variable using algebra,
and here I remember the equation that connects v s and t: velocity is equal to displacement divided
by time. So that is going to be the equation that you use. Again, the way that you identify
that is to find an equation with your unknown, the things that you know, and nothing else. A common
point of confusion is that some students assume that the equation has to have the unknown on one
side all by itself. They'll say, like, "Oh okay this is the equation for velocity, but where's
the equation for time?" but physics doesn't really work that way. Any physics equation can be used to
solve for any variable inside of that physics equation, no matter where that variable is,
if we do know all the other variables. So we don't really think in terms of what is the equation for
velocity or time or displacement, we just look for which equations connect which concepts. So
in this case it's v is displacement over time. The next step is to solve, which means to isolate the
unknown variables using algebra. So I want to isolate my unknown variable t and get it on one
side of the equation by itself, so this is my equation, and to get t out of the denominator
I'm going to start by multiplying both sides by t. When I do that, the ts over here cancel out and I
get t times v is equal to s, but I'm still trying to get t by itself, so the next step is to divide
both sides by v, and my final answer is time is equal to displacement divided by velocity. This
will always be true for constant velocity in the same situations where velocity is equal to
displacement over time is true. So we now have our equation and we're ready to substitute our
numbers. So we're going to substitute, plug in your givens into your equation and calculate
your answer. So I'm just going to plug in my given numbers. I know displacement and I know velocity,
so when I plug that in I get an answer of 30 seconds, and that is how you do the GUESS method.
So one more time, in given information you write down all the variables that you're given and set
variable is equal to this, and the reason why you're writing the variables as well as their
values is that you want to use those variables to identify your equation later. After you write down
your givens, write down your unknown variable and say this variable is equal to question mark. Next,
identify the equation using your givens and your unknowns. Finally solve by isolating the
unknown variable into one side of the equation and then substitute your numbers in to solve
the problem. So that's the basic structure of how we use the GUESS method in physics.