Mastering Vector Addition: A Comprehensive Guide to Physics

♪♪ Nice place, huh? What does this have to do with physics?

We're about to find out. ♪♪ By now, you know that vectors have both

a magnitude and direction. These are the key elements we need to know when we're navigating,

whether it's on a hike or in an airplane. In this segment, we're going to learn how to add vectors mathematically,

and see what that shows us about how far and in what direction we've gone or need to go. I'm going to use this information to find a cache

that I see on my geocaching app. So, that's where this park comes in. You can also resolve vectors graphically,

using a coordinate plane, as we did in another segment in this course. But if you use math, specifically, algebra,

you get more precise information. Why do we want to be as precise as we can sometimes? Pilots use vectors to navigate to get to a destination

the fastest way, without getting lost. Air traffic controllers deal in vectors all the time to direct those airplanes to land safely.

And the military relies on them to coordinate troop movements, communicating vital information about missions.

So, accuracy counts. Let's do a really simple example of resolving vectors with math first.

I can walk you through, literally, how to add vectors in one dimension, and get a resultant.

A result that tells us how far we've traveled, and in what direction. Let's make the positive direction to the right.

I'll walk 3 meters to the left, or negative 3 meters. That's a vector.

I go six meters back to the right, or positive 6 meters. That's another vector, all in a line.

So that's one dimension. So how far did I go? I ended up positive 3 meters from the origin, right?

With me so far? I know, super simple, right? Even when we're resolving vectors using math,

by the way, you can still draw your vectors if you'd like. For some people, it makes it easier to see how the vectors are interacting.

It's up to you. Anyway, that's all there is to resolving vectors in one dimension.

But vectors can be in any direction of any magnitude. And that means what? That we might be dealing with two dimensions.

So how does that work? Let's look at that geocache app and get information I need that will tell me

where I have to go to find a cache, and how far away it is. So, here's the location.

I'll map out a route to get me there. Okay, I figure I need to bike 125 meters due north, then 65 meters due west, then 15 meters south.

The cool thing about resolving vectors is that it will tell me the shortest distance from where I start to where I finish.

So, if I want to get the cache the most efficient way possible and give myself a break on the biking,

why not go for it? Let's look at the long way graphically, so we can see what that looks like.

This shows the 125 meters north, the 65 meters west, and the 15 meters south. Now let's take a look at the numbers.

And we can see how we can make our route more efficient. To resolve these vectors, we need to know two things, remember?

That's right, magnitude and direction. Let's start by breaking down the vectors into components. We can use this chart to do that.

When we fill it in, we see what part of each vector lies in the x direction, and what part of each vector lies in the y direction.

Let's make our positive direction to lie along the positive x-axis and the positive y-axis. The x component of the 125 meter north vector is 0 meters,

since it lies 100% in the y direction. The y component is equal to positive 125 meters. Now let's look at our 65 meter vector.

The x component is negative 65 meters, since it is wholly in the negative x direction. The y component of this vector is 0 meters.

The 15 meter vector has an x component of 0 meters, and a y component of negative 15 meters, since it lies totally in the negative y direction.

Now let's add those together. First, in the x direction, and then in the y direction. When we add up all of the components in the x direction,

we get negative 65 meters. When we add up all of the components in the y direction, we get positive 110 meters.

We'll use these totals to find both magnitude and direction of our resultant. Let's do magnitude first.

To get the magnitude, the distance I have to go, we use the Pythagorean Theorem. You remember.

The square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides. That's a-squared plus b-squared equals c-squared,

where c is the hypotenuse, and a and b are the opposite and adjacent sides. Since you're looking for the magnitude of the hypotenuse,

you need to rearrange the equation to c equals the square root of a-squared plus b-squared. Now let's walk through the steps to use the equation.

Step 1, square the length of the opposite side. Negative 65.0 meters squared. Step 2, square the length of the adjacent side.

Positive 110 meters squared. Step 3, add the answers from steps 1 and 2. Step 4, take the square root of the sum from step 3.

And we get 127.8 meters as the distance I need to travel. But I also have to know what direction to go in, right?

There's an equation we can use for that, which gives us the angle that describes the direction. This will involve a little trigonometry,

but stick with me. The direction we need is the angle between the first leg and the resultant vector.

We know that the tangent of this angle is the side opposite divided by the side adjacent. That's the total y distance

divided by the total x distance, which is positive 110 meters, divided by negative 65 meters.

Since we don't know the angles, we need to use what is called an inverse of the trig function.

In this case, the arc tangent, also written tan to the negative 1, to determine the angle.

So, to find the angle from our starting vector to our resultant vector, we use the ArcTan of positive 110.0

divided by negative 65 meters. We get negative 59.4 degrees. We know that our vector lies in the second quadrant,

since x has a negative value, and y, a positive one. When a vector lies in the 2nd quadrant,

add 180 degrees to the value calculated to find our angle, with respect to the positive x-axis. Negative 59.4 plus 180 equals 120.6 degrees,

with respect to the positive x-axis. So how does this information translate to finding our direction with our compass?

Since true north is 0 degrees, or 360 degrees on a compass, if we subtract 90 from 120.6 degrees, we find our resultant bearing is 30.6 degrees from North.

Subtracting this from 360 gives us a compass bearing of 329.4 degrees in a northwesterly direction.

I can look at my compass, take a bearing of 329.4 degrees, and go 127.8 meters and be at my geocache.

But for now, that's all I need to know. I can pretty much bike straight there, courtesy of resolving three vectors,

because I have the exact information about magnitude and direction, just by using a little math.

Oh, and one more thing, which is pretty cool. I could add the vectors in any order, and I would come up with the same resultant.

So you can switch them around like this... or like this, and you won't go wrong. For a look at more advanced two-dimensional problems,

see the Closer Look. That's it for this segment of "Physics in Motion," and we'll see you guys next time.

For more practice problems, lab activities and note-taking guides, check out the "Physics in Motion" toolkit.