Cross Product - Video Tutorials & Practice Problems

Computing the Cross Product

Welcome back everyone. So you may recall in a previous video, we talked about how to find the dot product between two vectors. And this was an operation where you took two vectors and multiplied them together. And it gave you a scalar result. Well, in this video, we're going to be learning about something called the cross product. And just like the dot product, the cross product is a way to multiply two vectors together except rather than getting a scr result, you actually end up getting a vector result. Now this process for finding the cross product is pretty tedious, but we're going to be learning in this video that there actually are some patterns you can recognize in this operation. So let's just jump right into things. Now, when doing a cross product, you're taking two vectors and you're multiplying them. And the other vector that you get is always going to be perpendicular to the original vectors. So if we have these vectors U and V and we want to find their cross product, the cross product of U and V is going to be a vector that points perpendicular to the vectors that we already have. So it would look something like that. Now, like I mentioned doing this cross product can be a pretty tedious process. So to make sure we understand this, well, we're going to solve this by the steps. So let's say we have this example where we're told to find the cross product W which is equal to the cross product of U and V. Now I can see that we have vector U which is 201 and vector V which is zero negative 12. Now our first step with the cross product should be to write a matrix for the IJK components for each vector. Now, in this matrix, the ISJS and KS go on top. Now in the second row here, we're going to have our vector U and notice that vector U is the first thing that we see in this cross product operation. So we're going to take the first vector, we see we're going to write its component. So we have the I component which is two, we have the J component which is zero and then we have the K component which is one. So that's going to be vector U. Now, below that, we're going to write the components for vector V which I can see here are zero negative 12. So this is our first step. Now our second step is going to be to repeat the I and J columns outside of the matrix. So I'm going to repeat the I column, which I can see is 20. And I'm also going to repeat the J column, which I can see is zero, negative one. So that's our second step. Now, our third step is going to be to for each of the W components, which are these components right here to write UV minus VU for each of them. So this is going to be the pattern for each component. We're always going to have UV minus VU. And that's the same for the Z component. It's going to be UV minus VU. Now, the way we do the cross product operation is by using this matrix and multiplying down then up for each product. And that's what to create this kind of cross pattern, which you can think of it like a cross product. So we're going to start with I, this corresponds to the X component of our vector. If we start with I, we can cross down through here crossing through the J component and then we can cross up right there. This is what the pattern is going to look like. Now notice when I do this, I have the Y component multiplied by the Z component. And then I also have the Y component here multiplied by the Z component there. So it ends up being Yzyz. Now let's move on to the J unit vector which corresponds with the Y component. Now, if I start with J I can use the same cross pattern where I first cross down and then I cross up. Now notice when I do this with the J, I end up with the Z component times the X component and then I once again, end up with the Z component times the X component. That is going to be the Y component. Now, what I'm going to do from here is move on to the K component and for K I'll just use the same crossing pattern. I'll cross down first and I'll cross up. So we're going to have X times Y and then we're going to have X times Y. Now notice the pattern that happens here. Notice each time we did this cross down and up strategy, we always ended up with the unlike components that got multiplied together. So notice for the X component, we had Yzyz notice for the Y component, we had ZXZX and then notice for the Z component we had Xyxy. So as you can see when you do this cross product operation, you start to recognize a pattern. Now we can go ahead and actually figure out what our solution is now that we know what these components are. So we're going to have zero times two, which is zero minus negative, one times one, which is negative one and zero minus negative one is the same thing as zero plus one, which comes out to just one. That's the X component. Now, for the Y component. Well, let's go to J for J, we're going to have one times zero, which is zero minus two times two, which is 40 minus four is negative. And that's our Y component. Now let's move on to the K component. For K I can see that we have two times negative one, which is going to be negative two minus zero times zero, which is zero, negative two minus zero is negative two. So now we have our XY and Z components. So I just need to write them in this vector. So I can see that we're going to have one, I then negative four J and then negative two K. So this right here is what our vector is going to look like. And if I want to write this in the component form, it's going to be one negative four, negative two. So this right here is the solution to the problem and that is how you can find a cross product. So as you can see, it's pretty tedious. But by doing this cross down and up strategy on this matrix notice we were able to find it was the unlike components being multiplied each time. So if you just subtract those results like we did here, it will allow you to find the vector that you're looking for. So I hope you found this video helpful. Thanks for watching and please let me know if you have any questions.

If vectors $v ⃗=⟨3,1,0⟩$, $u ⃗=⟨0,-2,0⟩$, and $w ⃗=v ⃗×u ⃗$, find $w⃗⃗$.

• A. $w⃗=\langle0,0,-6\rangle$
• B. $w⃗=\langle0,-2,0\rangle$
• C. $w⃗=\langle0,0,6\rangle$
• D. $w⃗=\langle0,0,-2\rangle$

If vectors $a⃗=5î$, $b⃗=12k̂$ and $c⃗=a⃗\times b⃗$ , find $c⃗$.

• A. $c⃗=60ĵ$
• B. $c⃗=-60ĵ$
• C. $c⃗=5î+12k̂$
• D. $c⃗=60k̂$