About this video
Particles A and B have velocity vectors a and b, and we are told the magnitude of a equals the magnitude of b. The video explains that this means the particles have the same speed, because speed is the magnitude of velocity, but not necessarily the same direction. It rules out the other choices by showing what equal vectors or equal directions would look like.
You'll be able to
- interpret the statement |a| = |b| for velocity vectors.
- explain that speed is the magnitude of velocity.
- tell the difference between equal magnitudes and equal vectors.
Key words
- magnitude
- The size or length of a vector, written with double bars like ||a||.
- velocity
- A vector that gives both speed and direction of motion.
- speed
- The magnitude of velocity: how fast something moves, without direction.
- equivalent vectors
- Vectors with the same magnitude and the same direction.
Transcript
- [Instructor] We're told that particles A and B are moving along a plane. Their velocities are represented by the vectors, vector a and vector b respectively. Which option best describes the meaning of the following statement. Choose one answer. So pause this video and try to work through this on your own before we work through this together.
All right, now let's work through this together. So this is saying that the magnitude of vector a is equal to the magnitude of vector b. So we know that a vector is specified by both a magnitude and a direction. And this is just saying that the magnitudes are the same.
So for example, vector a could look like this, and vector b could look like this. It could do something like that where it has the same magnitude and the same direction. Or vector b might be in a completely different direction. The magnitudes being equivalent just tells us that the length of these arrows are the same, but we could have different directions here.
So what this tells me is that we have the same speed which is the magnitude of velocity, but not necessarily the same direction. Now let's look at the choices here. The first choice is that two particles move at the same speed and in the same direction. So we've already said that that's not necessarily the case.
In order for choice A to be correct, they would essentially have to be equivalent vectors. Choice A would be the case if we said that vector a is equal to vector b, then they would have to have the same magnitude and the same direction, the same magnitude and the same direction.
But that's not what they told us. They just told us that the magnitudes are the same, not necessarily the directions. So I'll rule that one out. The two particles move at the same speed, but not necessarily in the same direction. Yes, that's what we just talked about.
They have the same speed, which is the magnitude of velocity, but they didn't tell us anything about the direction, just the magnitudes. So I like this choice, but let's look at choice C. The two particles move in the same direction, but not necessarily at the same speed.
Well, we know they move at the same speed. That's what this is telling us. The magnitudes are the same. We just don't know anything about the direction. So I would rule this one out as well. In order for choice C to be the case, you would see something like this, maybe that is vector a right here, and then vector b would move in the same direction, but it would have a different magnitude.
And here you would visualize the magnitude as the length of the arrow. But that's not what they told us. They told us this right over there.
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