Video Transcript
Which of the following objects’ motion best represents Brownian motion?
Looking at our five answer options, we can figure out which one is best by considering what Brownian motion involves. This sort of motion is described as the apparently random motion of a particle driven by collisions with other particles. So, say that we had a bunch of essentially identical particles and that all these particles were in motion at different speeds and in different directions. Every so often, these particles would collide with one another. And if we track the motion of one of these particles, say, this one here highlighted in pink, then over time we would see this particle follow a path that’s determined by collisions with the other particles. Its motion would appear to be apparently random.
Reviewing our answer options, option (A) shows a particle that moves in a straight line. This could represent part of Brownian motion before this particle had collided with any other, but it’s not the best representation of Brownian motion. If it is Brownian motion, the time scale over which we see this motion is too short to recognize it for what it is. Answer choice (B) shows a particle that is rotating around an axis through its center. This also isn’t Brownian motion. It’s not apparently random motion driven by collisions. The particle in option (C) doesn’t exhibit motion at all. In Brownian motion though, a particle tends not to stay still.
Looking next to answer option (D), the particle here does exhibit apparently random motion. At any given point in the particle’s path, we couldn’t guess in which direction it will move next. This agrees with our definition of Brownian motion. Answer choice (D) looks promising. And to confirm that it will be our answer, let’s look at answer option (E). Here, the particle moves in a complete circle. So, it is moving, but this motion is not apparently random. Rather, it’s very orderly motion, and therefore it’s not a Brownian motion representation.
Of all the possible options, option (D) best represents Brownian motion.
