To consider the molecule in gas at rest, suppose a container of gas at rest.
We know that velocity is a vector quantity and it includes direction but speed is a scalar quantity and it does not include a direction.
Although the molecules are moving around a lot, they are only moving around within the fixed walls of the container. As the container isn't moving (given), the net velocity of the gas molecules is zero. Think about their random movements as vectors, then think about adding up all those vectors over time. They cancel each other out over time. So, the average velocity of a molecule in gas at rest is zero.
But it's only the directions that cancel each other out, which is why the average speed (ignoring direction) is non-zero.
To learn more about average speed refer to the link:
brainly.com/question/12322912
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