# Are my observations about the differences between average velocity and speed, in 1 dimension, correct?

I've been struggling to understand the differences between average velocity and average speed in 1 dimension; by doing some exercises I think I have figured it out, but i want to check if I'm correct.

This describes the movement of a particle through 4 positions: $$P_1, P_2, P_3, P_4$$. Each position has a respective coordinate $$x_1, x_2, x_3, x_4$$, and a respective moment in time $$t_1, t_2, t_3, t_4$$ in which the particle is located in that position.

Let's talk about the movement from $$P_1$$ to $$P_3$$:

The x component of average velocity is $$\frac{x_3 - x_1}{t_3 - t_1}$$. However, average speed seems to be $$\frac{|x_2-x_1|+|x_3-x_2|}{|t_2-t_1|+|t_3-t_2|}$$. I have some questions about the latter; As you may have seen, I used absolute value signs. Regarding the norm of the vectors (which describes the distance between the points) it is because I bumped into an exercise in which average speed, initial and final coordinates, and initial time were given, and I had to solve for final time. the final position was a negative number, and initial was positive, so I ended up getting a negative distance, which resulted in a negative final time, something which can not happen. So I concluded that average speed is about the movements in absolute value, which is reinforced by the fact it's an scalar. I'm sure putting absolute values to find the distances between points is correct, but I could be wrong, so ¿is this correct?. The ones I'm not so sure about are the absolute value signs in time, because up to this point I have not done an exercise with negative time involved, but I don't know if they exist, which if they didn't, would make abs. val. signs unnecesary ¿do you know if I have to put them? ¿will I find excercises with negative time involved?

About the movement from $$P_1$$ to $$P_2$$.

It seems x component of average velocity is $$\frac{x_2 - x_1}{t_2 - t_1}$$, and average speed is given by $$\frac{|x_2 - x_1|}{|t_2 - t_1|}$$ Is this correct?

• I'll keep saying it forever: the desire of the English to be special will keep confusing people. Velocity is the only vector whose modulus has a special name. Other languages do not make such distinction. Other vectors do not cause those problems: force can be $F$ or $\vec{F}$ depending on the context. So this is the same. Nov 23, 2018 at 20:10
• The easiest way to understand the difference is to think about 1-dimensional motion along a circle rather along a straight line. A particle moving in a circle with constant speed has zero average velocity. Nov 23, 2018 at 20:10
• @FGSUZ I assume you're joking about having the two words "speed" and "velocity" being anything to do with a desire to be special! The mean speed is not necessarily the same as the modulus of the mean velocity, as G. Smith illustrates. I'd also defend the usefulness of having separate terms "displacement" and "distance". Is English the only language to have these? Nov 23, 2018 at 22:33
• @PhilipWood of course it wasn't serious, hope I didn't offend you. But yep, this is an issue that keeps rising every season. I wanted to point out that everybody understands that force isn't the same as it modulus. Yet we all can use the single word "force" with both meanings and we don't get confused. So I don't se any utility on the speed/velocity distinction, rather, it confuses people over and over again. Nov 23, 2018 at 23:07
• No offence! Enjoyed the joke. But what about $distance$ and $displacement$? A useful distinction? Nov 23, 2018 at 23:40

• Thanks! But now I'm only left with just one doubt: The absolute value signs in the denominator of $\frac{|x_2-x_1|+|x_3-x_2|}{|t_2-t_1|+|t_3-t_2|}$, these are: $|t_2-t_1|+|t_3-t_2|$ are necessary? I ask this because I haven't bumped into an excercise with negative time, so if time is never negative, absolute value signs aren't necessary, but if time can be negative, then they might be. Nov 23, 2018 at 20:21