# Velocity versus speed and philosophizing thereof

Tags:
1. Sep 22, 2015

I asked a class of students what the difference was of velocity versus speed. The answer I was looking for was "direction" i.e. one is a vector. But a number of them said "Velocity is displacement divided by time, and speed is distance divided by time." Ignoring the second half of that assertion, I'm not sure what to say about the statement…because that is exactly what the equation says! (V=deltaX/t). To complicate things, "velocity" is ofttimes used colloquially and not in such a picky way. What would you say to the students who responded like this?

2. Sep 22, 2015

### andrewkirk

I would ask them what the difference is between displacement and distance. If the answer included that displacement involves direction (ie moved 10 metres to the East) whereas distance doesn't (ended up 10 metres from where it started), then I think the answer is good. They would still have captured the idea that velocity involves direction whereas speed does not.

3. Sep 22, 2015

In this case, displacement is "how far you are from where you started" so I don't believe they are thinking about direction very explicitly. Probably they are just looking at the formula --> I can't say the answer is wrong, but it's not really right. I guess one thing I could do is to dissect that and say "note that displacement implies a DIRECTION"...

4. Sep 22, 2015

### nasu

I think that before discussing velocity you should make sure they understand the difference between displacement, distance and distance traveled.

5. Sep 22, 2015

### CWatters

Perhaps ask them about situations where after some time the displacement is zero again.