How speed = distance/time

louis arthur

I was looking at how speed = distance/time. Then i started messing around with negative signs in the equation and it gave me strange things to think about. I can put a negative sign on the top or bottom of the fraction side and I have a negative speed. When I think about traveling at negative speeds across negative distance, that is acceptable as an object driving away from me with the transmission in reverse. But if the negative sign is on the time side of the fraction, I think about that object moving the same as the former object. This messes up my concept of the entropy in the motion of these objects since the latter would have exhaust rushing into the muffler. I looked at each variable equaling the other two, and the various ways a negative sign can change the real life concepts of what is actually happening. In the end I sat there dumb-founded at the significance of the negative sign being on particular sides of the fraction, how it relates to the actual physical processes and why I was taught that where the negative is in a fraction is insignificant. I am new to thinking about science and math and such. Is this a real thing that is considered by physicists and physics students?

louis arthur

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berkeman

Mentor
Don't sweat the negative signs in the equation that you are considering. Just as you imply, you can run the "video" of the situation backwards or forwards in many situations. Negative signs show up lots of times, and you just have to think some about what they mean. Like the concept of negative frequency in spectral plots....that took me a while to get my arms around.

Libertine

louis arthur said:
When I think about traveling at negative speeds across negative distance, that is acceptable as an object driving away from me with the transmission in reverse.
I don't think that's right. Speed and Distance are both scalar quantities - they have no direction to them. So (unlike velocity), a negative speed does not mean going backwards - it's not really comparable to anything in the real world.

Hootenanny

Staff Emeritus
Gold Member
Libertine said:
I don't think that's right. Speed and Distance are both scalar quantities - they have no direction to them. So (unlike velocity), a negative speed does not mean going backwards - it's not really comparable to anything in the real world.
As you quite rightly said, distance and hence speed are scalar quanities. However, it is therefore impossible to have a negative speed. Negative times do not usually occur because we are usually trying to find out what happens after an event (t=0), a ball being hit, or a projectile being fired etc.

louis arthur

is time a scalar quantity?

Hootenanny

Staff Emeritus
Gold Member
Under normal cricustances, time is a scalar. However, under some circustances, such as when considering relativity, time can be considered a vector quantity. An example of this would be the 'light cone' http://en.wikipedia.org/wiki/Light_cone

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