# Displacement and average velocity

## Homework Statement

A hiker travels south a long a straight path for 1.5 h with an average velocity of .75 km/h. then travels south for 2.5 h with an average velocity of .90 km/h. What is the hiker's displacement for the total trip?

## The Attempt at a Solution

For the answer I have 3.4 km to the south.
but hm, I think I copied the question wrong. I have a feeling that I was supposed to have another coordinate sign besides south... so I'm not sure.

cristo
Staff Emeritus
Well, your answer is correct (3.375 km i get before rounding) if the question is as you've written it!!

oh ok. cool.
but oh gosh, let me try it. but well I'm not really sure what to do.

cristo
Staff Emeritus
Ahh, so you mean that's the answer the book gives? Ok, well what equation do you know relating velocity, distance and time?

well its a review sheet, the teacher gave it to me.

distance/t
or do I use: av. velocity= total distance/time

cristo
Staff Emeritus
Yes, use av.velocity=distance/time, but remember to calculate the two parts separately, and then add them up at the end to arrive at a total distance.

awesome.
another question, for instance 1.5 h, ok do I convert that into minutes? and if so do I take the .5 as half or as 50, as in fifty minutes. 1 hour and fifty minutes or 1 and a half or 90 minutes or 110 minutes?
wow, I ask a lot of questions.

cristo
Staff Emeritus
Well, you would take 1.5h as one and a half hours, or 1 hr 30 mins. However, in this instance, you don't need to convert.

The units of velocity are given as km/hr, and the times in hr, so, if we look at the equation: s=vt, and consider putting the units in, we get s (km) = v(km/hr)*t(hr). Notice that the hr's will cancel, and give a result in km, which is what is required.

For calculations like this, as long as you keep to the same units throughout, the answer will be correct. (A problem would occur if, for example, the velocity was in km/hr and the time measured in seconds. In this case we would have to convert units)

Hope this is clear!

yeah thats what I noticed.
but well ok for km/h I get 1.65 and then for hours 4.
so this is my equation: 1.65 km/h /4h and then I get .4125...
eek. I know I'm doing something wrong.

cristo
Staff Emeritus