# Homework Help: Finding the Average velocity

1. Oct 13, 2008

### pinkxoxo

1. The problem statement, all variables and given/known data
A train moving at a constant speed of 51.0 km/h moves east for 38.0 min, then in a direction 53.0° east of north for 16.0 min, and finally west for 64.0 min. What is the average velocity of the train during this run?

Magnitude? km/h

Direction?
° (counterclockwise from east)

2. Relevant equations

I dont even know what equations to use. I know average velocity is displacement/time but idk where to start from

3. The attempt at a solution

No attempts im lost help please!

2. Oct 13, 2008

### gabbagabbahey

Like you said, average velocity=displacement/time. So you need to start by calculating the total displacement and the total time elapsed.

3. Oct 13, 2008

### pinkxoxo

ok the total time would be all the times added together but how do i find the total displacement

4. Oct 13, 2008

### pinkxoxo

T=118 mins but i need to convert to hours so thats 1.967 hours

5. Oct 13, 2008

### HallsofIvy

The displacement is the vector that goes straight from the initial point to the final point. You can find it from the information given by adding components or by trigonometry.

6. Oct 13, 2008

### gabbagabbahey

The total displacement will be the vector sum of the three individual displacements (i.e. $\vec{d}_{tot}=\vec{d}_1+\vec{d}_2+\vec{d}_3$) So start by computing those:

If the train travels 51.0 km/hr due East for 38.0min, what is its displacement $\vec{d}_1$? (You may as well call East the positive x-direction and North the positive y-direction)

7. Oct 13, 2008

### pinkxoxo

oooohhhh ok hold on so i calculated each of the displacements by dividing the v by the t given and added them all together and tried to divide by the total time in hours and i got 162.46 km/h but it says im wrong

8. Oct 13, 2008

### gabbagabbahey

That's because they don't all point in the same direction. They are vectors and need to be added like vectors, not scalars.

9. Oct 13, 2008

### pinkxoxo

ok i still got it wrong! GOD AND I HAVE 30 mins left to do this
and i need to finish my other probs

10. Oct 13, 2008

### gabbagabbahey

If you show me your work, I can tell you where you are going wrong (P.S. it's usually a bad idea to leave your physics assignments to the last minute ;0) )

11. Oct 13, 2008

### pinkxoxo

welll........i had a busy weekend lol. But i have been workin since 8 in the morning! but i wanna take the trig approach. but my work is this ok..........................(51/.633333)+(51/.267)+(51/1.067)<--but i made that neg bc it goes left? then i divided by all the times added together and converted to hours which was 1.967 hours

12. Oct 13, 2008

### gabbagabbahey

For starters, displacement=velocity*time not velocity/time.

Secondly, you are adding them as if they are all in the same direction, but they're not...the first one points east, the second point 53 deg east of north and the 3rd one point west. draw a picture to help you see what is going on.

13. Oct 13, 2008

### pinkxoxo

X_X NO WONDER and i drew the pic i just dont know wut to do with it..i was gonna try to use trig but im so freakin lost on this problem

14. Oct 13, 2008

### gabbagabbahey

hmmm... start by computing each of the 3 distances (just the distance, not the displacement)

15. Oct 13, 2008

### pinkxoxo

ok and i calculate by v=d/t so d=v/t which is wut i did earlier when i was dividing v by t and thats how i got my d's for example (51/.63333)

16. Oct 13, 2008

### gabbagabbahey

v=d/t implies d =v*t not v/t

17. Oct 13, 2008

### pinkxoxo

Thank u for all of your help but my thing is due and i requested an extension but i think im just gonna leave it at what i have and get an 88 thank u once again...I'm glad i got an account on this bc ppl are very helpful when it comes to my webassign problems

18. Oct 13, 2008

### pinkxoxo

and oh yeah ur right idk wtf i was thinking! it is v*t..(STUPID MISTAKE) i promise im not dumb just more of a calculus person as opposed to physics and i know that they r basically the same but there is a difference to me my mind just works differently maybe its bc calculus isnt polluted with words and laws of dimensions and stuff its just plain 'ol math and yeah nvm im going off on a tangent XD

19. Oct 13, 2008

### pinkxoxo

woah my other posts got deleted weird thanks for the help