# How to find the velocity of the train using the graph below?

• May 15th 2011, 05:43 AM
silvercats
How to find the velocity of the train using the graph below?
http://oi52.tinypic.com/1zg7oxy.jpg

I got few equations but not correct I guess
(1/2)v(60+60-(t1+t3))=432

1/3=v/t1
and 1=v/t3

after manipulating the answers I get a way off answer which is not even close to the answer in the book.explained details are appreciated
• May 15th 2011, 05:59 AM
alexmahone
Quote:

Originally Posted by silvercats
http://oi52.tinypic.com/1zg7oxy.jpg

I got few equations but not correct I guess
(1/2)v(60+60-(t1+t3))=432

1/3=v/t1
and 1=v/t3

after manipulating the answers I get a way off answer which is not even close to the answer in the book.explained details are appreciated

$\displaystyle \frac{1}{2}(120-3v-v)=432$

$\displaystyle v(60-2v)=432$

$\displaystyle 30v-v^2=216$

$\displaystyle v^2-30v+216=0$

$\displaystyle (v-18)(v-12)=0$

$\displaystyle v=18\ or\ 12$

But $\displaystyle v=18 \implies t_1=54\ and\ t_3=18$ (Contradiction since the times add up to more than 60.)

Therefore, $\displaystyle v=12$
• May 15th 2011, 09:34 PM
silvercats
That is great.thanks!