# velocity-time curve problem

• Jul 27th 2012, 08:03 AM
srirahulan
velocity-time curve problem
A train start to move from rest and then travel its first part with constant acceleration of $\displaystyle f_1$then travel the second part of the constant velocity of V then travel the third part of the constant deceleration of $\displaystyle f_2$ then it comes to rest.So draw a velocity - time curve and it travel constant velocity with the $\displaystyle \frac{4}{3}t$ of the total time and the average speed of the total travelling is KV.so find out the value of k? so please give me the ideas to do that.
• Jul 27th 2012, 08:25 AM
HallsofIvy
Re: velocity-time curve problem
Quote:

Originally Posted by srirahulan
A train start to move from rest and then travel its first part with constant acceleration of $\displaystyle f_1$then travel the second part of the constant velocity of V then travel the third part of the constant deceleration of $\displaystyle f_2$ then it comes to rest.So draw a velocity - time curve and it travel constant velocity with the $\displaystyle \frac{4}{3}t$ of the total time and the average speed of the total travelling is KV.so find out the value of k? so please give me the ideas to do that.

At constant acceleration $\displaystyle f_1$, after time t, the speed will be $\displaystyle f_1t$. You say "the first part" but you don't say what the time is or if "the first part" is the same length of the time as the other "parts". The graph of that is a straight line with slope $\displaystyle f_1$. Calling the length of time for the first part "$\displaystyle T_1$", the velocity at the end of the first part will be $\displaystyle f_1T_1$ which, presumably, is the constant speed of the "second part". Taking the length of time for the "second part" "$\displaystyle T_2$, the second part of the graph is a horizontal straight line with height V, from $\displaystyle T_1$ to $\displaystyle T_2$. The third part of the graph will be a straight line with slope $\displaystyle -f_1$. Of course, the train will stop after time interval $\displaystyle T_2$ where $\displaystyle V- f_3T_3- T_2)= 0$

I don't uderstand what the "it travel constant velocity with the $\displaystyle \frac{4}{3}t$ of the total time" means. You did not mention "t" before and 4/3 is larger than 1.
• Jul 27th 2012, 04:10 PM
srirahulan
Re: velocity-time curve problem
I am really sorry i assume the total time for travelling is t.when it travel in the constant velocity of V with 3/4t of the total time (sorry for mention 4/3)