velocity-time curve problem

**srirahulan** · July 27th 2012, 08:03 AM

A train start to move from rest and then travel its first part with constant acceleration of $f_1$ then travel the second part of the constant velocity of V then travel the third part of the constant deceleration of $f_2$ then it comes to rest.So draw a velocity - time curve and it travel constant velocity with the $\frac{4}{3}t$ 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.

**HallsofIvy** · July 27th 2012, 08:25 AM

Originally Posted by srirahulan

A train start to move from rest and then travel its first part with constant acceleration of $f_1$ then travel the second part of the constant velocity of V then travel the third part of the constant deceleration of $f_2$ then it comes to rest.So draw a velocity - time curve and it travel constant velocity with the $\frac{4}{3}t$ 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 $f_1$ , after time t, the speed will be $f_1t$ . 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 $f_1$ . Calling the length of time for the first part " $T_1$ ", the velocity at the end of the first part will be $f_1T_1$ which, presumably, is the constant speed of the "second part". Taking the length of time for the "second part" " $T_2$ , the second part of the graph is a horizontal straight line with height V, from $T_1$ to $T_2$ . The third part of the graph will be a straight line with slope $-f_1$ . Of course, the train will stop after time interval $T_2$ where $V- f_3T_3- T_2)= 0$

I don't uderstand what the "it travel constant velocity with the $\frac{4}{3}t$ of the total time" means. You did not mention "t" before and 4/3 is larger than 1.

**srirahulan** · July 27th 2012, 04:10 PM

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)

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