Quite Easy Question PLEASE HELP

[kajicky]

Im a father trying to help my daughter with a Math problem.. And I am feeeling a little dumbfounded with this one.
This is a summary of the info she has
g=9.8m/s2

Point A to B is 12km
uniform acceleration 3/4 of Total distance
deceleration 1/4 of Total distance
Total Time 20mins

Whats the max velocity of the train?

I would apreciate if some could help me to solve this and teach me how to do it, Ill look like a genius to my daughter.

Kind Regards

Kai

 

[cesiumfrog]

You haven't given all the information. For example, if the deceleration isn't uniform then the velocity can go arbitrarily high.

g seems irrelevant.

have ds=ds1(9km)+ds2(3km), t1=20min-t2, assume v@0km=v@12km=0 & const. a1,a2.
d(v^2)=v@9^2-0^2=2.a1.ds1=2.a2.ds2 => a2=3a1
v@9-0=at=a1(20min-t2)=3a1.t2 => t2=5 => v@9=..

Not sure whether you'll actually be able to figure that out.. anyway, this belongs in the homework forum.