Find Mass of Passenger Train in Wagon Collision

[coltsamuel96]

hello, can someone help me with this,
a wagon train of given mass collides with a passenger train at rest... the train loses 27% of kinetic energy after the collision... find the mass of the passenger train...
so what i did is, i put this formula for inelastic collision,v(final)= m1v1+m2v2/m1+m2... since v2 is in rest , m2=(m1v1/v(final))-m1... so my question is what will be the velocities before and after collision...i did, m2=(m1*1/.73)-m1...but i have a feeling that this is wrong... please help...

 

[Delphi51]

Welcome to PF, coltSamuel.

You can write ½ mv² = .73*½ mV² and find that the reduced velocity is a number multiplied by the original velocity.

I don't see how you can solve for the mass with unknown initial V, unknown final v for the second train and two unknown masses. One equation, 4 unknowns. Hopeless?
Do we know anything else? Do the two trains stick together after the collision?

It is always a good idea to write the question here exactly as it was given to you - maybe one of us can spot some information in there that you missed.

 

[coltsamuel96]

the mass of the wagon train is 3.18*10^4kg... and the trains stick together after the collision...

 

[brno17]

You need the initial speed of the train. What is it?

 

[coltsamuel96]

there´s no intial velocity... only thing given is the loss of K.E... (27%)

 

[Delphi51]

Okay, I think we have enough to do it now!

v(final)= m1v1+m2v2/m1+m2

is a good start. One of those v's must be zero since the passenger train was initially at rest. The final v is known to be a certain fraction of the initial velocity (I suggested you work it out with the KE formula earlier). I think you will then be able to cancel the remaining velocity, put in your known mass and have only one unknown remaining!