Railroad engine coupling with flatcar at 5 km/h

[almost__overnow]

A railroad diesel engine weights 4 times as much as a flatear. If a diesel coasts a 5km/h into a flatear that is initally at rest. How fast do the two coast after they couple together?

 

[Doc Al]

Hint: What's conserved in any collision?

 

[almost__overnow]

Momentum is conserved in any collison.

 

[Doc Al]

Right! That's all you need to solve this one.

 

[almost__overnow]

But don't I have to do some kind of math since it says how fast do the two coast after they couple together? =\

 

[Doc Al]

The fact that they couple together tells you that they have the same final speed. Now express the conservation of momentum mathematically and solve for that speed.

 

[almost__overnow]

okay wait one more question how do you find the kg since i already have the speed? thanks

 

[OlderDan]

okay wait one more question how do you find the kg since i already have the speed? thanks

The actual masses do not matter. All that matters is that the masses have a known ratio. If you do the problem using m for the mass of the flatear and 4m for the engine the m will divide out of the problem. There is no way for you to determine the value of m.