# Physics major help =(

1. Oct 19, 2006

### 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?

2. Oct 19, 2006

### Staff: Mentor

Hint: What's conserved in any collision?

3. Oct 19, 2006

### almost__overnow

Momentum is conserved in any collison.

4. Oct 19, 2006

### Staff: Mentor

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

5. Oct 19, 2006

### almost__overnow

But dont I have to do some kind of math since it says how fast do the two coast after they couple together? =\

6. Oct 19, 2006

### Staff: Mentor

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.

7. Oct 19, 2006

### almost__overnow

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

8. Oct 19, 2006

### OlderDan

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.