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Physics help +K

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50in07
Posts: 3070 - Karma: 1,471
I can't figure this problem out
 
A railroad diesel engine weighs four times as much as a freight car. If the diesel engine coasts at 27 km/h into a freight car that is at rest, how fast do the two coast after they couple?
Feb 26 2009 4:11PM
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camelbackfreeski
Posts: 75 - Karma: 64
This problem is a simple matter of the conservation of momentum [m*v].  The initial momentum before the collision is equal to the mass of the engine times its mass.  The momentum after the collision is the unknown speed of the connected masses times their combined bass.  The momentum before and after equal each other:

mv=mv

(4*27)=v(5)
v=21.6 km/h
Feb 26 2009 4:21PM
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chapz
Posts: 10182 - Karma: 66
are there any values for mass?

anyways, without mass, you cant find the velocity..


A railroad diesel engine weighs four times as much as a freight car. If the diesel engine coasts at 27 km/h into a freight car that is at rest, how fast do the two coast after they couple?

p1 + p2 = p3
(m1)(7.5m/s) + (m2)(0m/s)= (m1 + m2)(vf)
Feb 26 2009 4:21PM
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BMskier
Posts: 3363 - Karma: 365
holy shitt i just did the same problem in my physics class for hw 2 days ago. this is weird.
Feb 26 2009 4:22PM
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chapz
Posts: 10182 - Karma: 66
false... its not asking for the initial momentum of the moving vehicle.. and your units are wrong...
Feb 26 2009 4:27PM
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camelbackfreeski
Posts: 75 - Karma: 64
are you kidding me!?!?!?! we both used momentum in our answer!!!  The only difference between us is that you didn't realize that you just need to know the mass ratio of the two masses and that their actual masses are irrelevant!   The units don't matter as long as you stay consistent.  Why don't you let the mechanical engineering major do the physics homework...
Feb 26 2009 4:31PM
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50in07
Posts: 3070 - Karma: 1,471
Yea 21.6 was right thanks! 10/10
Feb 26 2009 4:37PM
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chapz
Posts: 10182 - Karma: 66
fair enough. i just guess my physics professors never taught me that... and they always claimed the units had to be m/s..
Feb 26 2009 5:07PM
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camelbackfreeski
Posts: 75 - Karma: 64
you only need to use SI units for certain calculations.  It can never hurt to use them.  You just have to make sure that your units end up with the correct dimensions at the end of the problem.  For instance, in the case you were finding a velocity which it distance/time.  any unit will work...say a beard second over months... (yes a beard second is a unit of distance...look it up)  You are close to the university of vermont you should come here if you like physics
Feb 26 2009 5:23PM
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