The discussion revolves around a conservation of momentum problem involving relative velocities, where the original poster is struggling with the correct assignment of velocity signs and directions for various objects, specifically a flatcar and two cars. The confusion stems from the assumptions made regarding the direction of motion and the implications of those assumptions on the momentum calculations.
The discussion is ongoing, with participants providing insights into the nature of velocity as a vector quantity and the conventions typically used in physics problems. There is a recognition of the need to clarify assumptions about direction and the relationship between the movements of the flatcar and the cars. Some guidance has been offered regarding the sign convention, but no consensus has been reached on the correct interpretation of the problem.
Participants note that the problem involves a scenario where the total momentum is initially zero, and there is an emphasis on the absence of external forces acting on the system. The original poster expresses uncertainty about their assumptions, indicating a need for further clarification on the relationships between the objects' motions.
From conservation of momentum.cipotilla said:Yes I took left to be positive, why should the flatcar be negative? Its moving to the right? How do I know that?
And that's why you get a negative mass for the flatcar, a nonsensical answer!I assumed that car A, car B and the flatcar had a constant final velocities to the left, which made them all positive.
The issue was not one of sign convention, but of assigning the correct direction to the flatcar's motion.PBR said:Usually, but not necessarily the sign convention is that velocities pointed left are NEGATIVE (viz. -2.55 m/s and -2.5 m/s). Velocities to the right are usually given a positive value as the problem does +.34 m/s. Using these values you will get the correct answer. Velocity is a vector quantity having magnitude and direction. In a 1-D problem, + is to the right and - is to the left, by convention. (You can reverse the convention, FOR ALL THE VELOCITIES IN THE PROBLEM, and you also get the right answer.)
At some point, physical intuition will kick in and that will save you time. If the cars go left, the flatcar must go right. (The problem creater assumed you would know this.)cipotilla said:Doc Al, you said I know the flatcar is moving to the right from conservation of momentum...Initially I don't know any of the directions for the velocities.
They start out from rest so total momentum equals zero. If everything moved in the same direction, then total momentum could not be zero!I assumed them and got an answer that was obviously wrong. So I know that my assumption is wrong. But how to I know from conservation of momentum that the cars are going in the oposite direction of the flatcar?
Doc Al said:At some point, physical intuition will kick in and that will save you time. If the cars go left, the flatcar must go right. (The problem creater assumed you would know this.)
They start out from rest so total momentum equals zero. If everything moved in the same direction, then total momentum could not be zero!
Before the cars move, everything is at rest. The momentum of flatcar + cars starts and remains zero since we presume no external force acts on the flatcar.cipotilla said:When the cars start to move, yes the momentum is zero, they are moving in no direction at all.
Let's say at time t=1s, they can all start moving in the same direction, we would have: (mv)A+(mv)B+(mv)F=constant
That constant would just be a larger number than if I assumed that the flatcar was going in the other direction, in which case that same equation would look like this:
(mv)A+(mv)B-(mv)F=constant
I don't see why one is wrong and the other isnt.
The train starts to move because the tracks exert a force on it.cipotilla said:What I am thinking is similar to the following situation:
Lets say that I were on a train, sitting in my seat, at rest while the train too is at rest (lets say its at a stop). Then after a couple of minutes the train starts to move again, it moves east.
No, the cases are different, due to the presence of an external force. To see this more clearly, let's use a different example: You are on a rowboat floating on a calm lake. (Assume no resistive force from the water on the boat.) If you start to walk east, what happens to the boat?I could get up from my seat and walk east too. I don't see why that's against physical intuition. The same thing for the cars, where car A and car B are me and the train is the flatcar.