# Momentum problem (should be pretty easy I think)

## Homework Statement

A 20 metric ton train moves toward the south at 50 m/s. What speed must it travel to have two times its original momentum?

## Homework Equations

Change in momentum = mass(FinalVelocity - InitialVelocity)

## The Attempt at a Solution

so I thought you would simply times 50 by 2 and get 100 to be the answer, but thats not right. First question is do you have to convert 20 metric tons to kilograms?

PhanthomJay
Science Advisor
Homework Helper
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## Homework Statement

A 20 metric ton train moves toward the south at 50 m/s. What speed must it travel to have two times its original momentum?

## Homework Equations

Change in momentum = mass(FinalVelocity - InitialVelocity)

## The Attempt at a Solution

so I thought you would simply times 50 by 2 and get 100 to be the answer, but thats not right. First question is do you have to convert 20 metric tons to kilograms?
did you put in the units for its speed?

you mean 50 m/s yeah?...if thats not what you mean then I'm confused

PhanthomJay
Science Advisor
Homework Helper
Gold Member
you mean 50 m/s yeah?...if thats not what you mean then I'm confused
No, I meant 100m/s. In order for the momentum vector to be twice its original momentum, the new momentum must have a magnitude of twice as much and a direction in the same direction as the original momentum vector. Since its momentum doubles, and the mass is constant, then its velocity must double. So the velocity is 100m/s south. The speed is 100m/s. Unless we both missed something.