# Conservation of Momentum/Linear Momentum

## Homework Statement

A man (weighing 915 N) stands on a long railroad flatcar (weighing 2805 N) as it rolls at 18.0 m/s in the positive direction of an x axis, with negligible friction. Then the man runs along the flatcar in the negative x direction at 40.00 m/s relative to the flatcar. What is the resulting increase in the speed of the flatcar?

Pi = Pf

## The Attempt at a Solution

(915+2805)g(18)=(915)g(-40+18)+(2805)g(v+18)
v=-1.3 m/s

But this is the wrong answer...what am I doing wrong???

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mjsd
Homework Helper
firstly, weights were given in Newton already, no needs to add an extra "g". secondly, if you do want to use momentum (somehow), you want the "mass" of the objects.

usually i get ma equations in variables n thn substitute the values.........suppose the man moves at a speed w towards left and the platform recoils at a speed V towards right...hence the speed of the man relative to the platform is V+w=v...or w= v-V....

taking the man n the platform to b inna system, there is no external horizontal force on the system, the linear momentum remians constant. thus
(M+m)18 = MV- mv...now solve n check

sorry, the eqn above shud hav been (M+m)18 = MV- mw

ooookay...the catch is , it will recoil wit V+18 and not just V....ill just try n let u know

If You look at what I posted above that is the equation that I used and it did not work...(M+m)18=M(v+18)+m(v+18) and it did not give me the right answer.