# Man jumps on moving wagon

AndrewMorris
Homework Statement:
My daughter was asked this question:
A person of mass 60 kg running at 8 m/s reaches a wagon of mass 80 kg going at 2 m/s and jumps onto it. Calculate the velocity of the wagon immediately after the person jumps on.
Relevant Equations:
total momentum before impact = total momentum after impact
Using conservation of momentum I would say (60 x 8) + (80 x 2) = (60 + 80) v
so v = 400/140 = 2.86 m/s.
However, this is not one of the multiple choice responses. What am I doing wrong?

Homework Helper
Gold Member
Setup looks right. But check the number 400.

• Orodruin
Staff Emeritus
Homework Helper
Gold Member
Apart from that, consider using symbolic calculations and only insert your numbers at the end. It often avoids this kind of mistakes. Or at least use units throughout.

• TSny
AndrewMorris
oh man, I used to be good at maths, honest. Thanks :-)

Homework Helper
Gold Member
oh man, I used to be good at maths, honest. Thanks :-)
It was just a careless error. Happens to us all 