Calculating Velocity from Conservation of Momentum: A Science Tutorial

[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?

 

[TSny]

Setup looks right. But check the number 400.

 

[Orodruin]

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.

 

[AndrewMorris]

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

 

[TSny]

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

It was just a careless error. Happens to us all