momentum and velocity help?

Question

My problem is this -

an astrounout w/ a mass of 80 kg is on a spacewalk. if he throws a 3 kg wrench a 5 m/s aw what speed and direction does the astrounout move

i know i use m1v1=m2v2 but i'm not sure how.
would i do
80 3
5 X

and then just do 5*3 = 15 / 80 = .1875
is that right?
thank you!

by the way thats 80 over 5 then 3 over X and i crossed multiplyed

Answer 1

It's a liner momentum problem so you can use the m1v1 = m2v2 equation.
You have to visualize this.
Now remember, momentum is a VECTOR so it must have direction.
Since they don't tell you which direction he throws the wrench, just assume any direction, i assumed left.
v1 = -5 m/s (negative 5 because it's moving to the left!)
m1 = 3 kg

v2 = ?
m2 = 80 kg

v1m1 = v2m2
v2 = v1m1 / m2

v2 = -0.1875 m/s

so obviously, he to continues to move to the left.
if you had a positive answer for the direction of the wrench (i/e moving right) you'd get a positive velocity for the astronaut as well. Either answer is right since they don't specify the direction of the astronaut when he threw the wrench.
The main point of this question is to test whether you know that momentum is a vector or not.

Answer 2

Uhhh... I don't get the additional comment, but the (5*3)/80 = 0.1875 is correct.