Vectors Problem (dealing with velocity)

[strag]

Homework Statement

A passenger at the rear of a train traveling at 15 m/s relative to Earth throws a baseball with a speed of 17 m/s in the direction opposite the motion of the train. What is the velocity of the baseball relative to Earth as it leaves the thrower's hand? Answer in units of m/s.

Homework Equations

The Attempt at a Solution

Vector of the train is going to the right (15 m/s).
Vector of the person throwing the ball is going to the left (17 m/s).

Adding the vectors 15 - 17 would give you -2 m/s. I'm having trouble with assigning direction to vectors, so please let me know if I assigned them right in this problem. Thanks!

 

[xcvxcvvc]

you can use any reference system you want. for example, you could have said the train was moving at -15m/s and that the ball was thrown at 17 m/s or that the train was moving at 15 m/s at 30 degrees and the ball was thrown at 17m/s 210 degrees.

you'll always end up with the right answer as long as you assign your reference clearly. of course, you're expected to choose the most convenient/intuitive reference. it'd make no intuitive sense, for example, to assign in this problem the 30/210 degree reference.

so yeah, you seem to have done it right.

 

[strag]

The sign in my last answer matter? Could you explain this in detail for me??