Change in momentum problem. BASIC, IMPORTANT, but i don not know help?

[teggenspiller]

Homework Statement

A 0.2 baseball if pitched with a velocity of 40 m/s and is then batted to the pitcher with a velocity of 60 m/s. What is the magnitude of change in the ball’s momentum?

A. 4 kg*m/s
B. 8 kg*m/s
C. 2 kg*m/s
D. 20 kg*m/s

Homework Equations

change in momentum = m(vf-vi)

The Attempt at a Solution

.2 (60-40) =4


4m/s, OR would it be 20, since .2*40=8 and .2*60=12
and add them together since they are in opposite dirctions, to get the TOTAL magnitude of change??

ANOTHER example:
A softball with a mass of 0.11 kg moves at a speed of 12 m/s. Then the ball is hit by a bat and rebounds in the opposite direction at a speed of 15 m/s. What is the change in momentum of the ball?

A. –1.3 kg*m/s
B. –1.6 kg*m/s
C. –0.33 kg*m/s
D. –3.0 kg*m/s


should i add the momenta, since different directions to get 3.0 (D) or subtract vf from vi to get C, .33

i know this seems like a dumb q that has probably an obvious answer, but little things like this could BREAK my grade in the test

 

[Doc Al]

Change is always final minus initial, so you don't want to add the momenta. But where you're messing up is not taking into consideration the direction of the velocities. (As you seem to suspect.)

Hint: Since the initial and final velocities are in different directions, give them different signs. (Pick one direction to be positive, the other negative. I'd take the initial direction as positive, unless told otherwise.)

 

[kuruman]

The final and initial velocities must have opposite signs because they are in opposite directions.