Impulse and average force of baseball

[amlovatos]

i don't understand wat I am doing wrong here is my problem
Choose the positive direction to be
from the pitcher to the plate (some answers
may be negative).
A pitcher throws a 0.13 kg baseball so that
it crosses home plate horizontally with a speed
of 31 m/s. It is hit straight back at the pitcher
with a final speed of 28 m/s.
What is the impulse delivered to the ball?
Answer in units of kg m/s
Find the average force exerted by the bat on
the ball if the two are in contact for 0.0011 s.
Answer in units of N

ok here is my work
Impulse=m(vf-vi)
.13(28-31)=-.39
then for average force i have
I=t*af
-.39=.0011*af
af=-354.5454545
but i change my signs to positive right?? either way my answer is wrong help please

 

[PhanthomJay]

i don't understand wat I am doing wrong here is my problem
Choose the positive direction to be
from the pitcher to the plate (some answers
may be negative).
A pitcher throws a 0.13 kg baseball so that
it crosses home plate horizontally with a speed
of 31 m/s. It is hit straight back at the pitcher
with a final speed of 28 m/s.
What is the impulse delivered to the ball?
Answer in units of kg m/s
Find the average force exerted by the bat on
the ball if the two are in contact for 0.0011 s.
Answer in units of N

ok here is my work
Impulse=m(vf-vi)
.13(28-31)=-.39

the problem asks you to consider pitcher to batter as the positive direction, thus, batter to pitcher is the negative direction. What is the direction of vf?

then for average force i have
I=t*af
-.39=.0011*af
af=-354.5454545
but i change my signs to positive right?? either way my answer is wrong help please

once you correct the value of impulse, your force will be correct and it will ahve a minus sign associated with it. What does the minus sign indicate about the direction of the force of the bat on the ball?