Impulse imparted by the bat to the ball?

[DrMcDreamy]

Question:

A(n) 0.55 kg softball is pitched at a speed of
20 m/s. The batter hits it back directly at the
pitcher at a speed of 27 m/s. The bat acts on
the ball for 0.02 s.
What is the magnitude of the impulse im-
parted by the bat to the ball?
Answer in units of N · s.

Equation:

I=m(v_f-v_i)

My Solution:

.55 kg (27 m/s - 20 m/s) = 3.85 kg m/s
I put in the answer but its incorrect. What did I do wrong?

 

[Doc Al]

Hint: Momentum and velocity are vectors--direction matters. (Check your signs.)

 

[DrMcDreamy]

So would it be:

.55 kg (-27 m/s - 20 m/s) = -25.85 kg m/s ?

 

[Doc Al]

Looks good. (But just give the magnitude.)

 

[DrMcDreamy]

Thank you!

 

[DrMcDreamy]

May I ask why "27 m/s" is a -27 m/s? I don't understand that aspect of the problem.

 

[Doc Al]

May I ask why "27 m/s" is a -27 m/s? I don't understand that aspect of the problem.

The sign is just a way of specifying direction. If the ball was originally moving (before hitting the bat) in the positive direction, after it gets hit it will be moving in the negative direction.