Why Is the Change in Momentum 8Ns Instead of -8Ns?

[thoradicus]

Homework Statement

A cricket bat strikes a ball of mass 0.16 kg traveling towards it. The ball initially hits the bat at a speed of 25m/s and returns along the same path with the same soeed. Time of impact is 0.003s
a)Determine change in momentum of the ball
b)Determine FOrce exterted on the ball

Homework Equations

a) momentum=mass*velocity
b)Ft=mv-mu

The Attempt at a Solution

What I am trying to ask is, given that v is negative to due to the ball returns in opposite direction, and u is positive then, shouldn't the awnser be: (.16)(-25)-(.16)(25)= -8Ns.

However, I am confused when the answer shows that it is 8Ns. Why is it so? Does questions like this account for magnitude only?
Is it still correct if i put -8 as the awnser?

 

[The1337gamer]

Impulse is a vector quantity which is why you have you get the answer -8Ns as opposed to +8Ns.

You could just define the initial velocity, u to be in the negative direction and the final velocity, v to be in the positive direction. That way you would get 8Ns.

If the question asks for the magnitude of impulse then you take the positive value. As long as you clearly define which direction is positive and which is negative, your answer is correct.

 

[thoradicus]

thanks