Calculating Impulse in a Collision: Positive or Negative Velocity?

[ph_lover3294]

Homework Statement

a tinnes ball of mass 150 gm moves with velocity 11.2 m/s to collide with a racket , after collision it is rebounded with vilocity 7 m/s ...find :-the impulse of the racket on the ball.

Homework Equations

I = m (delta) v


what i want to know .. is
when do we use the vilocity with the negative direction ??

i mean when we answer it

can we calculate

0.15 * ( 11.2-7)= 0.63 kg.m/s

or

0.15* (11.2-(-7))=2.73 kg.m/s

so what is the right answer ?!

 

[jgens]

Think about it. If we define the direction of the initial velocity to be positive, than what must the sign of the resultant final velocity be?

Hint: The ball's final velocity is opposite in direction to it's initial velocity.

 

[ph_lover3294]

i know

i thought that second answer is the right one as it is opposit in direction

but i found different answers in the books so i got confused

but also i want to understant ... we want to find the change in velocity so when we give the final one -ve sign we will add them so how ?

im sorry I am asking a lot of questios

thank u

 

[Doc Al]

The change in velocity is always V_final - V_initial.

The direction that you call negative is arbitrary. For example, if you call toward the racket as positive, then away from the racket must be negative.

Pick a sign convention, then determine V_final and V_initial.

 

[ph_lover3294]

thank u