1. Dec 29, 2008

ph_lover3294

1. The problem statement, all variables and given/known data

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.
2. Relevant 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 ???!

2. Dec 29, 2008

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.

3. Dec 31, 2008

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 im asking alot of questios

thank u

4. Dec 31, 2008

Staff: Mentor

The change in velocity is always Vfinal - Vinitial.

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 Vfinal and Vinitial.

5. Jan 3, 2009

thank u