Homework Help: Inelastic ball problem

1. Oct 11, 2011

aszymans

1. The problem statement, all variables and given/known data
Now the racquet ball is moving straight toward the wall at a velocity of vi = 12.4 m/s. The ball makes an inelastic collision with the solid wall and leaves the wall in the opposite direction at vf = -8.2 m/s. The ball exerts the same average force on the ball as before. Mass of ball =.247 kg

2. Relevant equations
What is the magnitude of the change in momentum of the racquet ball?

3. The attempt at a solution
so I took
m*v(initial)=m*v(final) and got 3.0628 and -2.0254 respectively. so I subtracted the two and got -5.0882. I dont feel like this is correct.

2. Oct 12, 2011

kuruman

What is incorrect is to say m*v(initial)=m*v(final) because that is simply not true. It is correct to say that the change in momentum Δ(mv)=m*v(final)-m*v(initial), so if the final momentum is negative and the initial momentum is positive, you end up essentially adding two negative numbers and that's that.

3. Oct 12, 2011

PeterO

Many strange "formulae" and calculations, but probably almost the correct answer.

Generally change in a quantity means Final - Initial [which you actually ended up doing!!].

Depending which direction you decide to call positive will mean a final answer of -5.etc, like you got in an unusual way, or +5.etc.

The question only asked for the magnitude of the change, so the question of a +ve or -ve answer is irrelevant.