# İmpulse from the string

1. Feb 2, 2017

### Arman777

1. The problem statement, all variables and given/known data
A ball of mass 1.00Kg is attached to a loose string fixed to a ceiling.The ball is released from the rest and falls 2.00m,where the string suddenly stops it.Find the impulse on it from the string.

2. Relevant equations.
Kinematic equations
$\vec J=\vec FΔt=Δ\vec P$

3. The attempt at a solution
+z direction is upward.
$ΔH=v_0t+\frac 1 2at^2$ so this becames

$-2m=0-\frac 1 2gt^2$
$t=0.638 s$

and

$-2m=v_0t-\frac 1 2gt^2$
we know $t$ so
$-2-1.99=v_0(0.638s)$
$v_0=-6.25\frac m s$

then
$\vec J=\vec FΔt=Δ\vec P$
so
$P_f=m(-6.25)$
$P_i=m0$
$ΔP=-m6.25=-6.25Ns=J$

answer says $6.25Ns$ so theres no minus sign...
I didnt understand why...The question says "Find the impulse on it from the string" so the thing I found is some other impulse which created by something else..?

2. Feb 2, 2017

### jbriggs444

The final momentum $P_f$ is what again? Did the problem statement not say that the string stops it?

3. Feb 2, 2017

### Arman777

Is it wrong ?.I just wrote the velocity of the ball...Sure,spring stops it.I see I guess.

4. Feb 3, 2017

### jbriggs444

What is the "initial" condition just before the string stops the ball?
What is the "final" condition just after the string stops the ball?

5. Feb 3, 2017

### Arman777

Ohh $P_i$ is not $0$ its $-mv$ so $0-(-m v)=mv$ I solved

6. Feb 3, 2017

Thank you