$4$:A Two objects travel directly toward each other. Mass $1=5\mathrm{.}00kg$ and has an initial velocity of $5\mathrm{.}00\frac{m}{s}$ i$.$ Mass $2=3\mathrm{.}00kg$ and has an initial velocity of $-1\mathrm{.}50\frac{m}{s}$ i$.$

a$)$ If the two masses collide each other and stick together after colliding, what is the final velocity of the combined mass?

b$)$ What is the change in kinetic energy regarding this collision?

$4$:B Two hockey balls $($ ball #$1$ and ball#$2)$ of identical mass are on a flat, horizontal ice hockey rink. Ball#$1$ is motionless; the second ball is moving at $2\mathrm{.}5\frac{m}{s}$ to the left. It collides with the motionless ball #$1.$ The balls have a mass of $15$ g $.$ After the collision, the ball #$1$ is moving at $2\mathrm{.}5\frac{m}{s},$ to the left.

What is the final velocity of the ball #$2?$