Elastic Collision of a 90kg ball

[Lyphta]

Homework Statement

A 90-g ball moving at 100 cm/s collides head-on with a stationary 10-g ball. Determine the speed of each after impact if (a) they stick together, (b) the collision is perfectly elastic.

Homework Equations

p=mv

The Attempt at a Solution

I already solved A, but I don't know how to do B. So far, I've gotten:

p=mv
= 90(100)
= 9000 g.cm/s

p=mv
= 10 (0)
= 0

so...
p1+p2=p3
m1 v1 + m2 v2 = (m1 +m2)v
90(100) + 0 = 100v
v= 90 cm/s

I don't understand how the answer can be 80 cm/s when they perfectly elastic. Shouldn't it be 90 cm/s?

 

[rl.bhat]

p1+p2=p3
m1 v1 + m2 v2 = (m1 +m2)v
90(100) + 0 = 100v
v= 90 cm/s
This answer is for (a). In the part (b) after impact both bodies will have different velocities. So wrigth down two equations. One for coservation of linear momentum and another for conservation of energy. Then solve for the new velocities of the two bodies.

 

[Shooting Star]

The Attempt at a Solution

I already solved A, but I don't know how to do B. So far, I've gotten:

One method has already been given. You can also use the fact that the relative velocity before collision is equal to the relative velocity after collision, because the collision is fully elastic.