Help with Momentum & Kinetic Energy Homework

[zvee_y]

Homework Statement

A 30-kg child moving at 4 m/s jumps onto a 50-kg sled that is initially at rest a long, frictionless, horizonal sheet of ice. After coasting at constant speed for a short time, the child jumps off the sled in such a way that she is ar rest with respect to the ice

Homework Equations

a.Determine the speed of the sled after the child jumps off it.
b.Determine the kinetic energy of the child-sled system when the child is at rest on the ice.

The Attempt at a Solution

a. So after the elastic collision, I use the equation
m1vi1+m2vi2=m1vf1+m2vf2 to solve for the speed by plug in: (30)(40)=(50)(vf), that means the child's speed after jump off the sled is 0. But then in b, the kinertic of her should be 0 too. I don't know if I did right.
Someone please help. Thank you

 

[PiratePhysicist]

Yes, you did it right for the momentum. And in the case where the child is at rest with respect to the ice, her kinetic energy is zero, however it asks for the kinetic energy of the child-sled system, which is not zero.

 

[zvee_y]

oh ok ... i got it...Thank you!