What Is the Wagon's Speed After an Elastic Collision?

[canicon25]

Homework Statement

A 150 kg cart moving at 13 m/s east collided with a 420 kg wagon moving at 5.0 m/s east.
The cart rebounded westward with a speed of 3.0 m/s . What was the speed of the wagon after the collision? Observe standard Cartesian coordinates.

Homework Equations

KE1+KE2=KE1'+KE2'
conservation of momentum
conservation of kinetic energy in elastic collisions

The Attempt at a Solution

(0.5)(150)(13)²+(0.5)(420)(5)²=(0.5)(150)(-3)²+(0.5)(420)v²
v=9.0m/s

Answer as given is 11m/s

 

[issacnewton]

the problem itself doesn't say that the collision is elastic. so just use conservation of momentum here.

 

[cupid.callin]

Homework Statement

A 150 kg cart moving at 13 m/s east collided with a 420 kg wagon moving at 5.0 m/s east.
The cart rebounded westward with a speed of 3.0 m/s . What was the speed of the wagon after the collision? Observe standard Cartesian coordinates.

Homework Equations

KE1+KE2=KE1'+KE2'
conservation of momentum
conservation of kinetic energy in elastic collisions

The Attempt at a Solution

(0.5)(150)(13)²+(0.5)(420)(5)²=(0.5)(150)(-3)²+(0.5)(420)v²
v=9.0m/s

Answer as given is 11m/s

IssacNewton is right !
Conservation of kinetic energy is valid only when coefficient of restitution(e) is 1
here its not specified, so you must use momentum conservation.

ALSO:
if e is given and is ≠1 then you may use:
(v₂ - v₁) = e(u₁ - u₂)