# Elastic collision problem, I need help to find the mistake

1. Sep 12, 2015

### ft92

1. The problem statement, all variables and given/known data

Two objects with equal masses undergo an elastic collision in 2 dimensions. Object 1 has an initial velocity u1 and object 2 is initially at rest. After the collision, object 1 is observed to have a velocity equal to 3/5 of it's initial velocity. What is the velocity of object 2 after the collision, v2, in units of u1?

3. The attempt at a solution

Let

m1 = m2 = m

initial speeds,
u1 = u1

u2 = 0

final speeds,

v1 = (3/5)*u1

v2 = ?

Apply conservation of momentum

m1*u1 + m2*u2 = m1*v1 + m2*v2

m*u1 + 0 = m*(3/5)*u1 + m*v2

u1 = (3/5)*u1 + v2

v2 = u1*(1- 3/5)

= (2/5)*u1 or 0.4*u1 <<<<<<<-----Answer (unfortunately it's wrong but I don't know why!)

2. Sep 13, 2015

### ehild

It is said that the collision is elastic and is two-dimensional- It should mean that the velocity of m1 is not parallel with the initial velocity. Check the original text if the final speed is 3/5 of the initial speed?

3. Sep 13, 2015

### ft92

yes it's 3/5 of the initial speed

4. Sep 13, 2015

### ehild

Assume that both objects have both x and y components and use energy conservation, too.