# Homework Help: Initial Velocity with angles

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1. Jul 20, 2016

### xxphysics

1. The problem statement, all variables and given/known data
Disk P (inertia 0.41 kg ) moves at an unknown velocity across a low-friction horizontal surface and collides with disk Q (inertia 0.75 kg ), which is initially at rest. After the collision, the two (now slightly dented) disks move apart without spinning. Velocity information is provided in the initial and final top-view diagrams in the (Figure 1) .

What was the initial velocity of disk P?
What fraction of the initial kinetic energy is converted during the collision?

2. Relevant equations
v1im1 + v2im2 = v1fm1 + v2fm2

3. The attempt at a solution
So I know you use conservation of momentum, but I'm not entirely sure how to set up the equations to include theta.

2. Jul 20, 2016

### TSny

Momentum is a vector quantity. So, the relevant equation is a vector equation

m1 v1i + m2 v2i = m1 v1f + m2 v2f

In terms of x components:

m1 v1ix + m2 v2ix = m1 v1fx + m2 v2fx

(There is a similar equation for the y components.)