Dog walking on a raft on water then suddenly halting

[totallyclone]

Homework Statement

A dog of mass 10.0kg is standing on a raft so that he is 20.0m from shore. He walks 8.00m along the raft towards shore and then halts. The raft has a mass of 40.0kg, and we can assume there is no friction between the raft and the water. How far is the dog from shore when he stops?

Homework Equations

E_K=1/2 mv²

The Attempt at a Solution

This is a really tricky question and I'm not quite sure how to proceed. I know that when the dog halts, the raft moves forward towards the shore?

 

[phinds]

What's tricky about it? What external force would cause the raft to have a net movement?

 

[totallyclone]

What's tricky about it? What external force would cause the raft to have a net movement?

I find this question tricky because maybe I'm overthinking it...

I think the external force would be what the dog applies. Then suddenly stopping, transfers the energy to the raft causing it to more forward towards the shore?

 

[phinds]

I find this question tricky because maybe I'm overthinking it...

I think the external force would be what the dog applies. Then suddenly stopping, transfers the energy to the raft causing it to more forward towards the shore?

How is the dog external? What is the dog acting on that is external to the raft?