Unsure how to begin

I'm not even sure how to begin this problem. Can someone help me out?

A dog, with a mass of 12.0 kg, is standing on a flatboat so that he is 29.7 m from the shore. He walks 8.7m on the boat toward the shore and then stops. The boat has a mass of 44.0kg. Assuming there is no friction between the boat and the water, how far is the dog from the shore now?

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Andrew Mason
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vroman said:
I'm not even sure how to begin this problem. Can someone help me out?

A dog, with a mass of 12.0 kg, is standing on a flatboat so that he is 29.7 m from the shore. He walks 8.7m on the boat toward the shore and then stops. The boat has a mass of 44.0kg. Assuming there is no friction between the boat and the water, how far is the dog from the shore now?
Think of the boat has having a constant mass/length. Work out the change in center of mass of the dog/boat relative to the center of the boat. What can you say about the position of that center of mass relative to the shore?

AM

Imagine that the shore is at the origin of the coordinate system and the dog+boat system lies on the x-axis.Because there is no friction between the boat and water , so there are no external forces.As a fact, in absence of external forces , cente of mass of the system remains stationary .Calculate the x-coordinate of the centre of mass before the dog moves and after the dog moves.Equate both the c.m coordinates and get the needed value.

X-coordinate of c.m can be calculate as:

$X_c_m = \frac {m_1 x_1 + m_2 x_2}{m_1+m_2}$