# Unsure how to begin

#### vroman

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

Homework Helper
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

#### Dr.Brain

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:

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

"Unsure how to begin"

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