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# A 67 kg man sits on the stern of a 4 m long boat.?

The prow of the boat touches the pier,

but the boat isn’t tied. The man notices his mistake, stands up and walks to the boat’s

prow, but by the time he reaches the prow,

it’s moved 1.93 m away from the pier.

Assuming no water resistance to the boat’s

motion, calculate the boat’s mass (not counting the man).

Answer in units of kg.

### 1 Answer

Relevance

- WhomeLv 78 months agoFavourite answer
The center of mass of the man-boat system will not move.

Let's assume that CoM for the boat is at its midpoint, and call its mass m

Calculations for the CoM both initially and after he moves are equal.

m(2.0) + 67(4.0) / (m + 67) = m(2 + 1.93) + 67(1.93) / (m + 67)

m(2.0) + 67(4.0) = m(2.0 + 1.93) + 67(1.93)

m(2.0) + 67(4.0) = m(2.0) + 1.93m + 67(1.93)

67(4.0 - 1.93) = 1.93m

m = 71.860... ≈ 72 kg

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