Conservation of Momentum and Relative Velocities

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Homework Statement


An 82-kg lumberjack stands at one end of a 380-kg floating log, as shown in the figure (Figure 1) . Both the log and the lumberjack are at rest initially.

If the lumberjack now trots toward the other end of the log with a speed of 2.7 m/s relative to the log, what is the lumberjack's speed relative to the shore? Ignore friction between the log and the water.

Express your answer using two significant figures.

Homework Equations


Conservation of Momentum

The Attempt at a Solution


m1v1' + m2v2' = 0
m1v1' = -m2v2'
(82)(2.7) = (-380)(v2')
v2' = -0.5826

velocity in respect to shore = 2.7 + (-0.5826) = 2.1 m/s
For some reason this answer is incorrect. I also tried 2.7 + 0.5826 = 3.3 m/s, but that is wrong also. Is there something that I am doing incorrectly before this last step?
 

Answers and Replies

  • #2
BvU
Science Advisor
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the 2.7 m/s is w.r.t the log, right ? But the 0.58 m/s can not be wrt the log. !
 

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