A rock from a boat - could you help me understand?

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Homework Help Overview

The problem involves a scenario where a man on a boat throws a rock, and the goal is to determine the resulting velocity of the boat based on the mass and velocity of the rock. The subject area pertains to the principles of momentum in physics.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to understand the derivation of the momentum equation used, questioning why the momentum of the boat and rock must balance. Other participants discuss the implications of conservation of momentum and its application to the scenario.

Discussion Status

The discussion includes attempts to clarify the underlying principles of momentum conservation. Some participants have provided references to related concepts, and there appears to be a productive exchange of ideas regarding the assumptions involved.

Contextual Notes

There is an indication that the participants are working under the assumption that the man and boat start at rest, which is crucial for applying the conservation of momentum principle.

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Homework Statement



A man on a boat throws behind him a rock of mass m1=2kg with a velocity of v1=18m/s. The mass of a man with a boat is m_2=90kg. What velocty v_2 will the boat go with?

Homework Equations



v2 = (m1*v1)/m2

The Attempt at a Solution



So I was upsent on the lesson where we were speaking about this and can't figure out where does the formula I wrote above come from. Why is it certain that m2v2=m1v1? Of course I know I can just plug the numbers inside and bam, I've got my answer but I'd rather know why does it work. Could you help?
 
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Well the eqaution implies that: mom boat - mom rock = 0
Why would that be?
 
It comes from the conservation of momentum. Since (presumably), the man and boat start out at rest, the total momentum is zero.

You might want to read this: Momentum Conservation in Explosions
 
Oh, I see. Thank you a lot!
 

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