Calculate the initial recoil spped of the boat

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Homework Help Overview

The discussion revolves around a physics problem involving momentum conservation and mechanical energy in the context of a man throwing a rock from a rowboat and the subsequent recoil of the boat. The problem also touches on a collision scenario involving two pucks and the conservation of kinetic energy.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of the recoil distance and the conservation of momentum in both scenarios. Questions are raised about the relationship between mechanical energy and external work, as well as the role of friction in the context of the boat's motion.

Discussion Status

Participants are actively engaging with the concepts of momentum conservation and energy loss. Some have provided insights into the principles at play, while others are seeking clarification on specific aspects of the problems, such as the significance of the recoil distance and the calculations involved.

Contextual Notes

There is mention of potential confusion regarding the recoil distance of the boat and the relationship between mechanical energy and friction. Participants are also considering the implications of momentum conservation in both scenarios presented.

HurricaneH
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1. A 75KG man stands in a 100kg rowboat at rest in still water. He faces the back of the boat and throws a 5kg rock out of the back at a speed of 20 m/s. The boat recoils forward 4.2 m..
Calculate the initial recoil spped of the boat, the loss in mecahnical energy due to friction, the effective coefficient of friction.

Ok, what throws me off is that 4.2 meters that it recoils...


2. A 0.3kg puck, at rest, is struck by a 0.2 kg puck moving along the xaxis with a speed of 2 m/'s. After the collision, the 0.2 kg puck has a speed of 1 m/s at an angle of 53 degrees to the xaxis. Find the velocity of the 0.3kg puck after the collision, find the fraction of KE lost in the collision.

Momentum would be equal before and after right?
 
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1. What's the mechanical energy in this case? What's the relation between mechanical energy and external work (friction in this case)? What's the relation between force of friction and mass?

2. Yup, momentum is always conserved.
 
HurricaneH said:
1. A 75KG man stands in a 100kg rowboat at rest in still water. He faces the back of the boat and throws a 5kg rock out of the back at a speed of 20 m/s. The boat recoils forward 4.2 m..
Calculate the initial recoil spped of the boat, the loss in mecahnical energy due to friction, the effective coefficient of friction.

Ok, what throws me off is that 4.2 meters that it recoils...


2. A 0.3kg puck, at rest, is struck by a 0.2 kg puck moving along the xaxis with a speed of 2 m/'s. After the collision, the 0.2 kg puck has a speed of 1 m/s at an angle of 53 degrees to the xaxis. Find the velocity of the 0.3kg puck after the collision, find the fraction of KE lost in the collision.

Momentum would be equal before and after right?

1. Remember that there is conservation of momentum to consider. Then you can use that information to calculate the loss of energy too.

2. Same principles :-)
 
HurricaneH said:
1. A 75KG man stands in a 100kg rowboat at rest in still water. He faces the back of the boat and throws a 5kg rock out of the back at a speed of 20 m/s. The boat recoils forward 4.2 m..
Calculate the initial recoil spped of the boat, the loss in mecahnical energy due to friction, the effective coefficient of friction.

Ok, what throws me off is that 4.2 meters that it recoils...

Conservation of momentum - a 5kg rock goes one direction, the boat and man conserve momentum by going the opposite direction at a given speed (initial recoil speed). The boat and man decelerate back to zero velocity (how much of a loss in mechanical energy is that). What coefficient of friction decelerates your boat from initial recoil speed to zero in 4.2 meters?
 

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