# Homework Help: Calculate the initial recoil spped of the boat

1. Dec 14, 2004

### HurricaneH

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?

2. Dec 14, 2004

### daster

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.

3. Dec 14, 2004

### cyby

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 :-)

4. Dec 15, 2004

### BobG

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?