Momentum and velocity

1. Mar 28, 2010

mybrohshi5

1. The problem statement, all variables and given/known data

A railroad handcar is moving along straight, frictionless tracks with negligible air resistance. In the following cases, the car initially has a total mass (car and contents) of 160 kg and is traveling east with a velocity of magnitude 4.70 m/s. Find the final velocity of the car in each case, assuming that the handcar does not leave the tracks.

An object with a mass of 28.0 kg is thrown backward out of the car with a velocity of 4.70 m/s relative to the initial motion of the car.

2. Relevant equations

p=mv

3. The attempt at a solution

the momentum of the system is

p = (160+28)(4.7) = 883.6

so

883.6 = 160(vf) + 28(-4.7+4.7)

883.6 = 160vf

vf = 5.52 m/s

Where did i got wrong? :(

Thank you for any help

2. Mar 28, 2010

rock.freak667

The total mass of the cart is 160kg, so your total momentum would be 160*4.7.

After it is thrown out, it would lose 28 kg. So the final mass is 160-28.

3. Mar 28, 2010

mybrohshi5

Thank you :)

i was reading the question wrong haha i always seem to do that.