Finding Final Velocity in Momentum and Velocity Problems

[mybrohshi5]

Homework Statement

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.

Homework Equations

p=mv

The Attempt at a Solution

the momentum of the system is

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

so

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

883.6 = 160v_f

v_f = 5.52 m/s

Where did i got wrong? :(

Thank you for any help

 

[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.

 

[mybrohshi5]

Thank you :)

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