# 2D Momentum Problem

by Probie1
Tags: momentum
 P: 38 1. The problem statement, all variables and given/known data Car 1 weighing 1000 kg crashes into the rear of parked car 2 weighing 1100 kg and stops. Car2 moves ahead 2 ms2. What is the velocity of car1 at impact? 2. Relevant equations (m1v1+m2v2)=(m1v3+m2v4) 3. The attempt at a solution (m2V4)/m1 (1100*2)/1000 2.2ms2= 2200/1000 Okay, now I know this is right, but what I don't know is how do they arrive at the equation (m2v4)/m1 ????
P: 317
 Quote by Probie1 1. The problem statement, all variables and given/known data Car 1 weighing 1000 kg crashes into the rear of parked car 2 weighing 1100 kg and stops. Car2 moves ahead 2 ms2. What is the velocity of car1 at impact? 2. Relevant equations (m1v1+m2v2)=(m1v3+m2v4) 3. The attempt at a solution (m2V4)/m1 (1100*2)/1000 2.2ms2= 2200/1000 Okay, now I know this is right, but what I don't know is how do they arrive at the equation (m2v4)/m1 ????
If you examine your original conservation of momentum equation, let $v_1, v_3$ be the initial and final speeds of car 1, respectively, and let $v_2,v_4$ be the initial and final speeds of car 2, respectively. From the context of the problem, what must be true about speeds $v_2$ and $v_3$?
 P: 38 (1000*2.2+1100*0)=(1000*0+1100*2) V2 and v3 have no velocity so they have no momentum...correct?
P: 317

## 2D Momentum Problem

 Quote by Probie1 V2 and v3 have no velocity so they have no momentum...correct?
Correct.
 P: 38 So because they have no momentum they cancel out which means you rewrite the equation to solve for velocity1 (1000*2.2+1100*0)=(1000*0+1100*2) (1000)=(1100*2) (m1)=(m2v4) (m2V4)/m1 (1100*2)/1000 2.2ms2= 2200/1000 Is that how it is done?
 P: 317 Yes, that's the idea. Conservation of momentum dictates $m_1 v_1 = m_2 v_4$, solving for the initial speed is just an algebra problem.
 P: 38 Thanks Steely Dan... that wasn't as hard as I thought it would be... mind you that was an easy momentum problem for you, but difficult for me. I imagine it would be harder if you had 1 vehcile t-bone another and they both have pre and post impact velocities.
P: 38