# Homework Help: Momentum and percent kinetic energy loss

1. Apr 8, 2016

### physicsstudent30192

1. The problem statement, all variables and given/known data
1.) A 1200 kg car travelling at 20 m/s collides with a stationary 1400 kg car. The two cars lock together. Determine the speed of the vehicles immediately after the collision if 80% of the initial kinetic energy is converted to heat and sound during the collision

2.) Must all kinetic energy be lost in a collision for the collision to be considered completely inelastic? Explain
2. Relevant equations
Ek = 1/2mv^2

3. The attempt at a solution
For both of these questions I have the answers to them in the book but I can't seem to arrive at/understand the solution.
1.) 80% loss so..
4/5 Ekintial = Ekfinal
(4/5) * (1/2)(1200 kg)(20 m/s)2 + 0 = (1/2)(1200 kg + 1400 kg)(v2)
v^2 = 147.6923 m/s
v = 12.15 m/s
The answer should be 6.1 m/s according to my textbook though.

2. ) "No. As much as possible must be lost without violating the Law of Conversation of momentum" is the answer in the book.

Does this mean the maximum energy lost while the momentum of the system is equal for both the initial and final?

also, how do I know if an equation is completely inelastic rather than just inelastic?

Last edited: Apr 8, 2016
2. Apr 8, 2016

### drvrm

check ,why using lost energy for calculation?

3. Apr 8, 2016

### drvrm

if the dissipative forces /inelastic deformations are there then one can say the collision is inelastic-the proof is non equality of energy before and energy after the event. however momentum is conserved.
completely inelastic means bodies joined/stick together during collision due to inelastic forces.

4. Apr 8, 2016

### physicsstudent30192

Oh wow, I should read over the questions better...changed the 4/5 to 1/5 and it worked. Thanks drvrm for both answers.