# Homework Help: Energy Loss in Head on Collision of Two Cars w/Same Speed and Mass

1. Sep 25, 2011

### mrsteve

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

Two railroad cars, each with a mass of 7,000kg and traveling at 90 km/h collide head on and come to rest. How much mechanical energy is lost in this collision?

2. Relevant equations

$$KE=\frac{1}{2}mv^2$$

3. The attempt at a solution

$$KE=\frac{1}{2}(7000)(90)^2=2.835*10^7$$

Mechanical energy = KE+PE. PE = 0.

Energy lost = 2KE or $$5.67*10^7$$

Book answer is $$4*10^6$$

Where am I going wrong? Thanks..

2. Sep 25, 2011

### lewando

If your result is in joules, the velocity units of km/h is not helping.

3. Sep 25, 2011

### Staff: Mentor

Convert the speed to standard units of m/s.

4. Sep 25, 2011

### mrsteve

Ahh... So 90 km/h = 25 m/s.

$$\frac{1}{2}(7000)(25)^2=2.1875*10^6 => 2.1875*10^6(2) = 4.375*10^6 => 4*10^6$$

Thanks!