Quote:
Originally Posted by CamaroSkooter
Hey, I'm a civil engineer too 
So, both of y'all are correct. But you're comparing apples to oranges. 
If two cars weigh 1,000kg and are travelling towards each other at 60 kph, then car A's speed will look like 120kph to the driver of car B.
However, the formula for kinetic energy is:
Notice how v is squared?
That means that you can't simply add the velocities of two cars travelling 60kph and assume that a collision between the two would be the same as a single car going 120kph hitting an imovable object.
Look at it this way, the two cars going 60kph would have a kinetic energy of:
60kph = 16.66 m/s
0.5(1000)(16.66^2) + 0.5(1000)(16.66^2) = 277,777.77 Joules
The single car going 120kph would have a kinetic energy of:
120kph = 33.33 m/s
0.5(1000)(33.33^2) = 555,555.55 Joules
So, yeah, hopefully seeing the math clears up some confusion |
I stand corrected and humbled. You're right! I had a PE tell me once that that's why another engineer should always check your work. He said too often engineers get "locked in" a certain way of thinking and can't see the obvious error they've made. So good job, and thanks.
Oh, btw, I must have been ill during that lecture on 1/2 mv^2.
JK