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Q & A: force in a collison

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Most recent answer: 06/14/2017
Q:
It's been 50 years since I took high school physics, but I saw something yesterday that got me thinking about the Force equation F=ma, particularly the acceleration part. On the side of the road I saw a car that had obviously hit something a high velocity. The front end was almost completely destroyed, with the hood bent almost in half and almost a 90 degree angle. My thought was there was a lot of force involved. With regard to the force and acceleration I thought there were 3 scenarios: 1. the car was accelerating; 2. the car might have been in cruise control, having a constant velocity but no acceleration; or 3. the car was attempting to slow down, and therefore de-accelerating. Thinking through the math, scenario 1 appears to be the only one where F is positive. Scenario 2, with zero acceleration indicates a force of zero, and scenario 3 indicates a negative force. I know this cannot be accurate, so what am I missing?
- Bill Morton (age 67)
75056
A:

Think of how that car smashed. That big force on it was obviously approximately pointing from the front of the car toward the back. That caused its acceleration to point backwards, which is why the car lost its forward velocity. If you want to call forwards positive, then you'd call the acceleration negative. 

Mike W.


(published on 06/14/2017)

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