# How to determine the "a" in f=ma or, a=f/m. If a 2200 lb car

1. Aug 9, 2015

### Mellmann

How to determine the "a" in f=ma or, a=f/m. If a 2200 lb car Is stationary and is struck by a 3300 lb vehicle and is propelled 27 feetfrom the impact? The 3300 lb vehicle had to be traveling how fast?

2. Aug 9, 2015

### Orodruin

Staff Emeritus
Your question is ill defined, which acceleration are you talking about? Phisics is more than simply finding formulas to put numbers into, you need to understand what you are doing and when different theories apply.

3. Aug 9, 2015

### Mellmann

Vehicle 1 = 2200 lb
Vehicle 2 = 3300 lb
If veh. 1 is moved 27 feet upon impact from veh. 2. How many miles/hr. Was veh. 2 traveling, or can it not be determined?

4. Aug 9, 2015

### Orodruin

Staff Emeritus
The question is still incomplete. You are missing a lot of important information. Vehicle accident reconstruction is a much more complicated subject than just taking 3 numbers and getting something out of it.

5. Aug 9, 2015

### Mellmann

Thanks, was not looking for hypotheses, or other variables to include, just in a perfect scenario, no other factors.
I found: An estimate of the speed a car travels found by measuring its skid marks:
The formula r=2√5L can be used, where r is the speed, in miles per hour, and L is the length of the skid marks, in feet.
This appears to be more of a "questimate", too many factors unanswered, what if no skid marks & the force from the impact is projected into the other vehicle?

Last edited: Aug 9, 2015
6. Aug 9, 2015

### HallsofIvy

It doesn't work that way. If you used "perfect" physics laws, ignoring everything but "F= ma", then the impacted car would never stop. In a real situation, you have to take into account energy lost to the "crumpling" of the car as well as friction with the road.

7. Aug 9, 2015

Thanks!