• Support PF! Buy your school textbooks, materials and every day products Here!

Police Problem

  • #1
1,235
1
The question is whether a driver is exceeding a 30 miles/hr speed limit before he makes an emergency stop. The length of the skid marks on the road is 19.2 feet. The police assumed that the maximum deacceleration of the car would not exceed the acceleration of a freely falling body and arrested the driver for speeding. Was this correct to do?

So the acceleration of a freely falling body is [itex] a = -32 \frac{ft}{sec^{2}} [/itex]. So [itex] v_{x}^{2} = v_{x}_{0}^{2} + 2a_{x}(x-x_{0}}) [/itex]. Since [itex] v_{x} = 0 [/itex], [itex] 0 = v_{x}_{0}^{2} - 64(19.2) [/itex]. So [itex] v_{x}_{0}^{2} = 1228.8 [/itex] and [itex] v_{x}_{0} = 35 [/itex]. So he was speeding. Is this correct?

Thanks
 

Answers and Replies

  • #2
Tide
Science Advisor
Homework Helper
3,076
0
Be careful with your units! You used 32 ft/s^2 for the acceleration and 19.2 feet for the distance. What are the units of v? :)
 
  • #3
1,235
1
[itex] v_{x}_{0} = 35 \frac{ft}{sec}[/itex]. Is this correct?

Thanks
 
  • #4
Tide
Science Advisor
Homework Helper
3,076
0
So, was the driver speeding?
 
  • #5
1,235
1
no he wasnt. he was only traveling 23.86 miles per hour. I converted ft/sec to miles/hour.
 

Related Threads on Police Problem

  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
2
Views
34K
Replies
11
Views
1K
Replies
11
Views
5K
Replies
1
Views
4K
Replies
1
Views
5K
  • Last Post
Replies
1
Views
4K
Replies
11
Views
6K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
8
Views
5K
Top