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Kinematic Question -- Car braking to avoid hitting a fox

  • #1

Homework Statement



Andrew is driving his van to deliver groceries in Elmvale. As he travels along Hwy 92, a fox stops on the road. Andrew is travelling at 75 km/h and is 42 m away from the fox when he applies his brakes. It takes him 3.4 s to stop.

a) How far did Andrew travel before stopping?

b) Did Andrew hit the fox? Explain your answer by including a calculation.

c) If a typical van is 4.2 m in length, how many van lengths did it take Andrew to stop?

Homework Equations


Δd = v2+v1/2 (Δt)


The Attempt at a Solution


[/B]
Given:
v1 = 75 km/h
convert 75 km/h to m/s = 20.8 m/s

Δd = 42 m

Δt = 3.4 s


a) Δd = v2+v1/2 (Δt)
Δd = (0 m/s + 20.8 m/s) /2 x (3.4s)
Δd = 35.36 m

Andrew travelled 35.36 m before stopping.

b) 42 m - 35.36 m = 6.64 m
Andrew stoped 6.64 m in front of the fox and therefore did not hit it.

c) 35.36 m / 4.2 m = 8.4 van lengths
It took approximately 8 van lengths for Andrew to stop.

Please look over this and tell me if it is correct. Thanks :)
 

Answers and Replies

  • #2
TSny
Homework Helper
Gold Member
12,393
2,835
It all looks good.

The formula Δd = v2+v1/2 (Δt) should be written with parentheses in the numerator as Δd = (v2+v1)/2 (Δt) in order to indicate the proper order of operations.
 
Last edited:
  • #3
Ok, thanks for checking it out :)
 

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