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Car skidding to a stop

  • #1

Homework Statement


The driver of a car is travelling on a wet, level road, and slams on the brakes. The car slides 88M to a stop, with a coefficient of friction of 0.42. What was the initial speed of the car?


Homework Equations


KE = .5mv2
Δd = v1Δt + .5aΔt2
Any other kinematics, dynamics, and work/energy/power equations.

The Attempt at a Solution


I have tried subbing equations together but I always end up with 3 variables, v1, a, and Δt. I feel like mass is required for any solution, because then friction could be calculated and the rest would be simple.
 

Answers and Replies

  • #2
537
1
well so there's only one force acting on the car during this time, and that's the force due to friction. Try and get the acceleration of the car from that.
 
  • #3
But you can't find the force of friction without the normal force (requiring mass), and if you wanted to find acceleration you would need time, correct?
 
  • #4
537
1
Use Newton's second law of motion and the definition of the force of friction.
 
  • #5
How would I use that, Fnet=ma doesn't really help as I have none of those values.
 
  • #6
537
1
Well you know that the only force acting on the car is the force due to friction, this is

Ffric = μFN = μmg

and you know that the sum of all the forces acting on something is

Fnet = ma

what can you get from that?
 
  • #7
My brain is telling me:
μg = a
But I have the strangest feeling that's wrong.
Thanks for the help by the way.
 
  • #8
537
1
your brain is correct :)

Since we know that Fnet = ma and that Ffric is the only force acting on the car and we also know that Fnet = Ffric

we can therefore say that ma = μmg

and therefore a = μg

from that you can use one of the basic equations of constant acceleration.
 
  • #9
Thanks, I understand how to do the problem now, and that should help me with other ones.
 

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