Finding the distance given the intital velocity and the coefficient of friction

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Homework Help Overview

The problem involves a car traveling at an initial velocity of 15.5 m/s on a surface with a coefficient of friction of 0.1. The scenario presents a situation where the driver must stop the car to avoid hitting a boy who has run into the road 125 m ahead. The original poster is attempting to determine the stopping distance of the car.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the need to find the acceleration due to friction and its relationship to the forces acting on the car. There are attempts to apply kinematic equations and considerations of energy, with some participants questioning the necessity of mass in the calculations.

Discussion Status

The discussion is ongoing, with participants exploring different approaches to find acceleration and its implications for the stopping distance. Some guidance has been offered regarding the role of mass and friction, but there is no explicit consensus on the method to be used.

Contextual Notes

There is an emphasis on adhering to specific equations from the kinematics and dynamics unit as per the original poster's instructions from their teacher, which may limit the approaches discussed.

lenalee.9520
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Homework Statement


A car is traveling at 15.5m/s travels on a surface where μ=0.1. The driver sees a boy run into the road 125 m ahead. She immedietly applies the brakes, locking the wheel, and skids to a stop. Where will the car be when it stops? (i.e:Will it hit the boy?)

I know I should start by figuring out the acceleration and then use

Vfinal^2=V inital^2 + 2ad

But, I am having trouble finding the acceleration.

Homework Equations


Fnet=ma
Ffriction= μFN
Vfinal^2=V inital^2 + 2ad


The Attempt at a Solution

 
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welcome to pf!

hi lenalee.9520! welcome to pf! :smile:

(try using the X2 button just above the Reply box :wink:)
lenalee.9520 said:
… But, I am having trouble finding the acceleration.

call the mass "m"

then find the force, and divide by m :wink:

(the force will be µ times … ? :smile:)
 
Well I ended up with

Ffriction= -mgµ

Is this correct?
 
Last edited:
call the mass "m" ! :rolleyes:

(it'll cancel out in the end)
 
Ffriction= -mgµ

Is what I got...
But how does it cancel out??
 
You could identify it as an energy related problem.
 
Last edited:
azizlwl said:
You should identify it as an energy related problem.

Well, my teacher told the class that we should only need equations from the kinematics and dynamics unit, so I've been trying to stick to those things. (I have to turn this in, so I would like to follow his instructions as much as possible, for the sake of my mark)
 
As tiny-tim said, call the mass 'm'
Then if you apply force to it, what is the acceleration?
 
\sumF = ma = \mukmg
Once you solve for the acceleration, the rest is just a 'timeless' kinematics problem.
Vf2 = 0 = Vo2 + 2aΔx
 
  • #10
lenalee.9520 said:
Ffriction= -mgµ

Is what I got...
But how does it cancel out??

acceleration = Ffriction/m ? :wink:
 

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